1 <?xml version="1.0"?> <!-- -*- sgml -*- --> 2 <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" 4 [ <!ENTITY % vg-entities SYSTEM "../../docs/xml/vg-entities.xml"> %vg-entities; ]> 5 6 7 <chapter id="cg-manual" xreflabel="Cachegrind: a cache and branch-prediction profiler"> 8 <title>Cachegrind: a cache and branch-prediction profiler</title> 9 10 <para>To use this tool, you must specify 11 <option>--tool=cachegrind</option> on the 12 Valgrind command line.</para> 13 14 <sect1 id="cg-manual.overview" xreflabel="Overview"> 15 <title>Overview</title> 16 17 <para>Cachegrind simulates how your program interacts with a machine's cache 18 hierarchy and (optionally) branch predictor. It simulates a machine with 19 independent first-level instruction and data caches (I1 and D1), backed by a 20 unified second-level cache (L2). This exactly matches the configuration of 21 many modern machines.</para> 22 23 <para>However, some modern machines have three levels of cache. For these 24 machines (in the cases where Cachegrind can auto-detect the cache 25 configuration) Cachegrind simulates the first-level and third-level caches. 26 The reason for this choice is that the L3 cache has the most influence on 27 runtime, as it masks accesses to main memory. Furthermore, the L1 caches 28 often have low associativity, so simulating them can detect cases where the 29 code interacts badly with this cache (eg. traversing a matrix column-wise 30 with the row length being a power of 2).</para> 31 32 <para>Therefore, Cachegrind always refers to the I1, D1 and LL (last-level) 33 caches.</para> 34 35 <para> 36 Cachegrind gathers the following statistics (abbreviations used for each statistic 37 is given in parentheses):</para> 38 <itemizedlist> 39 <listitem> 40 <para>I cache reads (<computeroutput>Ir</computeroutput>, 41 which equals the number of instructions executed), 42 I1 cache read misses (<computeroutput>I1mr</computeroutput>) and 43 LL cache instruction read misses (<computeroutput>ILmr</computeroutput>). 44 </para> 45 </listitem> 46 <listitem> 47 <para>D cache reads (<computeroutput>Dr</computeroutput>, which 48 equals the number of memory reads), 49 D1 cache read misses (<computeroutput>D1mr</computeroutput>), and 50 LL cache data read misses (<computeroutput>DLmr</computeroutput>). 51 </para> 52 </listitem> 53 <listitem> 54 <para>D cache writes (<computeroutput>Dw</computeroutput>, which equals 55 the number of memory writes), 56 D1 cache write misses (<computeroutput>D1mw</computeroutput>), and 57 LL cache data write misses (<computeroutput>DLmw</computeroutput>). 58 </para> 59 </listitem> 60 <listitem> 61 <para>Conditional branches executed (<computeroutput>Bc</computeroutput>) and 62 conditional branches mispredicted (<computeroutput>Bcm</computeroutput>). 63 </para> 64 </listitem> 65 <listitem> 66 <para>Indirect branches executed (<computeroutput>Bi</computeroutput>) and 67 indirect branches mispredicted (<computeroutput>Bim</computeroutput>). 68 </para> 69 </listitem> 70 </itemizedlist> 71 72 <para>Note that D1 total accesses is given by 73 <computeroutput>D1mr</computeroutput> + 74 <computeroutput>D1mw</computeroutput>, and that LL total 75 accesses is given by <computeroutput>ILmr</computeroutput> + 76 <computeroutput>DLmr</computeroutput> + 77 <computeroutput>DLmw</computeroutput>. 78 </para> 79 80 <para>These statistics are presented for the entire program and for each 81 function in the program. You can also annotate each line of source code in 82 the program with the counts that were caused directly by it.</para> 83 84 <para>On a modern machine, an L1 miss will typically cost 85 around 10 cycles, an LL miss can cost as much as 200 86 cycles, and a mispredicted branch costs in the region of 10 87 to 30 cycles. Detailed cache and branch profiling can be very useful 88 for understanding how your program interacts with the machine and thus how 89 to make it faster.</para> 90 91 <para>Also, since one instruction cache read is performed per 92 instruction executed, you can find out how many instructions are 93 executed per line, which can be useful for traditional profiling.</para> 94 95 </sect1> 96 97 98 99 <sect1 id="cg-manual.profile" 100 xreflabel="Using Cachegrind, cg_annotate and cg_merge"> 101 <title>Using Cachegrind, cg_annotate and cg_merge</title> 102 103 <para>First off, as for normal Valgrind use, you probably want to 104 compile with debugging info (the 105 <option>-g</option> option). But by contrast with 106 normal Valgrind use, you probably do want to turn 107 optimisation on, since you should profile your program as it will 108 be normally run.</para> 109 110 <para>Then, you need to run Cachegrind itself to gather the profiling 111 information, and then run cg_annotate to get a detailed presentation of that 112 information. As an optional intermediate step, you can use cg_merge to sum 113 together the outputs of multiple Cachegrind runs into a single file which 114 you then use as the input for cg_annotate. Alternatively, you can use 115 cg_diff to difference the outputs of two Cachegrind runs into a signel file 116 which you then use as the input for cg_annotate.</para> 117 118 119 <sect2 id="cg-manual.running-cachegrind" xreflabel="Running Cachegrind"> 120 <title>Running Cachegrind</title> 121 122 <para>To run Cachegrind on a program <filename>prog</filename>, run:</para> 123 <screen><![CDATA[ 124 valgrind --tool=cachegrind prog 125 ]]></screen> 126 127 <para>The program will execute (slowly). Upon completion, 128 summary statistics that look like this will be printed:</para> 129 130 <programlisting><![CDATA[ 131 ==31751== I refs: 27,742,716 132 ==31751== I1 misses: 276 133 ==31751== LLi misses: 275 134 ==31751== I1 miss rate: 0.0% 135 ==31751== LLi miss rate: 0.0% 136 ==31751== 137 ==31751== D refs: 15,430,290 (10,955,517 rd + 4,474,773 wr) 138 ==31751== D1 misses: 41,185 ( 21,905 rd + 19,280 wr) 139 ==31751== LLd misses: 23,085 ( 3,987 rd + 19,098 wr) 140 ==31751== D1 miss rate: 0.2% ( 0.1% + 0.4%) 141 ==31751== LLd miss rate: 0.1% ( 0.0% + 0.4%) 142 ==31751== 143 ==31751== LL misses: 23,360 ( 4,262 rd + 19,098 wr) 144 ==31751== LL miss rate: 0.0% ( 0.0% + 0.4%)]]></programlisting> 145 146 <para>Cache accesses for instruction fetches are summarised 147 first, giving the number of fetches made (this is the number of 148 instructions executed, which can be useful to know in its own 149 right), the number of I1 misses, and the number of LL instruction 150 (<computeroutput>LLi</computeroutput>) misses.</para> 151 152 <para>Cache accesses for data follow. The information is similar 153 to that of the instruction fetches, except that the values are 154 also shown split between reads and writes (note each row's 155 <computeroutput>rd</computeroutput> and 156 <computeroutput>wr</computeroutput> values add up to the row's 157 total).</para> 158 159 <para>Combined instruction and data figures for the LL cache 160 follow that. Note that the LL miss rate is computed relative to the total 161 number of memory accesses, not the number of L1 misses. I.e. it is 162 <computeroutput>(ILmr + DLmr + DLmw) / (Ir + Dr + Dw)</computeroutput> 163 not 164 <computeroutput>(ILmr + DLmr + DLmw) / (I1mr + D1mr + D1mw)</computeroutput> 165 </para> 166 167 <para>Branch prediction statistics are not collected by default. 168 To do so, add the option <option>--branch-sim=yes</option>.