Date:	Thu, 3 Jul 2008 05:27:51 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Andy Whitcroft <apw@...dowen.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, NeilBrown <neilb@...e.de>,
	babydr@...y-dragons.com, cl@...ux-foundation.org,
	lee.schermerhorn@...com, a.beregalov@...il.com,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org
Subject: Re: [problem] raid performance loss with 2.6.26-rc8 on 32-bit x86 (bisected)

One-line-summary: kswapd is almost certainly broken on !NUMA and a candidate
patch is below

On (02/07/08 18:49), Dan Williams didst pronounce:
> On Tue, 2008-07-01 at 22:18 -0700, Mel Gorman wrote:
> > > Let us take commit 8b3e6cdc out of the equation and just look at raid0
> > > performance:
> > >
> > > revision   2.6.25.8-fc8 54a6eb5c 54a6eb5c-nohighmem 2.6.26-rc8
> > >            279          278      273                277
> > >            281          278      275                277
> > >            281          113      68.7               66.8
> > >            279          69.2     277                73.7
> > >            278          75.6     62.5               80.3
> > > MB/s (avg) 280          163      191                155
> > > % change   0%           -42%     -32%               -45%
> > > result     base         bad      bad                bad
> > >
> > 
> > Ok, based on your other mail, 54a6eb5c here is a bisection point. The good
> > figures are on par with the "good" kernel with some disasterous runs leading
> > to a bad average. The thing is, the bad results are way worse than could be
> > accounted for by two-zonelist alone.  In fact, the figures look suspiciously
> > like only 1 disk is in use as they are roughly quartered. Can you think of
> > anything that would explain that?
> 
> raid0 in contrast to raid5 is fairly simple it just remaps the bio.
> Then it is up to the block layer to do some merging before it hits the
> block device driver.  Not much room for error, especially since single
> drive performance seems to be unaffected.  The data seems to show that
> the vm is not queuing pages to be written back fast enough.
> 

That is exactly the case but I think I know why now.

> > Can you also confirm that using a bisection
> > point before two-zonelist runs steadily and with high performance as expected
> > please? This is to rule out some other RAID patch being a factor.
> 
> The immediately preceeding commit 18ea7e71 always runs with high
> performance, and raid0 was virtually untouched during the merge window:
> 
> $ git shortlog v2.6.25..v2.6.26-rc8 drivers/md/raid0.c
> Neil Brown (1):
>       Remove blkdev warning triggered by using md
> 
> > It would be worth running vmstat during the tests so we can see if IO
> > rates are dropping from an overall system perspective. If possible,
> > oprofile data from the same time would be helpful to see does it show up
> > where we are getting stuck.
> 
> I watched /proc/vmstat during a dd run, and in the good case
> nr_writeback never drops below 21,000.  In the bad case it may briefly
> break a few hundred pages.
> 
> This picture generated by seekwatcher shows that there are gaps in i/o
> submission:
> 
> 	http://userweb.kernel.org/~djbw/md0-bad2.png
> 
> Compared to a good run:
> 
> 	http://userweb.kernel.org/~djbw/md0-good.png
> 
> What is interesting is that I was only able to get this data with the
> following command:
> 
> ./seekwatcher -W -t md0.trace -o md0-bad2.png -p 'dd if=/dev/zero of=/dev/md0 bs=1024k count=2048' -d /dev/md0 -d /dev/sdb
> 
> When I tried to watch all four disks, i.e. 5 blktrace tasks running
> instead of 2:
> 
> ./seekwatcher -W -t md0.trace -o md0-bad2.png -p 'dd if=/dev/zero of=/dev/md0 bs=1024k count=2048' -d /dev/md0 -d /dev/sdb -d /dev/sdc -d /dev/sdd -d /dev/sde
> 
> ...it always measured good performance, as if having more tasks running
> stimulates the vm to keep writeback going at a high rate.  Could
> writeback be getting stalled on one cpu or more cpu's?
> 

Yes, we're stalling because the two-zonelist patch is breaking kswapd on
!NUMA machines. The impact is that kswapd never reclaims and processes always
directly reclaim. The result is you see large stalls.

Alexander, I believe this may also be why lockdep is triggering and you're
seeing periodic stalls in the XFS tests. The lockdep warning is a red herring
but the fact it's easier to trigger could be because kswapd is not working
and direct reclaim triggers the warning easily.

Alexander and Dan, could you test the patch below and let me know if it
fixes your respective problems please?

===

Subject: [PATCH] Do not overwrite nr_zones on !NUMA when initialising zlcache_ptr

With the two-zonelist patches on !NUMA machines, there really is only one
zonelist as __GFP_THISNODE is meaningless. However, during initialisation, the
assumption is made that two zonelists exist when initialising zlcache_ptr. The
result is that pgdat->nr_zones is always 0. As kswapd uses this value to
determine what reclaim work is necessary, the result is that kswapd never
reclaims. This causes processes to stall frequently in low-memory situations
as they always direct reclaim.  This patch initialises zlcache_ptr correctly.

Signed-off-by: Mel Gorman <mel@....ul.ie>
--- 
 page_alloc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.26-rc8-clean/mm/page_alloc.c linux-2.6.26-rc8-fix-kswapd-on-numa/mm/page_alloc.c
--- linux-2.6.26-rc8-clean/mm/page_alloc.c	2008-06-24 18:58:20.000000000 -0700
+++ linux-2.6.26-rc8-fix-kswapd-on-numa/mm/page_alloc.c	2008-07-02 21:14:16.000000000 -0700
@@ -2328,7 +2328,8 @@ static void build_zonelists(pg_data_t *p
 static void build_zonelist_cache(pg_data_t *pgdat)
 {
 	pgdat->node_zonelists[0].zlcache_ptr = NULL;
-	pgdat->node_zonelists[1].zlcache_ptr = NULL;
+	if (NUMA_BUILD)
+		pgdat->node_zonelists[1].zlcache_ptr = NULL;
 }
 
 #endif	/* CONFIG_NUMA */

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