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Subject[regression, 3.0-rc1] dentry cache growth during unlinks, XFS performance way down

I just booted up a 3.0-rc1 kernel, and mounted an XFS filesystem
with 50M files in it. Running:

$ for i in /mnt/scratch/*; do sudo /usr/bin/time rm -rf $i 2>&1 & done

runs an 8-way parallel unlink on the files. Normally this runs at
around 80k unlinks/s, and it runs with about 500k-1m dentries and
inodes cached in the steady state.

The steady state behaviour with 3.0-rc1 is that there are around 10m
cached dentries - all negative dentries - consuming about 1.6GB of
RAM (of 4GB total). Previous steady state was, IIRC, around 200MB of
dentries. My initial suspicions are that the dentry unhashing
changeѕ may be the cause of this...

Performance is now a very regular peak/trough patten with a period
of about 20s, where the peak is about 80k unlinks/s, and the trough
is around 20k unlinks/s. The runtime of the 50m inode delete has
gone from around 10m on 2.6.39, to:

11.71user 470.08system 15:07.91elapsed 53%CPU (0avgtext+0avgdata 133184maxresident)k
0inputs+0outputs (30major+497228minor)pagefaults 0swaps
11.50user 468.30system 15:14.35elapsed 52%CPU (0avgtext+0avgdata 133168maxresident)k
0inputs+0outputs (42major+497268minor)pagefaults 0swaps
11.34user 466.66system 15:26.04elapsed 51%CPU (0avgtext+0avgdata 133216maxresident)k
0inputs+0outputs (18major+497121minor)pagefaults 0swaps
12.14user 470.46system 15:26.60elapsed 52%CPU (0avgtext+0avgdata 133216maxresident)k
0inputs+0outputs (44major+497309minor)pagefaults 0swaps
12.06user 463.74system 15:28.84elapsed 51%CPU (0avgtext+0avgdata 133232maxresident)k
0inputs+0outputs (25major+497046minor)pagefaults 0swaps
11.37user 468.18system 15:29.07elapsed 51%CPU (0avgtext+0avgdata 133184maxresident)k
0inputs+0outputs (55major+497056minor)pagefaults 0swaps
11.69user 474.46system 15:47.45elapsed 51%CPU (0avgtext+0avgdata 133232maxresident)k
0inputs+0outputs (61major+497284minor)pagefaults 0swaps
11.32user 476.93system 16:05.14elapsed 50%CPU (0avgtext+0avgdata 133184maxresident)k
0inputs+0outputs (30major+497225minor)pagefaults 0swaps

About 16 minutes. I'm not sure yet whether this change of cache
behaviour is the cause of the entire performance regression, but
it's a good chance that it is a contributing factor.

Christoph, it appears that there is a significant increase in log
forces during this unlink workload compared to 2.6.39, and that's
possibly where the performance degradation is coming from. I'm going
to have to bisect, I think.

The 8-way create rate for the 50m inodes is down by 10% as well, but I
don't think that has anything to do with dentry cache behaviour -
log write throughput is up by a factor of 3x over 2.6.39. Christoph,
I think that this is once again due to an increase in log forces,
but I need to do more analysis to be sure...


Dave Chinner
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