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