Re: [PATCH 0/3] Make workingset detection logic memcg aware

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On 2015/08/08 22:05, Vladimir Davydov wrote:
> On Fri, Aug 07, 2015 at 10:38:16AM +0900, Kamezawa Hiroyuki wrote:
>> On 2015/08/06 17:59, Vladimir Davydov wrote:
>>> On Wed, Aug 05, 2015 at 10:34:58AM +0900, Kamezawa Hiroyuki wrote:
>>>> I wonder, rather than collecting more data, rough calculation can help the situation.
>>>> for example,
>>>>
>>>>     (refault_disatance calculated in zone) * memcg_reclaim_ratio < memcg's active list
>>>>
>>>> If one of per-zone calc or per-memcg calc returns true, refault should be true.
>>>>
>>>> memcg_reclaim_ratio is the percentage of scan in a memcg against in a zone.
>>>
>>> This particular formula wouldn't work I'm afraid. If there are two
>>> isolated cgroups issuing local reclaim on the same zone, the refault
>>> distance needed for activation would be reduced by half for no apparent
>>> reason.
>>
>> Hmm, you mean activation in memcg means activation in global LRU, and it's not a
>> valid reason. Current implementation does have the same issue, right ?
>>
>> i.e. when a container has been hitting its limit for a while, and then, a file cache is
>> pushed out but came back soon, it can be easily activated.
>>
>> I'd like to confirm what you want to do.
>>
>>   1) avoid activating a file cache when it was kicked out because of memcg's local limit.
>
> No, that's not what I want. I want pages of the workingset to get
> activated on refault no matter if they were evicted on global memory
> pressure or due to hitting a memory cgroup limit.
>

Sure.

>>   2) maintain acitve/inactive ratio in memcg properly as global LRU does.
>>   3) reclaim shadow entry at proper timing.
>>
>> All ? hmm. It seems that mixture of record of global memory pressure and of local memory
>> pressure is just wrong.
>
> What makes you think so? An example of misbehavior caused by this would
> be nice to have.
>

By design, memcg's LRU aging logic is independent from global memory allocation/pressure.


Assume there are 4 containers(using much page-cache) with 1GB limit on 4GB server,
   # contaienr A  workingset=600M   limit=1G (sleepy)
   # contaienr B  workingset=300M   limit=1G (work often)
   # container C  workingset=500M   limit=1G (work slowly)
   # container D  workingset=1.2G   limit=1G (work hard)
  
container D can drive the zone's distance counter because of local memory reclaim.
If active/inactive = 1:1, container D page can be activated.
At kswapd(global reclaim) runs, all container's LRU will rotate.

Possibility of refault in A, B, C is reduced by conainer D's counter updates.

But yes, some _real_  test are required.

>>
>> Now, the record is
>> a??a??a??a??
>> a??a??a??a??eviction | node | zone | 2bit.
>>
>> How about changing this as
>>
>>          0 |eviction | node | zone | 2bit
>>          1 |eviction |  memcgid    | 2bit
>>
>> Assume each memcg has an eviction counter, which ignoring node/zone.
>> i.e. memcg local reclaim happens against memcg not against zone.
>>
>> At page-in,
>>          if (the 1st bit is 0)
>>                  compare eviction counter with zone's counter and activate the page if needed.
>>          else if (the 1st bit is 1)
>>                  compare eviction counter with the memcg (if exists)
>
> Having a single counter per memcg won't scale with the number of NUMA
> nodes.
>
It doesn't matter, we can use lazy counter like pcpu counter because it's not needed to be very accurate.


>>                  if (current memcg == recorded memcg && eviction distance is okay)
>>                       activate page.
>>                  else
>>                       inactivate
>> At page-out
>>          if (global memory pressure)
>>                  record eviction id with using zone's counter.
>>          else if (memcg local memory pressure)
>>                  record eviction id with memcg's counter.
>>
>
> I don't understand how this is supposed to work when a memory cgroup
> experiences both local and global pressure simultaneously.
>

I think updating global distance counter by local reclaim may update counter too much.
Above is to avoid updating zone's counter and keep memcg's LRU active/inactive balanced.

> Also, what if a memory cgroup is protected by memory.low? Such a cgroup
> may have all its pages in the active list, because it is never scanned.

If LRU never scanned, all file caches tend to be in INACTIVE...it never refaults.

> This will affect the refault distance of other cgroups, making
> activations unpredictable.
>



Thanks,
-Kame







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