chrome中内存分配采用了第三方库tcmalloc，这个库主要提供给应用程序内存管理方面的优化，按资料说内存存取速度会从300ns降到50ns。更具体的关于这个tcmalloc的信息大家可以查网上的资料看看， 本人对tcmalloc的实现不是很熟，这里主要向熟悉一下chrome的代码，主要说说 tcmalloc如何集成到了chrome中，通过研究这个，一是看看如何使用tcmalloc，另外可以对window下VC的内存C runtime的库有更多的了解。
tcmalloc主要提供了自有的一套内存分配的函数，来替换我们常使用的new delete等VC++默认提供的内存分配实现。
chrome中的tcmalloc的源码位于third_party下，源代码通过GYP引入到chrome中的allocator项目中，我们重点可以看下 allocator_shim.cc中的代码。该文件中对外提供了内存分配的malloc, free和reallocator等函数，用来接管系统默认提供的内存非配函数。
extern "C" {
void* malloc(size_t size) __THROW {
void* ptr;
for (;;) {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
switch (allocator) {
case JEMALLOC:
ptr = je_malloc(size);
break;
case WINHEAP:
case WINLFH:
ptr = win_heap_malloc(size);
break;
case TCMALLOC:
default:
ptr = do_malloc(size);
break;
}
#else
// TCMalloc case.
ptr = do_malloc(size);
#endif
if (ptr)
return ptr;
if (!new_mode || !call_new_handler(true))
break;
}
return ptr;
}
void free(void* p) __THROW {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
switch (allocator) {
case JEMALLOC:
je_free(p);
return;
case WINHEAP:
case WINLFH:
win_heap_free(p);
return;
}
#endif
// TCMalloc case.
do_free(p);
}
void* realloc(void* ptr, size_t size) __THROW {
// Webkit is brittle for allocators that return NULL for malloc(0). The
// realloc(0, 0) code path does not guarantee a non-NULL return, so be sure
// to call malloc for this case.
if (!ptr)
return malloc(size);
void* new_ptr;
for (;;) {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
switch (allocator) {
case JEMALLOC:
new_ptr = je_realloc(ptr, size);
break;
case WINHEAP:
case WINLFH:
new_ptr = win_heap_realloc(ptr, size);
break;
case TCMALLOC:
default:
new_ptr = do_realloc(ptr, size);
break;
}
#else
// TCMalloc case.
new_ptr = do_realloc(ptr, size);
#endif
// Subtle warning: NULL return does not alwas indicate out-of-memory. If
// the requested new size is zero, realloc should free the ptr and return
// NULL.
if (new_ptr || !size)
return new_ptr;
if (!new_mode || !call_new_handler(true))
break;
}
return new_ptr;
}
通过这三个函数的代码我们可以看出，chrome提供了进行内存库切换的枚举， 不同的枚举值对应不同的具体malloc的实现转发，具体由JEMalloc, tcmalloc和默认等几种不同的内存库可以使用。
按照我们的经验，如果直接编译的话，那么就会有两份的malloc的实现，此时链接是不会通过的，chrome中如何解决？
chrome下的base中allocator相关的项目有4个，其中有一个名为libcmt的项目，其中只有一个python的脚本， 代码如下：
#!/usr/bin/env python # Copyright (c) 2012 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
#
# This script takes libcmt.lib for VS2005/08/10/12/13 and removes the allocation
# related functions from it.
#
# Usage: prep_libc.py <VCLibDir> <OutputDir> <arch>
#
# VCLibDir is the path where VC is installed, something like:
# C:\Program Files\Microsoft Visual Studio 8\VC\lib
# OutputDir is the directory where the modified libcmt file should be stored.
# arch is either 'ia32' or 'x64' import os
import shutil
import subprocess
import sys def run(command, filter=None):
"""Run |command|, removing any lines that match |filter|. The filter is
to remove the echoing of input filename that 'lib' does."""
popen = subprocess.Popen(
command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
out, _ = popen.communicate()
for line in out.splitlines():
if filter and line.strip() != filter:
print line
return popen.returncode def main():
bindir = 'SELF_X86'
objdir = 'INTEL'
vs_install_dir = sys.argv[1]
outdir = sys.argv[2]
if "x64" in sys.argv[3]:
bindir = 'SELF_64_amd64'
objdir = 'amd64'
vs_install_dir = os.path.join(vs_install_dir, 'amd64')
output_lib = os.path.join(outdir, 'libcmt.lib')
shutil.copyfile(os.path.join(vs_install_dir, 'libcmt.lib'), output_lib)
shutil.copyfile(os.path.join(vs_install_dir, 'libcmt.pdb'),
os.path.join(outdir, 'libcmt.pdb')) vspaths = [
'build\\intel\\mt_obj\\',
'f:\\dd\\vctools\\crt_bld\\' + bindir + \
'\\crt\\src\\build\\' + objdir + '\\mt_obj\\',
'F:\\dd\\vctools\\crt_bld\\' + bindir + \
'\\crt\\src\\build\\' + objdir + '\\mt_obj\\nativec\\\\',
'F:\\dd\\vctools\\crt_bld\\' + bindir + \
'\\crt\\src\\build\\' + objdir + '\\mt_obj\\nativecpp\\\\',
'f:\\binaries\\Intermediate\\vctools\\crt_bld\\' + bindir + \
'\\crt\\prebuild\\build\\INTEL\\mt_obj\\cpp_obj\\\\',
] objfiles = ['malloc', 'free', 'realloc', 'new', 'delete', 'new2', 'delete2',
'align', 'msize', 'heapinit', 'expand', 'heapchk', 'heapwalk',
'heapmin', 'sbheap', 'calloc', 'recalloc', 'calloc_impl',
'new_mode', 'newopnt', 'newaopnt']
for obj in objfiles:
for vspath in vspaths:
cmd = ('lib /nologo /ignore:4006,4014,4221 /remove:%s%s.obj %s' %
(vspath, obj, output_lib))
run(cmd, obj + '.obj') if __name__ == "__main__":
sys.exit(main())
这段代码主要 是从libcmt.lib中移出 malloc, free, new 等 的obj的默认实现， 就可以使用到我们所提供的malloc了。