linux-kernel - Re: Kswapd in 3.2.0-rc5 is a CPU hog

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Date:	Tue, 27 Dec 2011 11:15:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	nowhere <nowhere@...kenden.ath.cx>, Michal Hocko <mhocko@...e.cz>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Kswapd in 3.2.0-rc5 is a CPU hog

On Sat, 24 Dec 2011 07:45:03 +1100
Dave Chinner <david@...morbit.com> wrote:

> On Fri, Dec 23, 2011 at 03:04:02PM +0400, nowhere wrote:
> > В Пт., 23/12/2011 в 21:20 +1100, Dave Chinner пишет:
> > > On Fri, Dec 23, 2011 at 01:01:20PM +0400, nowhere wrote:
> > > > В Чт., 22/12/2011 в 09:55 +1100, Dave Chinner пишет:
> > > > > On Wed, Dec 21, 2011 at 10:52:49AM +0100, Michal Hocko wrote:

> > Here is the report of trace-cmd while dd'ing
> > https://80.237.6.56/report-dd.xz
> 
> Ok, it's not a shrink_slab() problem - it's just being called ~100uS
> by kswapd. The pattern is:
> 
> 	- reclaim 94 (batches of 32,32,30) pages from iinactive list
> 	  of zone 1, node 0, prio 12
> 	- call shrink_slab
> 		- scan all caches
> 		- all shrinkers return 0 saying nothing to shrink
> 	- 40us gap
> 	- reclaim 10-30 pages from inactive list of zone 2, node 0, prio 12
> 	- call shrink_slab
> 		- scan all caches
> 		- all shrinkers return 0 saying nothing to shrink
> 	- 40us gap
> 	- isolate 9 pages from LRU zone ?, node ?, none isolated, none freed
> 	- isolate 22 pages from LRU zone ?, node ?, none isolated, none freed
> 	- call shrink_slab
> 		- scan all caches
> 		- all shrinkers return 0 saying nothing to shrink
> 	40us gap
> 
> And it just repeats over and over again. After a while, nid=0,zone=1
> drops out of the traces, so reclaim only comes in batches of 10-30
> pages from zone 2 between each shrink_slab() call.
> 
> The trace starts at 111209.881s, with 944776 pages on the LRUs. It
> finishes at 111216.1 with kswapd going to sleep on node 0 with
> 930067 pages on the LRU. So 7 seconds to free 15,000 pages (call it
> 2,000 pages/s) which is awfully slow....
> 
> vmscan gurus - time for you to step in now...
>
 
Can you show /proc/zoneinfo ? I want to know each zone's size.

Below is my memo.

In trace log, priority = 11 or 12. Then, I think kswapd can reclaim memory
to satisfy "sc.nr_reclaimed >= SWAP_CLUSTER_MAX" condition and loops again.

Seeing balance_pgdat() and trace log, I guess it does

	wake up

	shrink_zone(zone=0(DMA?))     => nothing to reclaim.
		shrink_slab()
	shrink_zone(zone=1(DMA32?))   => reclaim 32,32,31 pages 
		shrink_slab()
	shrink_zone(zone=2(NORMAL?))  => reclaim 13 pages. 
		srhink_slab()

	sleep or retry.

Why shrink_slab() need to be called frequently like this ?

BTW. I'm sorry if I miss something ...Why only kswapd reclaims memory
while 'dd' operation ? (no direct relcaim by dd.)
Is this log record cpu hog after 'dd' ?

Thanks,
-Kame











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