linux overcommit flag

linux中有一個overcomit的配置，這個配置關係到進程在過多申請memory資源的時候，系統的表現（啓發式容許，不檢查，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

詳細的理解能夠參見這個博客： http://linuxperf.com/?p=102

在實際項目中，若是機器是跑redis的通常都會把overcomit開成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的時候，也會設置成1。避免ffmepg一些進程在申請內存的時候發生內部錯誤。

根本緣由是進程申請了這麼多memory，實際不必定佔用這麼多memory，並且物理內存頁的分配發生在使用的瞬間，而不是在申請的瞬間。因此咱們能夠容許系統overcommit，知足進程申請，能夠經過cgroup memory來限制用戶使用內存的上限。