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