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Date:   Mon, 9 Mar 2020 06:14:55 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Tyler Sanderson <tysand@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, virtualization@lists.linux-foundation.org,
        Wei Wang <wei.w.wang@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Nadav Amit <namit@vmware.com>, Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for
 VIRTIO_BALLOON_F_DEFLATE_ON_OOM
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On Mon, Mar 09, 2020 at 10:03:14AM +0100, David Hildenbrand wrote:
> On 08.03.20 05:47, Tyler Sanderson wrote:
> > Tested-by: Tyler Sanderson <tysand@google.com>
> > 
> > Test setup: VM with 16 CPU, 64GB RAM. Running Debian 10. We have a 42
> > GB file full of random bytes that we continually cat to /dev/null.
> > This fills the page cache as the file is read. Meanwhile we trigger
> > the balloon to inflate, with a target size of 53 GB. This setup causes
> > the balloon inflation to pressure the page cache as the page cache is
> > also trying to grow. Afterwards we shrink the balloon back to zero (so
> > total deflate = total inflate).
> > 
> > Without patch (kernel 4.19.0-5):
> > Inflation never reaches the target until we stop the "cat file >
> > /dev/null" process. Total inflation time was 542 seconds. The longest
> > period that made no net forward progress was 315 seconds (see attached
> > graph).
> > Result of "grep balloon /proc/vmstat" after the test:
> > balloon_inflate 154828377
> > balloon_deflate 154828377
> > 
> > With patch (kernel 5.6.0-rc4+):
> > Total inflation duration was 63 seconds. No deflate-queue activity
> > occurs when pressuring the page-cache.
> > Result of "grep balloon /proc/vmstat" after the test:
> > balloon_inflate 12968539
> > balloon_deflate 12968539
> > 
> > Conclusion: This patch fixes the issue. In the test it reduced
> > inflate/deflate activity by 12x, and reduced inflation time by 8.6x.
> > But more importantly, if we hadn't killed the "grep balloon
> > /proc/vmstat" process then, without the patch, the inflation process
> > would never reach the target.
> > 
> > Attached is a png of a graph showing the problematic behavior without
> > this patch. It shows deflate-queue activity increasing linearly while
> > balloon size stays constant over the course of more than 8 minutes of
> > the test.
> 
> Thanks a lot for the extended test!


Given we shipped this for a long time, I think the best way
to make progress is to merge 1/3, 2/3 right now, and 3/3
in the next release.

> -- 
> Thanks,
> 
> David / dhildenb