</para> 169 170 </sect2> 171 172 173 <sect2 id="cg-manual.outputfile" xreflabel="Output File"> 174 <title>Output File</title> 175 176 <para>As well as printing summary information, Cachegrind also writes 177 more detailed profiling information to a file. By default this file is named 178 <filename>cachegrind.out.<pid></filename> (where 179 <filename><pid></filename> is the program's process ID), but its name 180 can be changed with the <option>--cachegrind-out-file</option> option. This 181 file is human-readable, but is intended to be interpreted by the 182 accompanying program cg_annotate, described in the next section.</para> 183 184 <para>The default <computeroutput>.<pid></computeroutput> suffix 185 on the output file name serves two purposes. Firstly, it means you 186 don't have to rename old log files that you don't want to overwrite. 187 Secondly, and more importantly, it allows correct profiling with the 188 <option>--trace-children=yes</option> option of 189 programs that spawn child processes.</para> 190 191 <para>The output file can be big, many megabytes for large applications 192 built with full debugging information.</para> 193 194 </sect2> 195 196 197 198 <sect2 id="cg-manual.running-cg_annotate" xreflabel="Running cg_annotate"> 199 <title>Running cg_annotate</title> 200 201 <para>Before using cg_annotate, 202 it is worth widening your window to be at least 120-characters 203 wide if possible, as the output lines can be quite long.</para> 204 205 <para>To get a function-by-function summary, run:</para> 206 207 <screen>cg_annotate <filename></screen> 208 209 <para>on a Cachegrind output file.</para> 210 211 </sect2> 212 213 214 <sect2 id="cg-manual.the-output-preamble" xreflabel="The Output Preamble"> 215 <title>The Output Preamble</title> 216 217 <para>The first part of the output looks like this:</para> 218 219 <programlisting><![CDATA[ 220 -------------------------------------------------------------------------------- 221 I1 cache: 65536 B, 64 B, 2-way associative 222 D1 cache: 65536 B, 64 B, 2-way associative 223 LL cache: 262144 B, 64 B, 8-way associative 224 Command: concord vg_to_ucode.c 225 Events recorded: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 226 Events shown: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 227 Event sort order: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 228 Threshold: 99% 229 Chosen for annotation: 230 Auto-annotation: off 231 ]]></programlisting> 232 233 234 <para>This is a summary of the annotation options:</para> 235 236 <itemizedlist> 237 238 <listitem> 239 <para>I1 cache, D1 cache, LL cache: cache configuration. So 240 you know the configuration with which these results were 241 obtained.</para> 242 </listitem> 243 244 <listitem> 245 <para>Command: the command line invocation of the program 246 under examination.</para> 247 </listitem> 248 249 <listitem> 250 <para>Events recorded: which events were recorded.</para> 251 252 </listitem> 253 254 <listitem> 255 <para>Events shown: the events shown, which is a subset of the events 256 gathered. This can be adjusted with the 257 <option>--show</option> option.</para> 258 </listitem> 259 260 <listitem> 261 <para>Event sort order: the sort order in which functions are 262 shown. For example, in this case the functions are sorted 263 from highest <computeroutput>Ir</computeroutput> counts to 264 lowest. If two functions have identical 265 <computeroutput>Ir</computeroutput> counts, they will then be 266 sorted by <computeroutput>I1mr</computeroutput> counts, and 267 so on. This order can be adjusted with the 268 <option>--sort</option> option.</para> 269 270 <para>Note that this dictates the order the functions appear. 271 It is <emphasis>not</emphasis> the order in which the columns 272 appear; that is dictated by the "events shown" line (and can 273 be changed with the <option>--show</option> 274 option).</para> 275 </listitem> 276 277 <listitem> 278 <para>Threshold: cg_annotate 279 by default omits functions that cause very low counts 280 to avoid drowning you in information. In this case, 281 cg_annotate shows summaries the functions that account for 282 99% of the <computeroutput>Ir</computeroutput> counts; 283 <computeroutput>Ir</computeroutput> is chosen as the 284 threshold event since it is the primary sort event. The 285 threshold can be adjusted with the 286 <option>--threshold</option> 287 option.</para> 288 </listitem> 289 290 <listitem> 291 <para>Chosen for annotation: names of files specified 292 manually for annotation; in this case none.</para> 293 </listitem> 294 295 <listitem> 296 <para>Auto-annotation: whether auto-annotation was requested 297 via the <option>--auto=yes</option> 298 option. In this case no.</para> 299 </listitem> 300 301 </itemizedlist> 302 303 </sect2> 304 305 306 <sect2 id="cg-manual.the-global" 307 xreflabel="The Global and Function-level Counts"> 308 <title>The Global and Function-level Counts</title> 309 310 <para>Then follows summary statistics for the whole 311 program:</para> 312 313 <programlisting><![CDATA[ 314 -------------------------------------------------------------------------------- 315 Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 316 -------------------------------------------------------------------------------- 317 27,742,716 276 275 10,955,517 21,905 3,987 4,474,773 19,280 19,098 PROGRAM TOTALS]]></programlisting> 318 319 <para> 320 These are similar to the summary provided when Cachegrind finishes running. 321 </para> 322 323 <para>Then comes function-by-function statistics:</para> 324 325 <programlisting><![CDATA[ 326 -------------------------------------------------------------------------------- 327 Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw file:function 328 -------------------------------------------------------------------------------- 329 8,821,482 5 5 2,242,702 1,621 73 1,794,230 0 0 getc.c:_IO_getc 330 5,222,023 4 4 2,276,334 16 12 875,959 1 1 concord.c:get_word 331 2,649,248 2 2 1,344,810 7,326 1,385 . . . vg_main.c:strcmp 332 2,521,927 2 2 591,215 0 0 179,398 0 0 concord.c:hash 333 2,242,740 2 2 1,046,612 568 22 448,548 0 0 ctype.c:tolower 334 1,496,937 4 4 630,874 9,000 1,400 279,388 0 0 concord.c:insert 335 897,991 51 51 897,831 95 30 62 1 1 ???:??? 336 598,068 1 1 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__flockfile 337 598,068 0 0 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__funlockfile 338 598,024 4 4 213,580 35 16 149,506 0 0 vg_clientmalloc.c:malloc 339 446,587 1 1 215,973 2,167 430 129,948 14,057 13,957 concord.c:add_existing 340 341,760 2 2 128,160 0 0 128,160 0 0 vg_clientmalloc.c:vg_trap_here_WRAPPER 341 320,782 4 4 150,711 276 0 56,027 53 53 concord.c:init_hash_table 342 298,998 1 1 106,785 0 0 64,071 1 1 concord.c:create 343 149,518 0 0 149,516 0 0 1 0 0 ???:tolower@@GLIBC_2.0 344 149,518 0 0 149,516 0 0 1 0 0 ???:fgetc@@GLIBC_2.0 345 95,983 4 4 38,031 0 0 34,409 3,152 3,150 concord.c:new_word_node 346 85,440 0 0 42,720 0 0 21,360 0 0 vg_clientmalloc.c:vg_bogus_epilogue]]></programlisting> 347 348 <para>Each function 349 is identified by a 350 <computeroutput>file_name:function_name</computeroutput> pair. If 351 a column contains only a dot it means the function never performs 352 that event (e.g. the third row shows that 353 <computeroutput>strcmp()</computeroutput> contains no 354 instructions that write to memory). The name 355 <computeroutput>???