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

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Message-ID: <20190123110254.GU4087@dhcp22.suse.cz>
Date:   Wed, 23 Jan 2019 12:02:54 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, hannes@...xchg.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: vmscan: do not iterate all mem cgroups for
 global direct reclaim

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?

-- 
Michal Hocko
SUSE Labs