1 <html devsite> 2 <head> 3 <title>Implementing ART Just-In-Time (JIT) Compiler</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 26 <p> 27 Android 7.0 adds a just-in-time (JIT) compiler with code profiling to Android 28 runtime (ART) that constantly improves the performance of Android apps as they 29 run. The JIT compiler complements ART's current ahead-of-time (AOT) compiler and 30 improves runtime performance, saves storage space, and speeds app updates and 31 system updates. 32 </p> 33 34 <p> 35 The JIT compiler also improves upon the AOT compiler by avoiding system slowdown 36 during automatic application updates or recompilation of applications during 37 OTAs. This feature should require minimal device integration on the part of 38 manufacturers. 39 </p> 40 41 <p> 42 JIT and AOT use the same compiler with an almost identical set of optimizations. 43 The generated code might not be the same but it depends. JIT makes uses of 44 runtime type information and can do better inlining. Also, with JIT we sometimes 45 do OSR compilation (on stack replacement) which will again generate a bit 46 different code. 47 </p> 48 49 <h2 id="architectural-overview">Architectural Overview</h2> 50 51 <img src="/devices/tech/dalvik/images/jit-arch.png" alt="JIT architecture" width="633" id="JIT-architecture" /> 52 <p class="img-caption"> 53 <strong>Figure 1.</strong> JIT architecture - how it works 54 </p> 55 56 <h2 id="flow">Flow</h2> 57 58 <p> 59 JIT compilation works in this manner: 60 </p> 61 62 <ol> 63 <li>The user runs the app, which then triggers ART to load the .dex file. 64 <li>If the .oat file (the AOT binary for the .dex file) is available, ART uses 65 them directly. Note that .oat files are generated regularly. However, that does 66 not imply they contain compiled code (AOT binary). 67 <li>If no .oat file is available, ART runs through either JIT or an interpreter 68 to execute the .dex file. ART will always use the .oat files if available. 69 Otherwise, it will use the APK and extract it in memory to get to the .dex 70 incurring a big memory overhead (equal to the size of the dex files). 71 <li>JIT is enabled for any application that is not compiled according to the 72 "speed" compilation filter (which says, compile as much as you can from the 73 app). 74 <li>The JIT profile data is dumped to a file in a system directory. Only the 75 application has access to the directory. 76 <li>The AOT compilation (dex2oat) daemon parses that file to drive its 77 compilation.</li> 78 </ol> 79 80 <img src="/devices/tech/dalvik/images/jit-profile-comp.png" alt="Profile-guided comp" width="452" id="JIT-profile-comp" /> 81 <p class="img-caption"> 82 <strong>Figure 2.</strong> Profile-guided compilation 83 </p> 84 85 <img src="/devices/tech/dalvik/images/jit-daemon.png" alt="JIT daemon" width="718" id="JIT-daemon" /> 86 <p class="img-caption"> 87 <strong>Figure 3.</strong> How the daemon works 88 </p> 89 90 <p> 91 The Google Play service is an example used by other apps. These application tend 92 to behave more like shared libraries. 93 </p> 94 95 <h2 id="jit-workflow">JIT Workflow</h2> 96 <p> 97 See the following high-level overview of how JIT works in the next diagram. 98 </p> 99 100 <img src="/devices/tech/dalvik/images/jit-workflow.png" alt="JIT architecture" width="707" id="JIT-workflow" /> 101 <p class="img-caption"> 102 <strong>Figure 4.</strong> JIT data flow 103 </p> 104 105 <p> 106 This means: 107 </p> 108 109 <ul> 110 <li>Profiling information is stored in the code cache and subjected to garbage 111 collection under memory pressure. 112 <li>As a result, theres no guarantee the snapshot taken when the application is 113 in the background will contain the complete data (i.e. everything that was 114 JITed). 115 <li>There is no attempt to make sure we record everything as that will impact 116 runtime performance. 117 <li>Methods can be in three different states: <ul> 118 <li>interpreted (dex code) 119 <li>JIT compiled 120 <li>AOT compiled 121 <li>If both, JIT and AOT code exists (e.g. due to repeated de-optimizations), 122 the JITed code will be preferred. 123 <li>The memory requirement to run JIT without impacting foreground app 124 performance depends upon the app in question. Large apps will require more 125 memory than small apps. In general, big apps stabilize around 4 MB.</li></ul> 126 </li> 127 </ul> 128 129 <h2 id="tuning">Useful tips</h2> 130 131 <h3 id="turn-on-jit-logging">Turn on JIT logging</h3> 132 133 <pre class="devsite-click-to-copy"> 134 <code class="devsite-terminal">adb root</code> 135 <code class="devsite-terminal">adb shell stop</code> 136 <code class="devsite-terminal">adb shell setprop dalvik.vm.extra-opts -verbose:jit</code> 137 <code class="devsite-terminal">adb shell start</code> 138 </pre> 139 140 <h3 id="disable-jit-and-run-applications-in-interpreter">Disable JIT</h3> 141 142 <pre class="devsite-click-to-copy"> 143 <code class="devsite-terminal">adb root</code> 144 <code class="devsite-terminal">adb shell stop</code> 145 <code class="devsite-terminal">adb shell setprop dalvik.vm.usejit false</code> 146 <code class="devsite-terminal">adb shell start</code> 147 </pre> 148 149 <h3 id="force-compilation-of-a-specific-package">Force compilation of a specific 150 package</h3> 151 152 <p> 153 Check <code>$ adb shell cmd package compile</code> for usage. A few common use cases: 154 </p> 155 156 <ul> 157 <li>Profile-based: 158 <pre class="devsite-terminal devsite-click-to-copy"> 159 adb shell cmd package compile -m speed-profile -f my-package 160 </pre> 161 </li> 162 <li>Full: 163 <pre class="devsite-terminal devsite-click-to-copy"> 164 adb shell cmd package compile -m speed -f my-package 165 </pre> 166 </li> 167 </ul> 168 169 <h3 id="force-compilation-of-all-packages">Force compilation of all 170 packages</h3> 171 172 <ul> 173 <li>Profile-based: 174 <pre class="devsite-terminal devsite-click-to-copy"> 175 adb shell cmd package compile -m speed-profile -f -a 176 </pre> 177 </li> 178 <li>Full: 179 <pre class="devsite-terminal devsite-click-to-copy"> 180 adb shell cmd package compile -m speed -f -a 181 </pre> 182 </li> 183 </ul> 184 185 <h3 id="clear-profile-data-and-remove-compiled-code">Clear profile data and 186 remove compiled code</h3> 187 188 <ul> 189 <li>One package: 190 <pre class="devsite-terminal devsite-click-to-copy"> 191 adb shell cmd package compile --reset my-package 192 </pre> 193 </li> 194 <li>All packages 195 <pre class="devsite-terminal devsite-click-to-copy"> 196 adb shell cmd package compile --reset -a 197 </pre> 198 </li> 199 </ul> 200 201 </body> 202 </html> 203
