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

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Message-ID: <82a7690e181cf1995e733120dad828e628cb80d9.camel@surriel.com>
Date:   Fri, 09 Oct 2020 10:38:22 -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 Mon, 2020-10-05 at 13:32 -0400, Rik van Riel wrote:
> 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:

> > 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%.

Never mind that. On larger tests the effect seems to disappear,
probably because the logic in __swapin_nr_pages() already reduces
the number of pages read ahead to 2 on workloads with lots of
random access.

That reduces the latency effects observed.

Now we might
still see some memory waste due to decompressing
pages we don't need, but I have not seen any real effects from
that yet, either.

I think it may be time to focus on a larger memory waste with
zswap: leaving the compressed copy of memory around when we
decompress the memory at swapin time.  More aggressively freeing
the compressed memory will probably buy us more than reducing
readahead.

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