Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

From: Johannes Weiner <hannes@cmpxchg.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Michal Hocko <mhocko@suse.com>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	linux-mm@kvack.org, cgroups@vger.kernel.org,
	linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition
Date: Tue, 4 Apr 2017 18:29:52 -0400	[thread overview]
Message-ID: <20170404222952.GA28930@cmpxchg.org> (raw)
In-Reply-To: <20170404150703.742c49d73921df6369ed3dbd@linux-foundation.org>

On Tue, Apr 04, 2017 at 03:07:03PM -0700, Andrew Morton wrote:
> On Tue,  4 Apr 2017 18:00:52 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> > Since 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> > we noticed bigger IO spikes during changes in cache access patterns.
> > 
> > The patch in question shrunk the inactive list size to leave more room
> > for the current workingset in the presence of streaming IO. However,
> > workingset transitions that previously happened on the inactive list
> > are now pushed out of memory and incur more refaults to complete.
> > 
> > This patch disables active list protection when refaults are being
> > observed. This accelerates workingset transitions, and allows more of
> > the new set to establish itself from memory, without eating into the
> > ability to protect the established workingset during stable periods.
> > 
> > Fixes: 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: <stable@vger.kernel.org> # 4.7+
> 
> That's a pretty large patch and the problem has been there for a year. 
> I'm not sure that it's 4.11 material, let alone -stable.  Care to
> explain further?

The problem statement is a little terse, my apologies.

The workloads that were measurably affected for us were hit pretty bad
by it, with refault/majfault rates doubling and tripling during cache
transitions, and the machines sustaining half-hour periods of 100% IO
utilization, where they'd previously have sub-minute peaks at 60-90%.

Stateful services that handle user data tend to be more conservative
with kernel upgrades. As a result we hit most page cache issues with
some delay, as was the case here.

The severity seemed to warrant a stable tag, but I agree that holding
out until 4.11.1 is probably better, given the invasiveness of this.