linux - Cent OS: How do I turn off or reduce memory overcommitment, and is it safe to do it?

From time to time "my" server stalls because it runs out of both memory and swap space. (it keeps responding to ping but nothing more than that, not even ssh).

I'm told linux does memory overcommitment, which as far as I understand is the same as banks do with money: it grants to processes more memory than actually available, assuming that most processes won't actually use all the memory they ask, at least not all at the same time.

Please assume this is actually the cause why my system occasionally hangs, let's not discuss here whether or not this is the case
(see What can cause ALL services on a server to go down, yet still responding to ping? and how to figure out).

So,

1. how do I disable or reduce drastically memory overcommitment in CentOS? I've read there are two settings called vm.overcommit_memory (values 0, 1, or 2) and vm.overcommit_ratiom but I have no idea where I have to find and change them (some configuration file hopefully), what values should I try, and whether I need to reboot the server to make the changes effective.




2. and is it safe? What side effects could I expect?
   When googling for overcommit_memory I find scary things like people saying their server can't boot anymore....

Since what causes the sudden increase in memory usage is mysql because of queries that are made by php which in turn is called while serving http requests, I would expect just some php script to fail to complete and hence some 500 response from time to time when the server is too busy, which is a risk I can take (certainly better that have the whole server become inaccessible and have to hard reboot it).

Or can it really cause my server to be unable to reboot if I choose the wrong settings?

Answer

Memory overcommit can be disabled by vm.overcommit_memory=2

0 is the default mode, where kernel heuristically determines the allocation by calculating the free memory compared to the allocation request being made. And setting it to 1 enables the wizardry mode, where kernel always advertises that it has enough free memory for any allocation. Setting to 2, means that processes can only allocate up to a configurable amount (overcommit_ratio) of RAM and will start getting allocation failure or OOM messages when it goes beyond that amount.

Is it safe to do so, no. I haven't seen any proper use case where disabling memory overcommit actually helped, unless you are 100% certain of the workload and hardware capacity. In case you are interested, install kernel-docs package and go to /Documentation/sysctl/vm.txt to read more, or read it online.

If you set vm.overcommit_memory=2 then it will overcommit up to the percentage of physical RAM configured in vm.overcommit_ratio (default is 50%).

echo 0/1/2 > /proc/sys/vm/overcommit_memory 

This will not survive a reboot. For persistence, put this in /etc/sysctl.conf file:

vm.overcommit_memory=X

and run sysctl -p. No need to reboot.