README
1 Notes about the Chrome memory allocator. 2 3 Background 4 ---------- 5 We use this library as a generic way to fork into any of several allocators. 6 Currently we can, at runtime, switch between: 7 the default windows allocator 8 the windows low-fragmentation-heap 9 tcmalloc 10 jemalloc (the heap used most notably within Mozilla Firefox) 11 12 The mechanism for hooking LIBCMT in windows is rather tricky. The core 13 problem is that by default, the windows library does not declare malloc and 14 free as weak symbols. Because of this, they cannot be overriden. To work 15 around this, we start with the LIBCMT.LIB, and manually remove all allocator 16 related functions from it using the visual studio library tool. Once removed, 17 we can now link against the library and provide custom versions of the 18 allocator related functionality. 19 20 21 Source code 22 ----------- 23 This directory contains just the allocator (i.e. shim) layer that switches 24 between the different underlying memory allocation implementations. 25 26 The tcmalloc and jemalloc libraries originate outside of Chromium 27 and exist in ../../third_party/tcmalloc and ../../third_party/jemalloc 28 (currently, the actual locations are defined in the allocator.gyp file). 29 The third party sources use a vendor-branch SCM pattern to track 30 Chromium-specific changes independently from upstream changes. 31 32 The general intent is to push local changes upstream so that over 33 time we no longer need any forked files. 34 35 36 Adding a new allocator 37 ---------------------- 38 Adding a new allocator requires definition of the following five functions: 39 40 extern "C" { 41 bool init(); 42 void* malloc(size_t s); 43 void* realloc(void* p, size_t s); 44 void free(void* s); 45 size_t msize(void* p); 46 } 47 48 All other allocation related functions (new/delete/calloc/etc) have been 49 implemented generically to work across all allocators. 50 51 52 Usage 53 ----- 54 You can use the different allocators by setting the environment variable 55 CHROME_ALLOCATOR to: 56 "tcmalloc" - TC Malloc (default) 57 "jemalloc" - JE Malloc 58 "winheap" - Windows default heap 59 "winlfh" - Windows Low-Fragmentation heap 60
