linux-kernel - Re: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node

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Message-ID: <656725362af9bd757a281f0799a0bb9c9b2487bd.camel@surriel.com>
Date:   Fri, 02 Oct 2020 09:53:05 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Michal Hocko <mhocko@...e.com>,
        Sebastiaan Meijer <meijersebastiaan@...il.com>
Cc:     akpm@...ux-foundation.org, buddy.lumpkin@...cle.com,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mgorman@...e.de, willy@...radead.org
Subject: Re: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node

On Fri, 2020-10-02 at 09:03 +0200, Michal Hocko wrote:
> On Thu 01-10-20 18:18:10, Sebastiaan Meijer wrote:
> > (Apologies for messing up the mailing list thread, Gmail had fooled
> > me into
> > believing that it properly picked up the thread)
> > 
> > On Thu, 1 Oct 2020 at 14:30, Michal Hocko <mhocko@...e.com> wrote:
> > > On Wed 30-09-20 21:27:12, Sebastiaan Meijer wrote:
> > > > > yes it shows the bottleneck but it is quite artificial. Read
> > > > > data is
> > > > > usually processed and/or written back and that changes the
> > > > > picture a
> > > > > lot.
> > > > Apologies for reviving an ancient thread (and apologies in
> > > > advance for my lack
> > > > of knowledge on how mailing lists work), but I'd like to offer
> > > > up another
> > > > reason why merging this might be a good idea.
> > > > 
> > > > From what I understand, zswap runs its compression on the same
> > > > kswapd thread,
> > > > limiting it to a single thread for compression. Given enough
> > > > processing power,
> > > > zswap can get great throughput using heavier compression
> > > > algorithms like zstd,
> > > > but this is currently greatly limited by the lack of threading.
> > > 
> > > Isn't this a problem of the zswap implementation rather than
> > > general
> > > kswapd reclaim? Why zswap doesn't do the same as normal swap out
> > > in a
> > > context outside of the reclaim?

On systems with lots of very fast IO devices, we have
also seen kswapd take 100% CPU time without any zswap
in use.

This seems like a generic issue, though zswap does
manage to bring it out on lower end systems.

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