Re: [RFC v2 PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim

[Johannes Weiner]

On Wed, Jan 30, 2019 at 06:11:17AM +0800, 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.
> 
> A simple test with pref shows it may take around 220ms to iterate 8K memcgs
> in direct reclaim:
>              dd 13873 [011]   578.542919: vmscan:mm_vmscan_direct_reclaim_begin
>              dd 13873 [011]   578.758689: vmscan:mm_vmscan_direct_reclaim_end
> So for 400K, it may take around 11 seconds to iterate all memcgs.
> 
> Here just break the iteration once it reclaims enough pages as what
> memcg direct reclaim does.  This may hurt the fairness among memcgs.  But
> the cached iterator cookie could help to achieve the fairness more or
> less.
> 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

Looks sane to me, thanks Yang.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>