内容简介：翻译自：https://stackoverflow.com/questions/42502636/unable-to-save-in-background-redis-server
我有两台 redis 服务器在同一台机器上运行.第二个日志文件有几个实例,其中包含以下通知：
[50818] 19 Feb 06:41:05.007 * 10 changes in 300 seconds. Saving... [50818] 19 Feb 06:41:05.007 # Can't save in background: fork: Cannot allocate memory
相比之下,第一个日志文件仅包含成功的数据库保存.如果我内存不足,我估计两者都有类似的日志.令我感到困惑的是,只有一个有这个问题,另一个没有.任何线索？
此外,研究让我看到 this blog post ,它认为如果我在命令行上执行sysctl vm.overcommit_memory = 1,问题就可以得到改善.没有解释这有何帮助.有人可以在redis的背景下解释这里发生了什么吗？
Background saving is failing with a fork() error under Linux even if I’ve a lot of free RAM!
Short answer: echo 1 > /proc/sys/vm/overcommit_memory
And now the long one:
Redis background saving schema relies on the copy-on-write semantic of
fork in modern operating systems: Redis forks (creates a child
process) that is an exact copy of the parent. The child process dumps
the DB on disk and finally exits. In theory the child should use as
much memory as the parent being a copy, but actually thanks to the
copy-on-write semantic implemented by most modern operating systems
the parent and child process will share the common memory pages. A
page will be duplicated only when it changes in the child or in the
parent. Since in theory all the pages may change while the child
process is saving, Linux can’t tell in advance how much memory the
child will take, so if the overcommit_memory
setting is set to zero
fork will fail unless there is as much free RAM as required to really
duplicate all the parent memory pages, with the result that if you
have a Redis dataset of 3 GB and just 2 GB of free memory it will
fail. Setting overcommit_memory
to 1 says Linux to relax and perform
the fork in a more optimistic allocation fashion, and this is indeed
what you want for Redis.
A good source to understand how Linux Virtual Memory work and other
alternatives for overcommit_memory
and overcommit_ratio
is this
classic from Red Hat Magazine, “Understanding Virtual Memory”. Beware,
this article had 1 and 2 configuration values forovercommit_memory
reversed: refer to the 07001 man page for the right meaning of
the available values.
翻译自：https://stackoverflow.com/questions/42502636/unable-to-save-in-background-redis-server
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