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Date: Fri, 28 Nov 2008 16:02:21 +0900 (JST)
From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To: Rik van Riel <riel@...hat.com>
Cc: kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, mel@....ul.ie,
akpm@...ux-foundation.org
Subject: Re: [PATCH] vmscan: bail out of page reclaim after swap_cluster_max pages
Hi
I mesured some data in this week and I got some interesting data.
> > Kosaki, this should address the zone scanning pressure issue.
>
> hmmmm. I still don't like the behavior when priority==DEF_PRIORITY.
> but I also should explain by code and benchmark.
>
> therefore, I'll try to mesure this patch in this week.
1. many # of process reclaiming at that time
I mesure ten times following bench.
$ hackbench 140 process 300
rc6+stream: 2.6.28-rc6 +
vmscan-evict-streaming-io-first.patch (in -mm)
rvr: above + Rik's bailing out patch
kosaki: above + kosaki modify (attached the last in this mail)
result (unit: second)
rc6+stream rvr +kosaki patch
-----------------------------------------------------------
175.457 62.514 104.87
168.409 225.698 133.128
154.658 114.694 194.867
46.148 179.108 11.82
289.575 111.08 60.779
146.871 189.796 86.515
305.036 114.124 54.009
225.21 112.999 273.841
224.674 227.842 166.547
118.071 81.869 84.431
------------------------------------------
avg 185.4109 141.9724 117.0807
std 74.18484 55.93676126 73.28439987
min 46.148 62.514 11.82
max 305.036 227.842 273.841
OK.
Rik patch improve about 30% and my patch improve 20% more.
totally, We got about 50% improvement.
2. "communicate each other application" conflict the other
console A
$ dbench 100
console B
$ hackbench 130 process 300
hackbench result (unit :second)
rc6+stream rvr +kosaki
====================================================
588.74 57.084 139.448
569.876 325.063 52.233
427.078 295.259 53.492
65.264 132.873 59.009
136.636 136.367 319.115
221.538 76.352 187.937
244.881 125.774 158.846
37.523 115.77 122.17
182.485 382.376 105.195
273.983 299.577 130.478
----------------------------------------
avg 274.8004 194.6495 132.7923
std 184.4902365 111.5699478 75.88299814
min 37.523 57.084 52.233
max 588.74 382.376 319.115
That's more interesting.
-rc6 reclaim victory on min score. but also it has worst max score. why?
traditional reclaim assume following two case is equivalent.
case (1)
- task (a) spent 1 sec for reclaim.
- task (b) spent 1 sec for reclaim.
- task (c) spent 1 sec for reclaim.
case (2)
- task (a) spent 3 sec for reclaim.
- task (b) spent 0 sec for reclaim.
- task (c) spent 0 sec for reclaim.
However, when these task comminicate each other, it isn't correct.
if task (2)-(a) is dbench process, ok, you are lucky.
dbench process don't comminicate each other. that's performance is
decided from avarage performance.
one process slowdown don't become system slowdown.
then, hackbench and dbench get both good result.
In the other hand, if task (2)-(a) is hackbench process, you are unlucky.
hackbench process communicate each other.
then the performance is decided slowest process.
then, hackbench performance dramatically decreased although dbench
performance almost don't increased.
Therefore, I think case (1) is better.
So, rik patch and my patch improve perfectly different reclaim aspect.
In general, kernel reclaim processing has several key goals.
(1) if system has droppable cache, system shouldn't happen oom kill.
(2) if system has avaiable swap space, system shouldn't happen
oom kill as poosible as.
(3) if system has enough free memory, system shouldn't reclaim any page
at all.
(4) if memory pressure is lite, system shouldn't cause heavy reclaim
latency to application.
rik patch improve (3), my (this mail) modification improve (4).
BTW, reclaim throttle patch has one another improvement likes
Hanns's "vmscan: serialize aggressive reclaimers" patch.
actually, rik patch improvement (3). but it isn't perfect.
if 10000 thread call reclaim at that time, system reclaim 32*10000 pages.
it is definitly too much.
but it is obviously offtopic :)
Rik, could you please merge my modify into your patch?
======
In past, HPC guys want to improve lite reclaim latency multiple times.
(e.g. http://marc.info/?l=linux-mm&m=121418258514542&w=2)
because their workload has following characteristics.
- their workload make many process or many thread.
- typically, write() is called at job ending phase.
then, many file cache isn't reused.
- In their parallel job, any process comminicate each other.
then, system performance is decide from slowest thread.
then, large reclaim latency decrease system performance directly.
Actually, kswapd background reclaim and direct reclaim have perfectly
different purpose and goal.
background reclaim
- kswapd don't need latency overhead reducing.
it isn't observe from end user.
- kswapd shoudn't only increase free pages, but also should
zone balancing.
foreground reclaim
- it used application task context.
- as possible as, it shouldn't increase latency overhead.
- this reclaiming purpose is to make the memory for _own_ taks.
for other tasks memory don't need to concern.
kswap does it.
Almost developer don't payed attention for HPC in past.
However, in these days, # of cpus increase rapidly. and
parallel processing technique become commonly.
(e.g. now, gcc has OpenMP feature support directly)
Therefore, we shouldn't ignore parallel application modern days.
o remove priority==DEF_PRIORITY condision
o shrink_zones() also should have bailing out feature.
---
mm/vmscan.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
Index: b/mm/vmscan.c
===================================================================
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1469,7 +1469,7 @@ static void shrink_zone(int priority, st
* freeing target can get unreasonably large.
*/
if (sc->nr_reclaimed > sc->swap_cluster_max &&
- priority < DEF_PRIORITY && !current_is_kswapd())
+ !current_is_kswapd())
break;
}
@@ -1534,6 +1534,8 @@ static void shrink_zones(int priority, s
}
shrink_zone(priority, zone, sc);
+ if (sc->nr_reclaimed > sc->swap_cluster_max)
+ break;
}
}
--
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