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

[Andrew Morton <akpm@osdl.org>]

Judith Lebzelter <judith@osdl.org> 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.
> 
> 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)
> 

Well one would expect writes to be much faster than reads because writes
usually do not involve performing physical I/O, and when pagecache
writeback finally happens it has vastly more data to work with and hence
can schedule I/O more efficiently.

Unless you are using O_SYNC or fsync(), in which case ignore the above.

The regression within random write performance is unexpected.  Can you
please provide a URL to the current version of the test tool, and a
description of how you are invoking it?  What sort of I/O system, what
filesystem, etc.

Thanks.