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