Linux OOM一二三

本是古典何须时尚 2024-04-18 09:30 34阅读 0赞

Linux开发一般会遇到“/proc/sys/vm/overcommit\_memory”，即文件/etc/sysctl.conf中的vm.overcommit\_memory，Overcommit的意思如同其字面意思，即进程可申请超出可用内存大小的内存（对进程而言实为虚拟内存，一个进程占用的虚拟内存空间通常比物理空间要大，甚至可能大许多）。overcommit\_memory有三种取值（注：overcommit\_memory并不控制OOM，是否开启OOM由**panic\_on\_oom**控制）：

<table style="width:426.1pt;"> 
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 <td style="vertical-align:top;width:135.35pt;"> overcommit_memory取值 </td> 
 <td style="vertical-align:top;width:290.75pt;"> 含义 </td> 
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 <td style="vertical-align:top;width:135.35pt;"> 0 </td> 
 <td style="vertical-align:top;width:290.75pt;"> 系统默认值。在程序请求分配内存，比如C++程序调用malloc或new时，先检查是否有足够的内存。如果没有足够满足请求的内存，则分配请求失败。 </td> 
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 <td style="vertical-align:top;width:135.35pt;"> 1 </td> 
 <td style="vertical-align:top;width:290.75pt;"> 启用Overcommit，即进程可申请超出CommitLimit大小的内存。 </td> 
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 <td style="vertical-align:top;width:135.35pt;"> 2 </td> 
 <td style="vertical-align:top;width:290.75pt;"> 关闭Overcommit，即申请的内存大小不能超过CommitLimit。行为和/proc/sys/vm/overcommit_ratio的值相关，/proc/sys/vm/overcommit_ratio的默认值为50。 </td> 
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权威参考：

[https://www.kernel.org/doc/Documentation/vm/**overcommit**\-accounting][https_www.kernel.org_doc_Documentation_vm_overcommit_-accounting]

官方原文：

<table style="width:426.1pt;"> 
 <tbody> 
 <tr> 
 <td style="vertical-align:top;width:115.7pt;"> overcommit_memory取值 </td> 
 <td style="vertical-align:top;width:310.4pt;"> 含义 </td> 
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 <td style="vertical-align:top;width:115.7pt;"> 0 </td> 
 <td style="vertical-align:top;width:310.4pt;"> Heuristic overcommit handling. Obvious overcommits of address space are refused. Used for a typical system. It ensures a seriously wild allocation fails while allowing overcommit to reduce swap usage. &nbsp;root is allowed to allocate slightly more memory in this mode. This is the default. </td> 
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 <td style="vertical-align:top;width:115.7pt;"> 1 </td> 
 <td style="vertical-align:top;width:310.4pt;"> Always overcommit. Appropriate for some scientific applications. Classic example is code using sparse arrays and just relying on the virtual memory consisting almost entirely of zero pages. </td> 
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 <td style="vertical-align:top;width:115.7pt;"> 2 </td> 
 <td style="vertical-align:top;width:310.4pt;"> Don't overcommit. The total address space commit for the system is not permitted to exceed swap + a configurable amount (default is 50%) of physical RAM. Depending on the amount you use, in most situations this means a process will not be killed while accessing pages but will receive errors on memory allocation as appropriate. &nbsp; Useful for applications that want to guarantee their memory allocations will be available in the future without having to initialize every page. </td> 
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系统是否行使OOM，由/proc/sys/vm/**panic\_on\_oom**的值决定，当/proc/sys/vm/panic\_on\_oom取值为1时表示关闭OOM，取值0时表示启用OOM。

如果将/proc/sys/vm/oom\_kill\_allocating\_task的值设置为1，则OOM时直接KILL当前正在申请内存的进程，否则OOM根据进程的oom\_adj和oom\_score来决定。

oom\_adj表示进程被OOM KILLER杀死的权重，取值“17~15”，值越大被KILL的概率越高，当进程的oom\_adj值为-17时，表示永远不会被OOM KILLER选中。

什么是Overcommit？当已申请的内存（Committed\_AS）大小超出CommitLimit值时即为Overcommit，执行命令“cat /proc/**meminfo** |grep CommitLimit”可查看CommitLimit的当前大小。

CommitLimit为系统当前可以申请的内存总大小，即内存分配上限，当overcommit\_memory值为2时，其值等于：SwapTotal + **MemTotal** \* overcommit\_ratio。

而Committed\_AS，表示所有进程**已申请**的内存总和。命令“sar -r”输出中的kbcommit对应/proc/meminfo中的Committed\_AS，而“%commit”值等于Committed\_AS/(MemTotal+SwapTotal)。

如果是大内存机器，可以考虑适当调大/proc/sys/vm/min\_free\_kbytes的值，但不能太大了，不然容易频繁触发内存回收，min\_free\_kbytes是内核保留空闲内存最小值，作用是保障必要时有足够内存使用。

[https_www.kernel.org_doc_Documentation_vm_overcommit_-accounting]: https://www.kernel.org/doc/Documentation/vm/overcommit-accounting