</computeroutput> is used if the the file name 356 and/or function name could not be determined from debugging 357 information. If most of the entries have the form 358 <computeroutput>???:???</computeroutput> the program probably 359 wasn't compiled with <option>-g</option>.</para> 360 361 <para>It is worth noting that functions will come both from 362 the profiled program (e.g. <filename>concord.c</filename>) 363 and from libraries (e.g. <filename>getc.c</filename>)</para> 364 365 </sect2> 366 367 368 <sect2 id="cg-manual.line-by-line" xreflabel="Line-by-line Counts"> 369 <title>Line-by-line Counts</title> 370 371 <para>There are two ways to annotate source files -- by specifying them 372 manually as arguments to cg_annotate, or with the 373 <option>--auto=yes</option> option. For example, the output from running 374 <filename>cg_annotate <filename> concord.c</filename> for our example 375 produces the same output as above followed by an annotated version of 376 <filename>concord.c</filename>, a section of which looks like:</para> 377 378 <programlisting><![CDATA[ 379 -------------------------------------------------------------------------------- 380 -- User-annotated source: concord.c 381 -------------------------------------------------------------------------------- 382 Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 383 384 . . . . . . . . . void init_hash_table(char *file_name, Word_Node *table[]) 385 3 1 1 . . . 1 0 0 { 386 . . . . . . . . . FILE *file_ptr; 387 . . . . . . . . . Word_Info *data; 388 1 0 0 . . . 1 1 1 int line = 1, i; 389 . . . . . . . . . 390 5 0 0 . . . 3 0 0 data = (Word_Info *) create(sizeof(Word_Info)); 391 . . . . . . . . . 392 4,991 0 0 1,995 0 0 998 0 0 for (i = 0; i < TABLE_SIZE; i++) 393 3,988 1 1 1,994 0 0 997 53 52 table[i] = NULL; 394 . . . . . . . . . 395 . . . . . . . . . /* Open file, check it. */ 396 6 0 0 1 0 0 4 0 0 file_ptr = fopen(file_name, "r"); 397 2 0 0 1 0 0 . . . if (!(file_ptr)) { 398 . . . . . . . . . fprintf(stderr, "Couldn't open '%s'.\n", file_name); 399 1 1 1 . . . . . . exit(EXIT_FAILURE); 400 . . . . . . . . . } 401 . . . . . . . . . 402 165,062 1 1 73,360 0 0 91,700 0 0 while ((line = get_word(data, line, file_ptr)) != EOF) 403 146,712 0 0 73,356 0 0 73,356 0 0 insert(data->;word, data->line, table); 404 . . . . . . . . . 405 4 0 0 1 0 0 2 0 0 free(data); 406 4 0 0 1 0 0 2 0 0 fclose(file_ptr); 407 3 0 0 2 0 0 . . . }]]></programlisting> 408 409 <para>(Although column widths are automatically minimised, a wide 410 terminal is clearly useful.)</para> 411 412 <para>Each source file is clearly marked 413 (<computeroutput>User-annotated source</computeroutput>) as 414 having been chosen manually for annotation. If the file was 415 found in one of the directories specified with the 416 <option>-I</option>/<option>--include</option> option, the directory 417 and file are both given.</para> 418 419 <para>Each line is annotated with its event counts. Events not 420 applicable for a line are represented by a dot. This is useful 421 for distinguishing between an event which cannot happen, and one 422 which can but did not.</para> 423 424 <para>Sometimes only a small section of a source file is 425 executed. To minimise uninteresting output, Cachegrind only shows 426 annotated lines and lines within a small distance of annotated 427 lines. Gaps are marked with the line numbers so you know which 428 part of a file the shown code comes from, eg:</para> 429 430 <programlisting><![CDATA[ 431 (figures and code for line 704) 432 -- line 704 ---------------------------------------- 433 -- line 878 ---------------------------------------- 434 (figures and code for line 878)]]></programlisting> 435 436 <para>The amount of context to show around annotated lines is 437 controlled by the <option>--context</option> 438 option.</para> 439 440 <para>To get automatic annotation, use the <option>--auto=yes</option> option. 441 cg_annotate will automatically annotate every source file it can 442 find that is mentioned in the function-by-function summary. 443 Therefore, the files chosen for auto-annotation are affected by 444 the <option>--sort</option> and 445 <option>--threshold</option> options. Each 446 source file is clearly marked (<computeroutput>Auto-annotated 447 source</computeroutput>) as being chosen automatically. Any 448 files that could not be found are mentioned at the end of the 449 output, eg:</para> 450 451 <programlisting><![CDATA[ 452 ------------------------------------------------------------------ 453 The following files chosen for auto-annotation could not be found: 454 ------------------------------------------------------------------ 455 getc.c 456 ctype.c 457 ../sysdeps/generic/lockfile.c]]></programlisting> 458 459 <para>This is quite common for library files, since libraries are 460 usually compiled with debugging information, but the source files 461 are often not present on a system. If a file is chosen for 462 annotation both manually and automatically, it 463 is marked as <computeroutput>User-annotated 464 source</computeroutput>. Use the 465 <option>-I</option>/<option>--include</option> option to tell Valgrind where 466 to look for source files if the filenames found from the debugging 467 information aren't specific enough.</para> 468 469 <para>Beware that cg_annotate can take some time to digest large 470 <filename>cachegrind.out.<pid></filename> files, 471 e.g. 30 seconds or more. Also beware that auto-annotation can 472 produce a lot of output if your program is large!</para> 473 474 </sect2> 475 476 477 <sect2 id="cg-manual.assembler" xreflabel="Annotating Assembly Code Programs"> 478 <title>Annotating Assembly Code Programs</title> 479 480 <para>Valgrind can annotate assembly code programs too, or annotate 481 the assembly code generated for your C program. Sometimes this is 482 useful for understanding what is really happening when an 483 interesting line of C code is translated into multiple 484 instructions.</para> 485 486 <para>To do this, you just need to assemble your 487 <computeroutput>.s</computeroutput> files with assembly-level debug 488 information. You can use compile with the <option>-S</option> to compile C/C++ 489 programs to assembly code, and then assemble the assembly code files with 490 <option>-g</option> to achieve this. You can then profile and annotate the 491 assembly code source files in the same way as C/C++ source files.</para> 492 493 </sect2> 494 495 <sect2 id="ms-manual.forkingprograms" xreflabel="Forking Programs"> 496 <title>Forking Programs</title> 497 <para>If your program forks, the child will inherit all the profiling data that 498 has been gathered for the parent.</para> 499 500 <para>If the output file format string (controlled by 501 <option>--cachegrind-out-file</option>) does not contain <option>%p</option>, 502 then the outputs from the parent and child will be intermingled in a single 503 output file, which will almost certainly make it unreadable by 504 cg_annotate.</para> 505 </sect2> 506 507 508 <sect2 id="cg-manual.annopts.warnings" xreflabel="cg_annotate Warnings"> 509 <title>cg_annotate Warnings</title> 510 511 <para>There are a couple of situations in which 512 cg_annotate issues warnings.</para> 513 514 <itemizedlist> 515 <listitem> 516 <para>If a source file is more recent than the 517 <filename>cachegrind.out.<pid></filename> file. 518 This is because the information in 519 <filename>cachegrind.out.