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Re: [libvirt] [PATCH] qemu: Relax hard RSS limit



On Tue, Jan 08, 2013 at 10:37:19AM +0100, Michal Privoznik wrote:
> Currently, if there's no hard memory limit defined for a domain,
> libvirt tries to calculate one, based on domain definition and magic
> equation and set it upon the domain startup. The rationale behind was,
> if there's a memory leak or exploit in qemu, we should prevent the
> host system trashing. However, the equation was too tightening, as it
> didn't reflect what the kernel counts into the memory used by a
> process. Since many hosts do have a swap, nobody hasn't noticed
> anything, because if hard memory limit is reached, process can
> continue allocating memory on a swap. However, if there is no swap on
> the host, the process gets killed by OOM killer. In our case, the qemu
> process it is.
> 
> To prevent this, we need to relax the hard RSS limit. Moreover, we
> should reflect more precisely the kernel way of accounting the memory
> for process. That is, even the kernel caches are counted within the
> memory used by a process (within cgroups at least). Hence the magic
> equation has to be changed:
> 
>   limit = 1.5 * (domain memory + total video memory) + (32MB for cache
>           per each disk) + 200MB
> ---
> 
> There is a bit more that should be taken into account, e.g. shared
> pages, where accounting is even more complicated:
> 
> "Shared pages are accounted on the basis of the first touch approach.
> The cgroup that first touches a page is accounted for the page." [1]
> 
> I don't we even want to try to reflect this in our code. That's why
> the coefficient of domain memory has been lifted from 1.02 to 1.5, in
> hope it will just be enough.
> 
> 1: http://www.kernel.org/doc/Documentation/cgroups/memory.txt
> 
>  src/qemu/qemu_cgroup.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
> index 7faf025..16a9d7c 100644
> --- a/src/qemu/qemu_cgroup.c
> +++ b/src/qemu/qemu_cgroup.c
> @@ -343,15 +343,18 @@ int qemuSetupCgroup(virQEMUDriverPtr driver,
>          unsigned long long hard_limit = vm->def->mem.hard_limit;
>  
>          if (!hard_limit) {
> -            /* If there is no hard_limit set, set a reasonable
> -             * one to avoid system trashing caused by exploited qemu.
> -             * As 'reasonable limit' has been chosen:
> -             *     (1 + k) * (domain memory + total video memory) + F
> -             * where k = 0.02 and F = 200MB. */
> +            /* If there is no hard_limit set, set a reasonable one to avoid
> +             * system trashing caused by exploited qemu.  As 'reasonable limit'
> +             * has been chosen:
> +             *     (1 + k) * (domain memory + total video memory) + (32MB for
> +             *     cache per each disk) + F
> +             * where k = 0.5 and F = 200MB.  The cache for disks is important as
> +             * kernel cache on the host side counts into the RSS limit. */
>              hard_limit = vm->def->mem.max_balloon;
>              for (i = 0; i < vm->def->nvideos; i++)
>                  hard_limit += vm->def->videos[i]->vram;
> -            hard_limit = hard_limit * 1.02 + 204800;
> +            hard_limit = hard_limit * 1.5 + 204800;
> +            hard_limit += vm->def->ndisks * 32768;
>          }
>  
>          rc = virCgroupSetMemoryHardLimit(cgroup, hard_limit);

ACK,

can't say I'm a fan of our heuristics but I don't see a better way
yet. Lets see how this new limit copes.

Daniel
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