OSDL aio-stress results on latest kernels show buffered random readissue

[Judith Lebzelter]

Hello;

I am running aio-stress on the most recent kernels and have
found that on linux-2.6.8, 2.6.9-rc2 and 2.6.9-rc2-mm4 the
performance of buffered random reads is poor compared to the
buffered random writes:

               2.6.8      2.6.9-rc2     2.6.9-rc2-mm4
             --------------------------------------------
random write 35.66 MB/s   34.80 MB/s    29.89 MB/s
random read   7.69 MB/s    7.50 MB/s     7.68 MB/s

** 2CPU hosts with striped Megaraid. 1G RAM. 4G File.


This shows up on our 4CPU host as well. (striped AACRAID.4G
RAM. 8G File):
             2.6.9-rc2     2.6.9-rc2-mm4   2.6.9-rc2-mm1
             -------------------------------------------
random write 31.36 MB/s     18.92 MB/s      18.97 MB/s
random read  11.13 MB/s      9.74 MB/s      11.05 MB/s


There seems to be an issue with the reads.  Usually, reads
should be at least as fast as writes of the same type.

Also, there seems to be a substantial drop-off in the performance
of AIO buffered-random writes in the mm kernels. (14% on 2CPU,
40% on 4CPU)

Regards;
Judith Lebzelter
OSDL


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