<pid></filename> is only 520 recorded with line numbers, so if the line numbers change at 521 all in the source (e.g. lines added, deleted, swapped), any 522 annotations will be incorrect.</para> 523 </listitem> 524 <listitem> 525 <para>If information is recorded about line numbers past the 526 end of a file. This can be caused by the above problem, 527 i.e. shortening the source file while using an old 528 <filename>cachegrind.out.<pid></filename> file. If 529 this happens, the figures for the bogus lines are printed 530 anyway (clearly marked as bogus) in case they are 531 important.</para> 532 </listitem> 533 </itemizedlist> 534 535 </sect2> 536 537 538 539 <sect2 id="cg-manual.annopts.things-to-watch-out-for" 540 xreflabel="Unusual Annotation Cases"> 541 <title>Unusual Annotation Cases</title> 542 543 <para>Some odd things that can occur during annotation:</para> 544 545 <itemizedlist> 546 <listitem> 547 <para>If annotating at the assembler level, you might see 548 something like this:</para> 549 <programlisting><![CDATA[ 550 1 0 0 . . . . . . leal -12(%ebp),%eax 551 1 0 0 . . . 1 0 0 movl %eax,84(%ebx) 552 2 0 0 0 0 0 1 0 0 movl $1,-20(%ebp) 553 . . . . . . . . . .align 4,0x90 554 1 0 0 . . . . . . movl $.LnrB,%eax 555 1 0 0 . . . 1 0 0 movl %eax,-16(%ebp)]]></programlisting> 556 557 <para>How can the third instruction be executed twice when 558 the others are executed only once? As it turns out, it 559 isn't. Here's a dump of the executable, using 560 <computeroutput>objdump -d</computeroutput>:</para> 561 <programlisting><![CDATA[ 562 8048f25: 8d 45 f4 lea 0xfffffff4(%ebp),%eax 563 8048f28: 89 43 54 mov %eax,0x54(%ebx) 564 8048f2b: c7 45 ec 01 00 00 00 movl $0x1,0xffffffec(%ebp) 565 8048f32: 89 f6 mov %esi,%esi 566 8048f34: b8 08 8b 07 08 mov $0x8078b08,%eax 567 8048f39: 89 45 f0 mov %eax,0xfffffff0(%ebp)]]></programlisting> 568 569 <para>Notice the extra <computeroutput>mov 570 %esi,%esi</computeroutput> instruction. Where did this come 571 from? The GNU assembler inserted it to serve as the two 572 bytes of padding needed to align the <computeroutput>movl 573 $.LnrB,%eax</computeroutput> instruction on a four-byte 574 boundary, but pretended it didn't exist when adding debug 575 information. Thus when Valgrind reads the debug info it 576 thinks that the <computeroutput>movl 577 $0x1,0xffffffec(%ebp)</computeroutput> instruction covers the 578 address range 0x8048f2b--0x804833 by itself, and attributes 579 the counts for the <computeroutput>mov 580 %esi,%esi</computeroutput> to it.</para> 581 </listitem> 582 583 <!-- 584 I think this isn't true any more, not since cost centres were moved from 585 being associated with instruction addresses to being associated with 586 source line numbers. 587 <listitem> 588 <para>Inlined functions can cause strange results in the 589 function-by-function summary. If a function 590 <computeroutput>inline_me()</computeroutput> is defined in 591 <filename>foo.h</filename> and inlined in the functions 592 <computeroutput>f1()</computeroutput>, 593 <computeroutput>f2()</computeroutput> and 594 <computeroutput>f3()</computeroutput> in 595 <filename>bar.c</filename>, there will not be a 596 <computeroutput>foo.h:inline_me()</computeroutput> function 597 entry. Instead, there will be separate function entries for 598 each inlining site, i.e. 599 <computeroutput>foo.h:f1()</computeroutput>, 600 <computeroutput>foo.h:f2()</computeroutput> and 601 <computeroutput>foo.h:f3()</computeroutput>. To find the 602 total counts for 603 <computeroutput>foo.h:inline_me()</computeroutput>, add up 604 the counts from each entry.</para> 605 606 <para>The reason for this is that although the debug info 607 output by GCC indicates the switch from 608 <filename>bar.c</filename> to <filename>foo.h</filename>, it 609 doesn't indicate the name of the function in 610 <filename>foo.h</filename>, so Valgrind keeps using the old 611 one.</para> 612 </listitem> 613 --> 614 615 <listitem> 616 <para>Sometimes, the same filename might be represented with 617 a relative name and with an absolute name in different parts 618 of the debug info, eg: 619 <filename>/home/user/proj/proj.h</filename> and 620 <filename>../proj.h</filename>. In this case, if you use 621 auto-annotation, the file will be annotated twice with the 622 counts split between the two.</para> 623 </listitem> 624 625 <listitem> 626 <para>Files with more than 65,535 lines cause difficulties 627 for the Stabs-format debug info reader. This is because the line 628 number in the <computeroutput>struct nlist</computeroutput> 629 defined in <filename>a.out.h</filename> under Linux is only a 630 16-bit value. Valgrind can handle some files with more than 631 65,535 lines correctly by making some guesses to identify 632 line number overflows. But some cases are beyond it, in 633 which case you'll get a warning message explaining that 634 annotations for the file might be incorrect.</para> 635 636 <para>If you are using GCC 3.1 or later, this is most likely 637 irrelevant, since GCC switched to using the more modern DWARF2 638 format by default at version 3.1. DWARF2 does not have any such 639 limitations on line numbers.</para> 640 </listitem> 641 642 <listitem> 643 <para>If you compile some files with 644 <option>-g</option> and some without, some 645 events that take place in a file without debug info could be 646 attributed to the last line of a file with debug info 647 (whichever one gets placed before the non-debug-info file in 648 the executable).</para> 649 </listitem> 650 651 </itemizedlist> 652 653 <para>This list looks long, but these cases should be fairly 654 rare.</para> 655 656 </sect2> 657 658 659 <sect2 id="cg-manual.cg_merge" xreflabel="cg_merge"> 660 <title>Merging Profiles with cg_merge</title> 661 662 <para> 663 cg_merge is a simple program which 664 reads multiple profile files, as created by Cachegrind, merges them 665 together, and writes the results into another file in the same format. 666 You can then examine the merged results using 667 <computeroutput>cg_annotate <filename></computeroutput>, as 668 described above. The merging functionality might be useful if you 669 want to aggregate costs over multiple runs of the same program, or 670 from a single parallel run with multiple instances of the same 671 program.</para> 672 673 <para> 674 cg_merge is invoked as follows: 675 </para> 676 677 <programlisting><![CDATA[ 678 cg_merge -o outputfile file1 file2 file3 ...]]></programlisting> 679 680 <para> 681 It reads and checks <computeroutput>file1</computeroutput>, then read 682 and checks <computeroutput>file2</computeroutput> and merges it into 683 the running totals, then the same with 684 <computeroutput>file3</computeroutput>, etc. The final results are 685 written to <computeroutput>outputfile</computeroutput>, or to standard 686 out if no output file is specified.</para> 687 688 <para> 689 Costs are summed on a per-function, per-line and per-instruction 690 basis. Because of this, the order in which the input files does not 691 matter, although you should take care to only mention each file once, 692 since any file mentioned twice will be added in twice.</para> 693 694 <para> 695 cg_merge does not attempt to check 696 that the input files come from runs of the same executable. It will 697 happily merge together profile files from completely unrelated 698 programs. It does however check that the 699 <computeroutput>Events:</computeroutput> lines of all the inputs are 700 identical, so as to ensure that the addition of costs makes sense. 