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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


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