linux-kernel - XFS cpu use and slowness

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Date:	Wed, 14 Oct 2009 06:44:22 -0600
From:	Thomas Fjellstrom <tfjellstrom@...w.ca>
To:	linux-kernel@...r.kernel.org
Subject: XFS cpu use and slowness

I've recently had a couple issues with XFS.

First off I've noticed a few times that XFS becomes VERY slow when its near 
full (a few GB free), even when it's not fragmented. Reads slow down to 
something on the order of 1MB/s, when at full speed I can generally pull 
200MB/s off the raid5 array (its been a long time since I've seen speed 
anywhere near that off of the file system).

And second, I see one or more of the xfsdatad threads using up a significant 
amount of CPU time (30%+) when running a test like: `iozone -s 8G -A` on my 
new 5 disk md raid0 array. Its pretty fast however (570MBs write, 380MB/s 
read). That said, flush-9:0 and kswapd0* also like to use up a lot of cpu at 
the same time (10-30% cpu each)

So it seems xfs is fast when its a fresh file system, but degrades over time 
(I have xfs_fsr set to run automatically every day on my existing xfs fs, I 
can't see it being very fragmented).

* I have no idea why kswapd0 would be using that much cpu, system has 4G ram, 
and most of it is file/disk cache. Doing the tests however seems to push more 
and more into swap. Started out as 0M, during the first test it moved up to 
1MB, and now during a second test its up to 14MB. Not a lot, but more than I 
expected.

-- 
Thomas Fjellstrom
tfjellstrom@...w.ca
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