Re: Memory under KVM?

[Avi Kivity <avi@redhat.com>]

On 12/12/2009 10:37 PM, Thomas Fjellstrom wrote:
>
> I have the opposite happen, when a VM is started, RES is usually lower than
> -m, which I find slightly odd. But makes sense if qemu/kvm don't actually
> allocate memory from the host till its requested the first time

That is the case.

> (if only it
> would return some of it afterwards, it would be even better).
>    

Use the balloon driver to return memory to the host.

> I just fully shut down and restarted on of my vms, which is set to use
> 128-256 MB ram max. RES is like 72MB on start, and VIRT is 454M. RES
> generally gets up around 120MB ram when its doing something.
>
> One thing I do find a little odd is one of my VMs which is allocated 512MB
> ram, has a VIRT of 826MB ram. I didn't realize that qemu had so many lib
> dependencies.

It's not just libraries, it's mostly glibc malloc() allocating huge 
pools per thread, as well as large thread stacks.

> Due to kvm not supporting giving memory back, besides by
> swapping large portions of unused guest ram, my host currently has over 1G
> used swap. Not particularly happy with that, but it doesn't seem to effect
> performance too much (except that it generally likes to swap host processes
> first, guest performance is decent, but host, not so much).
>    

The Linux vm prefers anonymous memory, so guests do get an advantage.

-- 
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