Re: [RFC PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim

[Kirill Tkhai]

On 23.01.2019 14:02, Michal Hocko wrote:
> On Wed 23-01-19 13:28:03, Kirill Tkhai wrote:
>> On 22.01.2019 23:09, Yang Shi wrote:
>>> In current implementation, both kswapd and direct reclaim has to iterate
>>> all mem cgroups.  It is not a problem before offline mem cgroups could
>>> be iterated.  But, currently with iterating offline mem cgroups, it
>>> could be very time consuming.  In our workloads, we saw over 400K mem
>>> cgroups accumulated in some cases, only a few hundred are online memcgs.
>>> Although kswapd could help out to reduce the number of memcgs, direct
>>> reclaim still get hit with iterating a number of offline memcgs in some
>>> cases.  We experienced the responsiveness problems due to this
>>> occassionally.
>>>
>>> Here just break the iteration once it reclaims enough pages as what
>>> memcg direct reclaim does.  This may hurt the fairness among memcgs
>>> since direct reclaim may awlays do reclaim from same memcgs.  But, it
>>> sounds ok since direct reclaim just tries to reclaim SWAP_CLUSTER_MAX
>>> pages and memcgs can be protected by min/low.
>>
>> In case of we stop after SWAP_CLUSTER_MAX pages are reclaimed; it's possible
>> the following situation. Memcgs, which are closest to root_mem_cgroup, will
>> become empty, and you will have to iterate over empty memcg hierarchy long time,
>> just to find a not empty memcg.
>>
>> I'd suggest, we should not lose fairness. We may introduce
>> mem_cgroup::last_reclaim_child parameter to save a child
>> (or its id), where the last reclaim was interrupted. Then
>> next reclaim should start from this child:
> 
> Why is not our reclaim_cookie based caching sufficient?

Hm, maybe I missed them. Do cookies already implement this functionality?

Kirill