From: Mel Gorman <mgorman@techsingularity.net>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Shantanu Goel <sgoel01@yahoo.com>, Chris Mason <clm@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Vlastimil Babka <vbabka@suse.cz>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 1/3] mm, vmscan: fix zone balance check in prepare_kswapd_sleep
Date: Thu, 23 Feb 2017 15:05:34 +0000 [thread overview]
Message-ID: <20170223150534.64fpsvlse33rj2aa@techsingularity.net> (raw)
In-Reply-To: <20170222070036.GA17962@bbox>
On Wed, Feb 22, 2017 at 04:00:36PM +0900, Minchan Kim wrote:
> > There are also more allocation stalls. One of the largest impacts was due
> > to pages written back from kswapd context rising from 0 pages to 4516642
> > pages during the hour the workload ran for. By and large, the patch has very
> > bad behaviour but easily missed as the impact on a UMA machine is negligible.
> >
> > This patch is included with the data in case a bisection leads to this area.
> > This patch is also a pre-requisite for the rest of the series.
> >
> > Signed-off-by: Shantanu Goel <sgoel01@yahoo.com>
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
>
> Hmm, I don't understand why we should bind wakeup_kcompactd to kswapd's
> short sleep point where every eligible zones are balanced.
> What's the correlation between them?
>
If kswapd is ready for a short sleep, eligible zones are balanced for
order-0 but not necessarily the originally requested order if kswapd
gave up reclaiming as compaction was ready to start. As kswapd is ready
to sleep for a short period, it's a suitable time for kcompactd to decide
if it should start working or not. There is no need for kswapd to be aware
of kcompactd's wakeup criteria.
> Can't we wake up kcompactd once we found a zone has enough free pages
> above high watermark like this?
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 26c3b405ef34..f4f0ad0e9ede 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3346,13 +3346,6 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
> * that pages and compaction may succeed so reset the cache.
> */
> reset_isolation_suitable(pgdat);
> -
> - /*
> - * We have freed the memory, now we should compact it to make
> - * allocation of the requested order possible.
> - */
> - wakeup_kcompactd(pgdat, alloc_order, classzone_idx);
> -
> remaining = schedule_timeout(HZ/10);
>
> /*
> @@ -3451,6 +3444,14 @@ static int kswapd(void *p)
> bool ret;
>
> kswapd_try_sleep:
> + /*
> + * We have freed the memory, now we should compact it to make
> + * allocation of the requested order possible.
> + */
> + if (alloc_order > 0 && zone_balanced(zone, reclaim_order,
> + classzone_idx))
> + wakeup_kcompactd(pgdat, alloc_order, classzone_idx);
> +
> kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
> classzone_idx);
That's functionally very similar to what happens already. wakeup_kcompactd
checks the order and does not wake for order-0. It also makes its own
decisions that include zone_balanced on whether it is safe to wakeup.
I doubt there would be any measurable difference from a patch like this
and to my mind at least, it does not improve the readability or flow of
the code.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2017-02-23 15:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-15 9:22 [PATCH 0/3] Reduce amount of time kswapd sleeps prematurely Mel Gorman
2017-02-15 9:22 ` [PATCH 1/3] mm, vmscan: fix zone balance check in prepare_kswapd_sleep Mel Gorman
2017-02-16 2:50 ` Hillf Danton
2017-02-22 7:00 ` Minchan Kim
2017-02-23 15:05 ` Mel Gorman [this message]
2017-02-24 1:17 ` Minchan Kim
2017-02-24 9:11 ` Mel Gorman
2017-02-27 6:16 ` Minchan Kim
2017-02-15 9:22 ` [PATCH 2/3] mm, vmscan: Only clear pgdat congested/dirty/writeback state when balanced Mel Gorman
2017-02-15 9:22 ` [PATCH 3/3] mm, vmscan: Prevent kswapd sleeping prematurely due to mismatched classzone_idx Mel Gorman
2017-02-16 6:23 ` Hillf Danton
2017-02-16 8:10 ` Mel Gorman
2017-02-16 8:21 ` Hillf Danton
2017-02-16 9:32 ` Mel Gorman
2017-02-20 16:34 ` Vlastimil Babka
2017-02-21 4:10 ` Hillf Danton
2017-02-20 16:42 ` Vlastimil Babka
2017-02-23 15:01 ` Mel Gorman
2017-03-01 9:04 ` Vlastimil Babka
2017-02-15 20:30 ` [PATCH 0/3] Reduce amount of time kswapd sleeps prematurely Andrew Morton
2017-02-15 21:29 ` Mel Gorman
2017-02-15 21:56 ` Andrew Morton
2017-02-15 22:15 ` Mel Gorman
2017-02-15 22:00 ` Vlastimil Babka
2017-03-09 7:56 [PATCH 0/3] Reduce amount of time kswapd sleeps prematurely v2 Mel Gorman
2017-03-09 7:56 ` [PATCH 1/3] mm, vmscan: fix zone balance check in prepare_kswapd_sleep Mel Gorman
2017-03-10 9:06 ` Vlastimil Babka
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