Re: [Qemu-discuss] qemu-kvm swapping even though vm.swappiness = 0

[Mike Lovell]

On 05/23/2014 02:39 AM, Horatiu N wrote:
> on a 64gb ram node with 1gb swap i have swappiness=0
> but still i see qemu-kvm swapping and when the swap fills (the 
> kernel?) kills the pids

first off, just in case there is confusion, the vm in vm.swapiness is 
not for virtual machine but for virtual memory. its not anything 
specific to qemu or virtual machines.

setting swappiness to 0 does not mean the kernel wont use swap. "A value 
of 0 instructs the kernel not to initiate swap until the amount of free 
and file-backed pages is less than the high water mark in a zone" [1]. 
unfortunately, i didn't readily see how to determine what the high water 
mark for a zone is. i am pretty sure the only way to make it so the 
kernel never uses swap is to not provide the kernel with any.

i'm not sure if the data you provided is what is intended. it looks like 
the sysctl command showing vm.swapiness was run on a different system 
(node) than the free command (fr04). i don't know if one was a vm 
running on the host or something else.

this overall looks like behavior i would expect from what you are 
describing. is there an issue with your qemu guests being killed because 
the host is running out of memory? it does seem odd that the guests 
would be getting killed from the oom-killer (out-of-memory) from the 
info you've given but i'm not sure if that is the memory info from when 
the oom-killer is doing its thing. if it is then i'm guessing there are 
some tweaks you can make to the variables for the host virtual memory 
system. there are a number of options that are kind of black magic to me 
and i would recommend doing some searching and reading of docs around 
the oom-killer and other virtual memory options.

mike

[1] 
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/sysctl/vm.txt