linux-kernel - Re: [RFC PATCH 0/5] Remove dependency on congestion

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Date:   Tue, 21 Sep 2021 12:18:17 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     dsterba@...e.cz, Matthew Wilcox <willy@...radead.org>,
        Linux-MM <linux-mm@...ck.org>, NeilBrown <neilb@...e.de>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/5] Remove dependency on congestion_wait in mm/

On Mon, Sep 20, 2021 at 04:11:52PM +0200, David Sterba wrote:
> On Mon, Sep 20, 2021 at 01:50:58PM +0100, Mel Gorman wrote:
> > On Mon, Sep 20, 2021 at 12:42:44PM +0100, Matthew Wilcox wrote:
> > > On Mon, Sep 20, 2021 at 09:54:31AM +0100, Mel Gorman wrote:
> > > > This has been lightly tested only and the testing was useless as the
> > > > relevant code was not executed. The workload configurations I had that
> > > > used to trigger these corner cases no longer work (yey?) and I'll need
> > > > to implement a new synthetic workload. If someone is aware of a realistic
> > > > workload that forces reclaim activity to the point where reclaim stalls
> > > > then kindly share the details.
> > > 
> > > The stereeotypical "stalling on I/O" problem is to plug in one of the
> > > crap USB drives you were given at a trade show and simply
> > > 	dd if=/dev/zero of=/dev/sdb
> > > 	sync
> > > 
> > 
> > The test machines are 1500KM away so plugging in a USB stick but worst
> > comes to the worst, I could test it on a laptop.
> 
> There's a device mapper target dm-delay [1] that as it says delays the
> reads and writes, so you could try to emulate the slow USB that way.
> 
> [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/delay.html

Ah, thanks for that tip. I wondered if something like this existed and
clearly did not search hard enough. I was able to reproduce the problem
without throttling but this could still be useful if examining cases
where there are 2 or more BDIs with variable speeds.

-- 
Mel Gorman
SUSE Labs