701 For example, it would be nonsensical for it to add a number indicating 702 D1 read references to a number from a different file indicating LL 703 write misses.</para> 704 705 <para> 706 A number of other syntax and sanity checks are done whilst reading the 707 inputs. cg_merge will stop and 708 attempt to print a helpful error message if any of the input files 709 fail these checks.</para> 710 711 </sect2> 712 713 714 <sect2 id="cg-manual.cg_diff" xreflabel="cg_diff"> 715 <title>Differencing Profiles with cg_diff</title> 716 717 <para> 718 cg_diff is a simple program which 719 reads two profile files, as created by Cachegrind, finds the difference 720 between them, and writes the results into another file in the same format. 721 You can then examine the merged results using 722 <computeroutput>cg_annotate <filename></computeroutput>, as 723 described above. This is very useful if you want to measure how a change to 724 a program affected its performance. 725 </para> 726 727 <para> 728 cg_diff is invoked as follows: 729 </para> 730 731 <programlisting><![CDATA[ 732 cg_diff file1 file2]]></programlisting> 733 734 <para> 735 It reads and checks <computeroutput>file1</computeroutput>, then read 736 and checks <computeroutput>file2</computeroutput>, then computes the 737 difference (effectively <computeroutput>file1</computeroutput> - 738 <computeroutput>file2</computeroutput>). The final results are written to 739 standard output.</para> 740 741 <para> 742 Costs are summed on a per-function basis. Per-line costs are not summed, 743 because doing so is too difficult. For example, consider differencing two 744 profiles, one from a single-file program A, and one from the same program A 745 where a single blank line was inserted at the top of the file. Every single 746 per-line count has changed. In comparison, the per-function counts have not 747 changed. The per-function count differences are still very useful for 748 determining differences between programs. Note that because the result is 749 the difference of two profiles, many of the counts will be negative; this 750 indicates that the counts for the relevant function are fewer in the second 751 version than those in the first version.</para> 752 753 <para> 754 cg_diff does not attempt to check 755 that the input files come from runs of the same executable. It will 756 happily merge together profile files from completely unrelated 757 programs. It does however check that the 758 <computeroutput>Events:</computeroutput> lines of all the inputs are 759 identical, so as to ensure that the addition of costs makes sense. 760 For example, it would be nonsensical for it to add a number indicating 761 D1 read references to a number from a different file indicating LL 762 write misses.</para> 763 764 <para> 765 A number of other syntax and sanity checks are done whilst reading the 766 inputs. cg_diff will stop and 767 attempt to print a helpful error message if any of the input files 768 fail these checks.</para> 769 770 <para> 771 Sometimes you will want to compare Cachegrind profiles of two versions of a 772 program that you have sitting side-by-side. For example, you might have 773 <computeroutput>version1/prog.c</computeroutput> and 774 <computeroutput>version2/prog.c</computeroutput>, where the second is 775 slightly different to the first. A straight comparison of the two will not 776 be useful -- because functions are qualified with filenames, a function 777 <function>f</function> will be listed as 778 <computeroutput>version1/prog.c:f</computeroutput> for the first version but 779 <computeroutput>version2/prog.c:f</computeroutput> for the second 780 version.</para> 781 782 <para> 783 When this happens, you can use the <option>--mod-filename</option> option. 784 Its argument is a Perl search-and-replace expression that will be applied 785 to all the filenames in both Cachegrind output files. It can be used to 786 remove minor differences in filenames. For example, the option 787 <option>--mod-filename='s/version[0-9]/versionN/'</option> will suffice for 788 this case.</para> 789 790 </sect2> 791 792 793 </sect1> 794 795 796 797 <sect1 id="cg-manual.cgopts" xreflabel="Cachegrind Command-line Options"> 798 <title>Cachegrind Command-line Options</title> 799 800 <!-- start of xi:include in the manpage --> 801 <para>Cachegrind-specific options are:</para> 802 803 <variablelist id="cg.opts.list"> 804 805 <varlistentry id="opt.I1" xreflabel="--I1"> 806 <term> 807 <option><![CDATA[--I1=<size>,<associativity>,<line size> ]]></option> 808 </term> 809 <listitem> 810 <para>Specify the size, associativity and line size of the level 1 811 instruction cache. </para> 812 </listitem> 813 </varlistentry> 814 815 <varlistentry id="opt.D1" xreflabel="--D1"> 816 <term> 817 <option><![CDATA[--D1=<size>,<associativity>,<line size> ]]></option> 818 </term> 819 <listitem> 820 <para>Specify the size, associativity and line size of the level 1 821 data cache.</para> 822 </listitem> 823 </varlistentry> 824 825 <varlistentry id="opt.LL" xreflabel="--LL"> 826 <term> 827 <option><![CDATA[--LL=<size>,<associativity>,<line size> ]]></option> 828 </term> 829 <listitem> 830 <para>Specify the size, associativity and line size of the last-level 831 cache.</para> 832 </listitem> 833 </varlistentry> 834 835 <varlistentry id="opt.cache-sim" xreflabel="--cache-sim"> 836 <term> 837 <option><![CDATA[--cache-sim=no|yes [yes] ]]></option> 838 </term> 839 <listitem> 840 <para>Enables or disables collection of cache access and miss 841 counts.</para> 842 </listitem> 843 </varlistentry> 844 845 <varlistentry id="opt.branch-sim" xreflabel="--branch-sim"> 846 <term> 847 <option><![CDATA[--branch-sim=no|yes [no] ]]></option> 848 </term> 849 <listitem> 850 <para>Enables or disables collection of branch instruction and 851 misprediction counts. By default this is disabled as it 852 slows Cachegrind down by approximately 25%. Note that you 853 cannot specify <option>--cache-sim=no</option> 854 and <option>--branch-sim=no</option> 855 together, as that would leave Cachegrind with no 856 information to collect.</para> 857 </listitem> 858 </varlistentry> 859 860 <varlistentry id="opt.cachegrind-out-file" xreflabel="--cachegrind-out-file"> 861 <term> 862 <option><![CDATA[--cachegrind-out-file=<file> ]]></option> 863 </term> 864 <listitem> 865 <para>Write the profile data to 866 <computeroutput>file</computeroutput> rather than to the default 867 output file, 868 <filename>cachegrind.out.<pid></filename>. The 869 <option>%p</option> and <option>%q</option> format specifiers 870 can be used to embed the process ID and/or the contents of an 871 environment variable in the name, as is the case for the core 872 option <option><xref linkend="opt.log-file"/></option>. 873 </para> 874 </listitem> 875 </varlistentry> 876 877 </variablelist> 878 <!-- end of xi:include in the manpage --> 879 880 </sect1> 881 882 883 884 <sect1 id="cg-manual.annopts" xreflabel="cg_annotate Command-line Options"> 885 <title>cg_annotate Command-line Options</title> 886 887 <!-- start of xi:include in the manpage --> 888 <variablelist id="cg_annotate.opts.list"> 889 890 <varlistentry> 891 <term> 892 <option><![CDATA[-h --help ]]></option> 893 </term> 894 <listitem> 895 <para>Show the help message.</para> 896 </listitem> 897 </varlistentry> 898 899 <varlistentry> 900 <term> 901 <option><![CDATA[--version ]]></option> 902 </term> 903 <listitem> 904 <para>Show the version number.