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



On Tue, Jan 08, 2013 at 04:42:00PM +0100, Michal Privoznik wrote:
> On 08.01.2013 16:24, Daniel P. Berrange wrote:
> > 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
> > 
> 
> Yeah, it's sort of magic. Pushed now. Thanks.

How does one turn off the limits?

Dave

> Michal
> 
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