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Date:   Fri, 28 Feb 2020 15:52:59 +0900
From:   Joonsoo Kim <js1304@gmail.com>
To:     Aaron Lu <aaron.lwe@gmail.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>, kernel-team@lge.com
Subject: Re: [PATCH v2 0/9] workingset protection/detection on the anonymous
 LRU list
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On Fri, Feb 28, 2020 at 01:57:26PM +0800, Aaron Lu wrote:
> On Fri, Feb 28, 2020 at 01:03:03PM +0900, Joonsoo Kim wrote:
> > Hello,
> > 
> > On Fri, Feb 28, 2020 at 11:23:58AM +0800, Aaron Lu wrote:
> > > On Thu, Feb 27, 2020 at 08:48:06AM -0500, Johannes Weiner wrote:
> > > > On Wed, Feb 26, 2020 at 07:39:42PM -0800, Andrew Morton wrote:
> > > > > It sounds like the above simple aging changes provide most of the
> > > > > improvement, and that the workingset changes are less beneficial and a
> > > > > bit more risky/speculative?
> > > > > 
> > > > > If so, would it be best for us to concentrate on the aging changes
> > > > > first, let that settle in and spread out and then turn attention to the
> > > > > workingset changes?
> > > > 
> > > > Those two patches work well for some workloads (like the benchmark),
> > > > but not for others. The full patchset makes sure both types work well.
> > > > 
> > > > Specifically, the existing aging strategy for anon assumes that most
> > > > anon pages allocated are hot. That's why they all start active and we
> > > > then do second-chance with the small inactive LRU to filter out the
> > > > few cold ones to swap out. This is true for many common workloads.
> > > > 
> > > > The benchmark creates a larger-than-memory set of anon pages with a
> > > > flat access profile - to the VM a flood of one-off pages. Joonsoo's
> > > 
> > > test: swap-w-rand-mt, which is a multi thread swap write intensive
> > > workload so there will be swap out and swap ins.
> > > 
> > > > first two patches allow the VM to usher those pages in and out of
> > > 
> > > Weird part is, the robot says the performance gain comes from the 1st
> > > patch only, which adjust the ratio, not including the 2nd patch which
> > > makes anon page starting from inactive list.
> > > 
> > > I find the performance gain hard to explain...
> > 
> > Let me explain the reason of the performance gain.
> > 
> > 1st patch provides more second chance to the anonymous pages.
> 
> By second chance, do I understand correctely this refers to pages on 
> inactive list get moved back to active list?

Yes.

> 
> > In swap-w-rand-mt test, memory used by all threads is greater than the
> > amount of the system memory, but, memory used by each thread would
> > not be much. So, although it is a rand test, there is a locality
> > in each thread's job. More second chance helps to exploit this
> > locality so performance could be improved.
> 
> Does this mean there should be fewer vmstat.pswpout and vmstat.pswpin
> with patch1 compared to vanilla?

It depends on the workload. If the workload consists of anonymous
pages only, I think, yes, pswpout/pswpin would be lower than vanilla
with patch #1. With large inactive list, we can easily find the
frequently referenced page and it would result in less swap in/out.

Thanks.