</para> 905 </listitem> 906 </varlistentry> 907 908 <varlistentry> 909 <term> 910 <option><![CDATA[--show=A,B,C [default: all, using order in 911 cachegrind.out.<pid>] ]]></option> 912 </term> 913 <listitem> 914 <para>Specifies which events to show (and the column 915 order). Default is to use all present in the 916 <filename>cachegrind.out.<pid></filename> file (and 917 use the order in the file). Useful if you want to concentrate on, for 918 example, I cache misses (<option>--show=I1mr,ILmr</option>), or data 919 read misses (<option>--show=D1mr,DLmr</option>), or LL data misses 920 (<option>--show=DLmr,DLmw</option>). Best used in conjunction with 921 <option>--sort</option>.</para> 922 </listitem> 923 </varlistentry> 924 925 <varlistentry> 926 <term> 927 <option><![CDATA[--sort=A,B,C [default: order in 928 cachegrind.out.<pid>] ]]></option> 929 </term> 930 <listitem> 931 <para>Specifies the events upon which the sorting of the 932 function-by-function entries will be based.</para> 933 </listitem> 934 </varlistentry> 935 936 <varlistentry> 937 <term> 938 <option><![CDATA[--threshold=X [default: 0.1%] ]]></option> 939 </term> 940 <listitem> 941 <para>Sets the threshold for the function-by-function 942 summary. A function is shown if it accounts for more than X% 943 of the counts for the primary sort event. If auto-annotating, also 944 affects which files are annotated.</para> 945 946 <para>Note: thresholds can be set for more than one of the 947 events by appending any events for the 948 <option>--sort</option> option with a colon 949 and a number (no spaces, though). E.g. if you want to see 950 each function that covers more than 1% of LL read misses or 1% of LL 951 write misses, use this option:</para> 952 <para><option>--sort=DLmr:1,DLmw:1</option></para> 953 </listitem> 954 </varlistentry> 955 956 <varlistentry> 957 <term> 958 <option><![CDATA[--auto=<no|yes> [default: no] ]]></option> 959 </term> 960 <listitem> 961 <para>When enabled, automatically annotates every file that 962 is mentioned in the function-by-function summary that can be 963 found. Also gives a list of those that couldn't be found.</para> 964 </listitem> 965 </varlistentry> 966 967 <varlistentry> 968 <term> 969 <option><![CDATA[--context=N [default: 8] ]]></option> 970 </term> 971 <listitem> 972 <para>Print N lines of context before and after each 973 annotated line. Avoids printing large sections of source 974 files that were not executed. Use a large number 975 (e.g. 100000) to show all source lines.</para> 976 </listitem> 977 </varlistentry> 978 979 <varlistentry> 980 <term> 981 <option><![CDATA[-I<dir> --include=<dir> [default: none] ]]></option> 982 </term> 983 <listitem> 984 <para>Adds a directory to the list in which to search for 985 files. Multiple <option>-I</option>/<option>--include</option> 986 options can be given to add multiple directories.</para> 987 </listitem> 988 </varlistentry> 989 990 </variablelist> 991 <!-- end of xi:include in the manpage --> 992 993 </sect1> 994 995 996 <sect1 id="cg-manual.diffopts" xreflabel="cg_diff Command-line Options"> 997 <title>cg_diff Command-line Options</title> 998 999 <!-- start of xi:include in the manpage --> 1000 <variablelist id="cg_diff.opts.list"> 1001 1002 <varlistentry> 1003 <term> 1004 <option><![CDATA[-h --help ]]></option> 1005 </term> 1006 <listitem> 1007 <para>Show the help message.</para> 1008 </listitem> 1009 </varlistentry> 1010 1011 <varlistentry> 1012 <term> 1013 <option><![CDATA[--version ]]></option> 1014 </term> 1015 <listitem> 1016 <para>Show the version number.</para> 1017 </listitem> 1018 </varlistentry> 1019 1020 <varlistentry> 1021 <term> 1022 <option><![CDATA[--mod-filename=<expr> [default: none]]]></option> 1023 </term> 1024 <listitem> 1025 <para>Specifies a Perl search-and-replace expression that is applied 1026 to all filenames. Useful for removing minor differences in paths 1027 between two different versions of a program that are sitting in 1028 different directories.</para> 1029 </listitem> 1030 </varlistentry> 1031 1032 </variablelist> 1033 <!-- end of xi:include in the manpage --> 1034 1035 </sect1> 1036 1037 1038 1039 1040 <sect1 id="cg-manual.acting-on" 1041 xreflabel="Acting on Cachegrind's Information"> 1042 <title>Acting on Cachegrind's Information</title> 1043 <para> 1044 Cachegrind gives you lots of information, but acting on that information 1045 isn't always easy. Here are some rules of thumb that we have found to be 1046 useful.</para> 1047 1048 <para> 1049 First of all, the global hit/miss counts and miss rates are not that useful. 1050 If you have multiple programs or multiple runs of a program, comparing the 1051 numbers might identify if any are outliers and worthy of closer 1052 investigation. Otherwise, they're not enough to act on.</para> 1053 1054 <para> 1055 The function-by-function counts are more useful to look at, as they pinpoint 1056 which functions are causing large numbers of counts. However, beware that 1057 inlining can make these counts misleading. If a function 1058 <function>f</function> is always inlined, counts will be attributed to the 1059 functions it is inlined into, rather than itself. However, if you look at 1060 the line-by-line annotations for <function>f</function> you'll see the 1061 counts that belong to <function>f</function>. (This is hard to avoid, it's 1062 how the debug info is structured.) So it's worth looking for large numbers 1063 in the line-by-line annotations.</para> 1064 1065 <para> 1066 The line-by-line source code annotations are much more useful. In our 1067 experience, the best place to start is by looking at the 1068 <computeroutput>Ir</computeroutput> numbers. They simply measure how many 1069 instructions were executed for each line, and don't include any cache 1070 information, but they can still be very useful for identifying 1071 bottlenecks.</para> 1072 1073 <para> 1074 After that, we have found that LL misses are typically a much bigger source 1075 of slow-downs than L1 misses. So it's worth looking for any snippets of 1076 code with high <computeroutput>DLmr</computeroutput> or 1077 <computeroutput>DLmw</computeroutput> counts. (You can use 1078 <option>--show=DLmr 1079 --sort=DLmr</option> with cg_annotate to focus just on 1080 <literal>DLmr</literal> counts, for example.) If you find any, it's still 1081 not always easy to work out how to improve things. You need to have a 1082 reasonable understanding of how caches work, the principles of locality, and 1083 your program's data access patterns. Improving things may require 1084 redesigning a data structure, for example.</para> 1085 1086 <para> 1087 Looking at the <computeroutput>Bcm</computeroutput> and 1088 <computeroutput>Bim</computeroutput> misses can also be helpful. 1089 In particular, <computeroutput>Bim</computeroutput> misses are often caused 1090 by <literal>switch</literal> statements, and in some cases these 1091 <literal>switch</literal> statements can be replaced with table-driven code. 1092 For example, you might replace code like this:</para> 1093 1094 <programlisting><![CDATA[ 1095 enum E { A, B, C }; 1096 enum E e; 1097 int i; 1098 ... 1099 switch (e) 1100 { 1101 case A: i += 1; 1102 case B: i += 2; 1103 case C: i += 3; 1104 } 1105 ]]></programlisting> 1106 1107 <para>with code like this:</para> 1108 1109 <programlisting><![CDATA[ 1110 enum E { A, B, C }; 1111 enum E e; 1112 enum E table[] = { 1, 2, 3 }; 1113 int i; 1114 ... 