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

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. Michal