tcmalloc使用的一点经验

之前写了一个多线程的应用，使用的boost asio异步，发现使用的过程中linux glibc自带的malloc性能很差，
替换为tcmlloc，性能得到比较大的提升。

但是最近突然发现有时候内存增长到几个G，开始以为是内存泄露的问题，用valgrind做了大量的测试，发现
内存虽然有时候会飙升到几个G，但是valgrind显示没有内存泄露。

最后怀疑是tcmalloc缓存了这部分内存，仔细研读了tcmlloc文档和部分源码，发现了这个

Releasing Memory Back to the System

By default, tcmalloc will release no-longer-used memory back to the kernel gradually, over time. The  MallocExtension::instance()->ReleaseFreeMemory();

You can also call SetMemoryReleaseRate() to change the tcmalloc_release_rate value at runtime, or GetMemoryReleaseRate to see what the current release rate is.

TCMALLOC_RELEASE_RATE	default: 1.0	Rate at which we release unused memory to the system, via madvise(MADV_DONTNEED), on systems that support it. Zero means we never release memory back to the system. Increase this flag to return memory faster; decrease it to return memory slower. Reasonable rates are in the range [0,10].

tcmalloc将逐渐的释放内存，释放的速度取决于tcmalloc_release_rate，合理情况下一般情况是0-10，另外可以手动的在你的application
中调用

   MallocExtension::instance()->ReleaseFreeMemory()

强制释放。

最后在代码中加入了ReleaseFreeMemory，问题解决