1115 i += table[e]; 1116 ]]></programlisting> 1117 1118 <para> 1119 This is obviously a contrived example, but the basic principle applies in a 1120 wide variety of situations.</para> 1121 1122 <para> 1123 In short, Cachegrind can tell you where some of the bottlenecks in your code 1124 are, but it can't tell you how to fix them. You have to work that out for 1125 yourself. But at least you have the information! 1126 </para> 1127 1128 </sect1> 1129 1130 1131 <sect1 id="cg-manual.sim-details" 1132 xreflabel="Simulation Details"> 1133 <title>Simulation Details</title> 1134 <para> 1135 This section talks about details you don't need to know about in order to 1136 use Cachegrind, but may be of interest to some people. 1137 </para> 1138 1139 <sect2 id="cache-sim" xreflabel="Cache Simulation Specifics"> 1140 <title>Cache Simulation Specifics</title> 1141 1142 <para>Specific characteristics of the cache simulation are as 1143 follows:</para> 1144 1145 <itemizedlist> 1146 1147 <listitem> 1148 <para>Write-allocate: when a write miss occurs, the block 1149 written to is brought into the D1 cache. Most modern caches 1150 have this property.</para> 1151 </listitem> 1152 1153 <listitem> 1154 <para>Bit-selection hash function: the set of line(s) in the cache 1155 to which a memory block maps is chosen by the middle bits 1156 M--(M+N-1) of the byte address, where:</para> 1157 <itemizedlist> 1158 <listitem> 1159 <para>line size = 2^M bytes</para> 1160 </listitem> 1161 <listitem> 1162 <para>(cache size / line size / associativity) = 2^N bytes</para> 1163 </listitem> 1164 </itemizedlist> 1165 </listitem> 1166 1167 <listitem> 1168 <para>Inclusive LL cache: the LL cache typically replicates all 1169 the entries of the L1 caches, because fetching into L1 involves 1170 fetching into LL first (this does not guarantee strict inclusiveness, 1171 as lines evicted from LL still could reside in L1). This is 1172 standard on Pentium chips, but AMD Opterons, Athlons and Durons 1173 use an exclusive LL cache that only holds 1174 blocks evicted from L1. Ditto most modern VIA CPUs.</para> 1175 </listitem> 1176 1177 </itemizedlist> 1178 1179 <para>The cache configuration simulated (cache size, 1180 associativity and line size) is determined automatically using 1181 the x86 CPUID instruction. If you have a machine that (a) 1182 doesn't support the CPUID instruction, or (b) supports it in an 1183 early incarnation that doesn't give any cache information, then 1184 Cachegrind will fall back to using a default configuration (that 1185 of a model 3/4 Athlon). Cachegrind will tell you if this 1186 happens. You can manually specify one, two or all three levels 1187 (I1/D1/LL) of the cache from the command line using the 1188 <option>--I1</option>, 1189 <option>--D1</option> and 1190 <option>--LL</option> options. 1191 For cache parameters to be valid for simulation, the number 1192 of sets (with associativity being the number of cache lines in 1193 each set) has to be a power of two.</para> 1194 1195 <para>On PowerPC platforms 1196 Cachegrind cannot automatically 1197 determine the cache configuration, so you will 1198 need to specify it with the 1199 <option>--I1</option>, 1200 <option>--D1</option> and 1201 <option>--LL</option> options.</para> 1202 1203 1204 <para>Other noteworthy behaviour:</para> 1205 1206 <itemizedlist> 1207 <listitem> 1208 <para>References that straddle two cache lines are treated as 1209 follows:</para> 1210 <itemizedlist> 1211 <listitem> 1212 <para>If both blocks hit --> counted as one hit</para> 1213 </listitem> 1214 <listitem> 1215 <para>If one block hits, the other misses --> counted 1216 as one miss.</para> 1217 </listitem> 1218 <listitem> 1219 <para>If both blocks miss --> counted as one miss (not 1220 two)</para> 1221 </listitem> 1222 </itemizedlist> 1223 </listitem> 1224 1225 <listitem> 1226 <para>Instructions that modify a memory location 1227 (e.g. <computeroutput>inc</computeroutput> and 1228 <computeroutput>dec</computeroutput>) are counted as doing 1229 just a read, i.e. a single data reference. This may seem 1230 strange, but since the write can never cause a miss (the read 1231 guarantees the block is in the cache) it's not very 1232 interesting.</para> 1233 1234 <para>Thus it measures not the number of times the data cache 1235 is accessed, but the number of times a data cache miss could 1236 occur.</para> 1237 </listitem> 1238 1239 </itemizedlist> 1240 1241 <para>If you are interested in simulating a cache with different 1242 properties, it is not particularly hard to write your own cache 1243 simulator, or to modify the existing ones in 1244 <computeroutput>cg_sim.c</computeroutput>. We'd be 1245 interested to hear from anyone who does.</para> 1246 1247 </sect2> 1248 1249 1250 <sect2 id="branch-sim" xreflabel="Branch Simulation Specifics"> 1251 <title>Branch Simulation Specifics</title> 1252 1253 <para>Cachegrind simulates branch predictors intended to be 1254 typical of mainstream desktop/server processors of around 2004.</para> 1255 1256 <para>Conditional branches are predicted using an array of 16384 2-bit 1257 saturating counters. The array index used for a branch instruction is 1258 computed partly from the low-order bits of the branch instruction's 1259 address and partly using the taken/not-taken behaviour of the last few 1260 conditional branches. As a result the predictions for any specific 1261 branch depend both on its own history and the behaviour of previous 1262 branches. This is a standard technique for improving prediction 1263 accuracy.</para> 1264 1265 <para>For indirect branches (that is, jumps to unknown destinations) 1266 Cachegrind uses a simple branch target address predictor. Targets are 1267 predicted using an array of 512 entries indexed by the low order 9 1268 bits of the branch instruction's address. Each branch is predicted to 1269 jump to the same address it did last time. Any other behaviour causes 1270 a mispredict.</para> 1271 1272 <para>More recent processors have better branch predictors, in 1273 particular better indirect branch predictors. Cachegrind's predictor 1274 design is deliberately conservative so as to be representative of the 1275 large installed base of processors which pre-date widespread 1276 deployment of more sophisticated indirect branch predictors. In 1277 particular, late model Pentium 4s (Prescott), Pentium M, Core and Core 1278 2 have more sophisticated indirect branch predictors than modelled by 1279 Cachegrind. </para> 1280 1281 <para>Cachegrind does not simulate a return stack predictor. It 1282 assumes that processors perfectly predict function return addresses, 1283 an assumption which is probably close to being true.</para> 1284 1285 <para>See Hennessy and Patterson's classic text "Computer 1286 Architecture: A Quantitative Approach", 4th edition (2007), Section 1287 2.3 (pages 80-89) for background on modern branch predictors.</para> 1288 1289 </sect2> 1290 1291 <sect2 id="cg-manual.annopts.accuracy" xreflabel="Accuracy"> 1292 <title>Accuracy</title> 1293 1294 <para>Valgrind's cache profiling has a number of 1295 shortcomings:</para> 1296 1297 <itemizedlist> 1298 <listitem> 1299 <para>It doesn't account for kernel activity -- the effect of system 1300 calls on the cache and branch predictor contents is ignored.