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

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Date:   Wed, 4 Apr 2018 21:08:15 -0700
From:   Buddy Lumpkin <buddy.lumpkin@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
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 Apr 3, 2018, at 2:12 PM, Matthew Wilcox <willy@...radead.org> wrote:
> 
> 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?
> 


Trying to distinguish between the monster and a high value task that you want
to run as quickly as possible would be challenging. I like your idea of using
renice. It probably makes sense to continue to run the first thread on each node
at a standard nice value, and run each additional task with a positive nice value.