linux-kernel - Re: [PATCH 0/2] mm,swap: skip swap readahead for instant IO (like zswap)

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Date:   Mon, 05 Oct 2020 13:32:58 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com, niketa@...com, sjenning@...hat.com,
        ddstreet@...e.org, konrad.wilk@...cle.com, hannes@...xchg.org
Subject: Re: [PATCH 0/2] mm,swap: skip swap readahead for instant IO (like
 zswap)

On Tue, 2020-09-22 at 10:12 -0700, Andrew Morton wrote:
> On Mon, 21 Sep 2020 22:01:46 -0400 Rik van Riel <riel@...riel.com>
> wrote:
> 
> > Both with frontswap/zswap, and with some extremely fast IO devices,
> > swap IO will be done before the "asynchronous" swap_readpage() call
> > has returned.
> > 
> > In that case, doing swap readahead only wastes memory, increases
> > latency, and increases the chances of needing to evict something
> > more
> > useful from memory. In that case, just skip swap readahead.
> 
> Any quantitative testing results?

I have test results with a real workload now.

Without this patch, enabling zswap results in about an 
8% increase in p99 request latency. With these patches,
the latency penalty for enabling zswap is under 1%.

Enabling zswap
allows us to give the main workload a
little more memory, since the spikes in memory demand
caused by things like system management software no 
longer cause large latency issues.

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