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SubjectRe: [RFC v2 PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim
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@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Looks sane to me, thanks Yang.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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