</para> 1301 </listitem> 1302 1303 <listitem> 1304 <para>It doesn't account for other process activity. 1305 This is probably desirable when considering a single 1306 program.</para> 1307 </listitem> 1308 1309 <listitem> 1310 <para>It doesn't account for virtual-to-physical address 1311 mappings. Hence the simulation is not a true 1312 representation of what's happening in the 1313 cache. Most caches and branch predictors are physically indexed, but 1314 Cachegrind simulates caches using virtual addresses.</para> 1315 </listitem> 1316 1317 <listitem> 1318 <para>It doesn't account for cache misses not visible at the 1319 instruction level, e.g. those arising from TLB misses, or 1320 speculative execution.</para> 1321 </listitem> 1322 1323 <listitem> 1324 <para>Valgrind will schedule 1325 threads differently from how they would be when running natively. 1326 This could warp the results for threaded programs.</para> 1327 </listitem> 1328 1329 <listitem> 1330 <para>The x86/amd64 instructions <computeroutput>bts</computeroutput>, 1331 <computeroutput>btr</computeroutput> and 1332 <computeroutput>btc</computeroutput> will incorrectly be 1333 counted as doing a data read if both the arguments are 1334 registers, eg:</para> 1335 <programlisting><![CDATA[ 1336 btsl %eax, %edx]]></programlisting> 1337 1338 <para>This should only happen rarely.</para> 1339 </listitem> 1340 1341 <listitem> 1342 <para>x86/amd64 FPU instructions with data sizes of 28 and 108 bytes 1343 (e.g. <computeroutput>fsave</computeroutput>) are treated as 1344 though they only access 16 bytes. These instructions seem to 1345 be rare so hopefully this won't affect accuracy much.</para> 1346 </listitem> 1347 1348 </itemizedlist> 1349 1350 <para>Another thing worth noting is that results are very sensitive. 1351 Changing the size of the the executable being profiled, or the sizes 1352 of any of the shared libraries it uses, or even the length of their 1353 file names, can perturb the results. Variations will be small, but 1354 don't expect perfectly repeatable results if your program changes at 1355 all.</para> 1356 1357 <para>More recent GNU/Linux distributions do address space 1358 randomisation, in which identical runs of the same program have their 1359 shared libraries loaded at different locations, as a security measure. 1360 This also perturbs the results.</para> 1361 1362 <para>While these factors mean you shouldn't trust the results to 1363 be super-accurate, they should be close enough to be useful.</para> 1364 1365 </sect2> 1366 1367 </sect1> 1368 1369 1370 1371 <sect1 id="cg-manual.impl-details" 1372 xreflabel="Implementation Details"> 1373 <title>Implementation Details</title> 1374 <para> 1375 This section talks about details you don't need to know about in order to 1376 use Cachegrind, but may be of interest to some people. 1377 </para> 1378 1379 <sect2 id="cg-manual.impl-details.how-cg-works" 1380 xreflabel="How Cachegrind Works"> 1381 <title>How Cachegrind Works</title> 1382 <para>The best reference for understanding how Cachegrind works is chapter 3 of 1383 "Dynamic Binary Analysis and Instrumentation", by Nicholas Nethercote. It 1384 is available on the <ulink url="&vg-pubs-url;">Valgrind publications 1385 page</ulink>.</para> 1386 </sect2> 1387 1388 <sect2 id="cg-manual.impl-details.file-format" 1389 xreflabel="Cachegrind Output File Format"> 1390 <title>Cachegrind Output File Format</title> 1391 <para>The file format is fairly straightforward, basically giving the 1392 cost centre for every line, grouped by files and 1393 functions. It's also totally generic and self-describing, in the sense that 1394 it can be used for any events that can be counted on a line-by-line basis, 1395 not just cache and branch predictor events. For example, earlier versions 1396 of Cachegrind didn't have a branch predictor simulation. When this was 1397 added, the file format didn't need to change at all. So the format (and 1398 consequently, cg_annotate) could be used by other tools.</para> 1399 1400 <para>The file format:</para> 1401 <programlisting><![CDATA[ 1402 file ::= desc_line* cmd_line events_line data_line+ summary_line 1403 desc_line ::= "desc:" ws? non_nl_string 1404 cmd_line ::= "cmd:" ws? cmd 1405 events_line ::= "events:" ws? (event ws)+ 1406 data_line ::= file_line | fn_line | count_line 1407 file_line ::= "fl=" filename 1408 fn_line ::= "fn=" fn_name 1409 count_line ::= line_num ws? (count ws)+ 1410 summary_line ::= "summary:" ws? (count ws)+ 1411 count ::= num | "."]]></programlisting> 1412 1413 <para>Where:</para> 1414 <itemizedlist> 1415 <listitem> 1416 <para><computeroutput>non_nl_string</computeroutput> is any 1417 string not containing a newline.</para> 1418 </listitem> 1419 <listitem> 1420 <para><computeroutput>cmd</computeroutput> is a string holding the 1421 command line of the profiled program.</para> 1422 </listitem> 1423 <listitem> 1424 <para><computeroutput>event</computeroutput> is a string containing 1425 no whitespace.</para> 1426 </listitem> 1427 <listitem> 1428 <para><computeroutput>filename</computeroutput> and 1429 <computeroutput>fn_name</computeroutput> are strings.</para> 1430 </listitem> 1431 <listitem> 1432 <para><computeroutput>num</computeroutput> and 1433 <computeroutput>line_num</computeroutput> are decimal 1434 numbers.</para> 1435 </listitem> 1436 <listitem> 1437 <para><computeroutput>ws</computeroutput> is whitespace.</para> 1438 </listitem> 1439 </itemizedlist> 1440 1441 <para>The contents of the "desc:" lines are printed out at the top 1442 of the summary. This is a generic way of providing simulation 1443 specific information, e.g. for giving the cache configuration for 1444 cache simulation.</para> 1445 1446 <para>More than one line of info can be presented for each file/fn/line number. 1447 In such cases, the counts for the named events will be accumulated.</para> 1448 1449 <para>Counts can be "." to represent zero. This makes the files easier for 1450 humans to read.</para> 1451 1452 <para>The number of counts in each 1453 <computeroutput>line</computeroutput> and the 1454 <computeroutput>summary_line</computeroutput> should not exceed 1455 the number of events in the 1456 <computeroutput>event_line</computeroutput>. If the number in 1457 each <computeroutput>line</computeroutput> is less, cg_annotate 1458 treats those missing as though they were a "." entry. This saves space. 1459 </para> 1460 1461 <para>A <computeroutput>file_line</computeroutput> changes the 1462 current file name. A <computeroutput>fn_line</computeroutput> 1463 changes the current function name. A 1464 <computeroutput>count_line</computeroutput> contains counts that 1465 pertain to the current filename/fn_name. A "fn=" 1466 <computeroutput>file_line</computeroutput> and a 1467 <computeroutput>fn_line</computeroutput> must appear before any 1468 <computeroutput>count_line</computeroutput>s to give the context 1469 of the first <computeroutput>count_line</computeroutput>s.</para> 1470 1471 <para>Each <computeroutput>file_line</computeroutput> will normally be 1472 immediately followed by a <computeroutput>fn_line</computeroutput>. But it 1473 doesn't have to be.</para> 1474 1475 <para>The summary line is redundant, because it just holds the total counts 1476 for each event. But this serves as a useful sanity check of the data; if 1477 the totals for each event don't match the summary line, something has gone 1478 wrong.</para> 1479 1480 </sect2> 1481 1482 </sect1> 1483 </chapter> 1484
