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

[Michal Privoznik]

On 08.01.2013 16:42, 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.
> 
> Michal

I've pushed this into v0.10.2-maint branch as well.

Michal