Re: OSDL aio-stress results on latest kernels show buffered random read issue

[Bill Davidsen <davidsen@tmr.com>]

Judith Lebzelter wrote:
> 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.

Usually not, since the write goes to cache in most cases while the read 
must really happen. The 40% slowdown is troubling, though, that's 
unexpected.


-- 
    -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me