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Message-ID: <28c262360911130648q7b615ad4if75b902ed25d5fbd@mail.gmail.com>
Date: Fri, 13 Nov 2009 23:48:57 +0900
From: Minchan Kim <minchan.kim@...il.com>
To: Mel Gorman <mel@....ul.ie>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Frans Pop <elendil@...net.nl>, Jiri Kosina <jkosina@...e.cz>,
Sven Geggus <lists@...hsschwanzdomain.de>,
Karol Lewandowski <karol.k.lewandowski@...il.com>,
Tobias Oetiker <tobi@...iker.ch>, linux-kernel@...r.kernel.org,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
Pekka Enberg <penberg@...helsinki.fi>,
Rik van Riel <riel@...hat.com>,
Christoph Lameter <cl@...ux-foundation.org>,
Stephan von Krawczynski <skraw@...net.com>,
"Rafael J. Wysocki" <rjw@...k.pl>,
Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH 5/5] vmscan: Take order into consideration when deciding
if kswapd is in trouble
On Fri, Nov 13, 2009 at 10:54 PM, Mel Gorman <mel@....ul.ie> wrote:
> On Fri, Nov 13, 2009 at 06:54:29PM +0900, KOSAKI Motohiro wrote:
>> > If reclaim fails to make sufficient progress, the priority is raised.
>> > Once the priority is higher, kswapd starts waiting on congestion.
>> > However, on systems with large numbers of high-order atomics due to
>> > crappy network cards, it's important that kswapd keep working in
>> > parallel to save their sorry ass.
>> >
>> > This patch takes into account the order kswapd is reclaiming at before
>> > waiting on congestion. The higher the order, the longer it is before
>> > kswapd considers itself to be in trouble. The impact is that kswapd
>> > works harder in parallel rather than depending on direct reclaimers or
>> > atomic allocations to fail.
>> >
>> > Signed-off-by: Mel Gorman <mel@....ul.ie>
>> > ---
>> > mm/vmscan.c | 14 ++++++++++++--
>> > 1 files changed, 12 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index ffa1766..5e200f1 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1946,7 +1946,7 @@ static int sleeping_prematurely(int order, long remaining)
>> > static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
>> > {
>> > int all_zones_ok;
>> > - int priority;
>> > + int priority, congestion_priority;
>> > int i;
>> > unsigned long total_scanned;
>> > struct reclaim_state *reclaim_state = current->reclaim_state;
>> > @@ -1967,6 +1967,16 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
>> > */
>> > int temp_priority[MAX_NR_ZONES];
>> >
>> > + /*
>> > + * When priority reaches congestion_priority, kswapd will sleep
>> > + * for a short time while congestion clears. The higher the
>> > + * order being reclaimed, the less likely kswapd will go to
>> > + * sleep as high-order allocations are harder to reclaim and
>> > + * stall direct reclaimers longer
>> > + */
>> > + congestion_priority = DEF_PRIORITY - 2;
>> > + congestion_priority -= min(congestion_priority, sc.order);
>>
>> This calculation mean
>>
>> sc.order congestion_priority scan-pages
>> ---------------------------------------------------------
>> 0 10 1/1024 * zone-mem
>> 1 9 1/512 * zone-mem
>> 2 8 1/256 * zone-mem
>> 3 7 1/128 * zone-mem
>> 4 6 1/64 * zone-mem
>> 5 5 1/32 * zone-mem
>> 6 4 1/16 * zone-mem
>> 7 3 1/8 * zone-mem
>> 8 2 1/4 * zone-mem
>> 9 1 1/2 * zone-mem
>> 10 0 1 * zone-mem
>> 11+ 0 1 * zone-mem
>>
>> I feel this is too agressive. The intention of this congestion_wait()
>> is to prevent kswapd use 100% cpu time.
As I said in reply of kosaki's patch, I can't understand point.
> Ok, I thought the intention might be to avoid dumping too many pages on
> the queue but it was already waiting on congestion elsewhere.
>
>> but the above promotion seems
>> break it.
>>
>> example,
>> ia64 have 256MB hugepage (i.e. order=14). it mean kswapd never sleep.
>> example2,
But, This is a true problem missed in my review.
Thanks, Kosaki.
>> order-3 (i.e. PAGE_ALLOC_COSTLY_ORDER) makes one of most inefficent
>> reclaim, because it doesn't use lumpy recliam.
>> I've seen 128GB size zone, it mean 1/128 = 1GB. oh well, kswapd definitely
>> waste cpu time 100%.
>>
>>
>> > +
>> > loop_again:
>> > total_scanned = 0;
>> > sc.nr_reclaimed = 0;
>> > @@ -2092,7 +2102,7 @@ loop_again:
>> > * OK, kswapd is getting into trouble. Take a nap, then take
>> > * another pass across the zones.
>> > */
>> > - if (total_scanned && priority < DEF_PRIORITY - 2)
>> > + if (total_scanned && priority < congestion_priority)
>> > congestion_wait(BLK_RW_ASYNC, HZ/10);
>>
>> Instead, How about this?
>>
>
> This makes a lot of sense. Tests look good and I added stats to make sure
> the logic was triggering. On X86, kswapd avoided a congestion_wait 11723
> times and X86-64 avoided it 5084 times. I think we should hold onto the
> stats temporarily until all these bugs are ironed out.
>
> Would you like to sign off the following?
>
> If you are ok to sign off, this patch should replace my patch 5 in
> the series.
I agree Kosaki's patch is more strightforward.
You can add my review sign, too.
Thanks for good patch, Kosaki. :)
--
Kind regards,
Minchan Kim
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