Linux Page Cache performance

[Ravi Wijayaratne <ravi_wija@yahoo.com>]

Hi All.

We are running dbench on a machine with Dual Xeon (Hyper threading 
turned off), 1GB RAM
and 4 Drive software RAID5. The kernel is 2.4.29-xfs 1.2. We are using 
LVM. However
similar test done using ext2 on a disk partiotion (no md or LVM) shows 

The throughput is find till the number of clients are Around 16. At 
that point the throughput
plummets about 40%. We are trying to avoid that and see how we could 
have a consistent throughput
perhaps sacrificing some peak performance.

One could argue that at the point the performance drops we are actualy 
beggining to see 
I/O requests missing page cache and going to disk. That is our current 
guess. We try to tune
the buffer age and the interval page buffer daemon runs, but had no 
effect on the curve.
So transactional meta data operations seems not be causing the bottle 
neck.

Are there any VM patches that smooths out the Page Cache dirty page 
swapping process so that we
wont see this sudden drop of through put, but could have a smoother 
transition. I ran a 
kernel profiler on the test and I dont see any dentry cache flushes or 
inode cache flushes.

Similar test done using ext2 on a disk partiotion (no md or LVM) shows 
us similar performances.
However for ext2 when we tweak the bdflush parameters in /proc 
(specifically the max age of meta
data buffers) we can push the point where the data throughput falls to 
around 40 clients. 

But we are more interested in finding out ways to solve the XFS case.

Any insights on this ?

Thanx
Ravi



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Ravi Wijayaratne

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