linux overcommit flag

linux中有一个overcomit的配置，这个配置关系到进程在过多申请memory资源的时候，系统的表现（启发式允许，不检查，or 阻止）

/proc/sys/vm/overcommit_memory
       This file contains the kernel virtual memory accounting mode.
       Values are:

              0: heuristic overcommit (this is the default)
              1: always overcommit, never check
              2: always check, never overcommit

       In mode 0, calls of mmap(2) with MAP_NORESERVE are not
       checked, and the default check is very weak, leading to the
       risk of getting a process "OOM-killed".

       In mode 2 (available since Linux 2.6), the total virtual
       address space that can be allocated (CommitLimit in /proc/mem‐
       info) is calculated as

           CommitLimit = (total_RAM - total_huge_TLB) *
                         overcommit_ratio / 100 + total_swap

详细的理解可以参见这个博客： http://linuxperf.com/?p=102

在实际项目中，如果机器是跑redis的一般都会把overcomit开成1 （https://redis.io/topics/faq#background-saving-fails-with-a-fork-error-under-linux-even-if-i-have-a-lot-of-free-ram）

Short answer: echo 1 > /proc/sys/vm/overcommit_memory :)

跑ffmepg的时候，也会设置成1。避免ffmepg一些进程在申请内存的时候发生内部错误。

根本原因是进程申请了这么多memory，实际不一定占用这么多memory，而且物理内存页的分配发生在使用的瞬间，而不是在申请的瞬间。所以我们可以允许系统overcommit，满足进程申请，可以通过cgroup memory来限制用户使用内存的上限。