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

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Date:   Tue, 3 Apr 2018 14:12:53 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Buddy Lumpkin <buddy.lumpkin@...cle.com>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, hannes@...xchg.org, riel@...riel.com,
        mgorman@...e.de, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node

On Tue, Apr 03, 2018 at 01:49:25PM -0700, Buddy Lumpkin wrote:
> > Yes, very much this.  If you have a single-threaded workload which is
> > using the entirety of memory and would like to use even more, then it
> > makes sense to use as many CPUs as necessary getting memory out of its
> > way.  If you have N CPUs and N-1 threads happily occupying themselves in
> > their own reasonably-sized working sets with one monster process trying
> > to use as much RAM as possible, then I'd be pretty unimpressed to see
> > the N-1 well-behaved threads preempted by kswapd.
> 
> The default value provides one kswapd thread per NUMA node, the same
> it was without the patch. Also, I would point out that just because you devote
> more threads to kswapd, doesn’t mean they are busy. If multiple kswapd threads
> are busy, they are almost certainly doing work that would have resulted in
> direct reclaims, which are often substantially more expensive than a couple
> extra context switches due to preemption.

[...]

> In my previous response to Michal Hocko, I described
> how I think we could scale watermarks in response to direct reclaims, and
> launch more kswapd threads when kswapd peaks at 100% CPU usage.

I think you're missing my point about the workload ... kswapd isn't
"nice", so it will compete with the N-1 threads which are chugging along
at 100% CPU inside their working sets.  In this scenario, we _don't_
want to kick off kswapd at all; we want the monster thread to clean up
its own mess.  If we have idle CPUs, then yes, absolutely, lets have
them clean up for the monster, but otherwise, I want my N-1 threads
doing their own thing.

Maybe we should renice kswapd anyway ... thoughts?  We don't seem to have
had a nice'd kswapd since 2.6.12, but maybe we played with that earlier
and discovered it was a bad idea?