Date:   Mon, 20 Sep 2021 13:50:58 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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 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. I considered using the
IO controller but I'm not sure that would throttle background writeback.
I dismissed doing this for a few reasons though -- the dirtying should
be rate limited based on the speed of the BDI so it will not necessarily
trigger the condition. It also misses the other interesting cases --
throttling due to excessive isolation and throttling due to failing to
make progress.

I've prototyped a synthetic case that uses 4..(NR_CPUS*4) workers. 1
worker measures mmap/munmap latency. 1 worker under fio is randomly reading
files. The remaining workers are split between fio doing random write IO
on separate files and anonymous memory hogs reading large mappings every
5 seconds. The aggregate WSS is approximately totalmem*2 split between 60%
anon and 40% file-backed (40% to be 2xdirty_ratio). After a warmup period
based on the writeback speed, it runs for 5 minutes per number of workers.

The primary metric of "goodness" will be the mmap latency because it's
the smallest worker that should be able to make quick progress and I
want to see how much it is interfered with during reclaim. I'll be
graphing the throttling times to see what processes get throttled and
for how long.

I was hoping though that there was a canonical realistic case that the
FS people use to stress the paths where the allocator fails to return
memory.  While my synthetic workload *might* work to trigger the cases,
I would prefer to have something that can compare this basic approach
with anything that is more clever.

Similarly, it would be nice to have a reasonable test case that phase
changes what memory is hot while there is heavy IO in the background to
detect whether the hot WSS is being properly protected. I used to use
memcached and a heavy writer to simulate this but it's weak because there
is no phase change so it's poor at evaluating vmscan.

> You can also set up qemu to have extremely slow I/O performance:
> https://serverfault.com/questions/675704/extremely-slow-qemu-storage-performance-with-qcow2-images
> 

Similar problem to the slow USB case, it's only catching one part of the
picture except now I have to worry about differences that are related
to the VM configuration (e.g. pinning virtual CPUs to physical CPUs
and replicating topology). Fine for a functional test, not so fine for
measuring if the patch is any good performance-wise.

-- 
Mel Gorman
SUSE Labs

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