Re: [RFC] Make the SCSI mempool allocations variable

[Jeremy Higdon <jeremy@sgi.com>]

On Tue, Mar 09, 2004 at 02:02:55PM -0500, James Bottomley wrote:
> Based on Jeremy's request to increase max_phys_segments, this is the way
> to do it (SCSI_MAX_PHYS_SEGMENTS) is the value that gets set as the
> queue's max_phys_segments.
> 
> I have to say that when I tried raising it to 256 and hammering a 10GB
> ext2 filesystem, I still didn't generate any >128 segment requests, so
> I'm dubious that raising it has any benefit at all, but feel free to try
> it and publish the figures.
> 
> I did wonder if lowering it might help improve the memory footprint for
> some embedded systems, so I set it up to be lowered as far as 32.
> 
> James


I got a chance to try this tonight and can report excellent results.
I modified the qla2xxx driver to have an sg_tablesize of 256 (default
is SG_ALL, which is 0xFF).  I also modified SCSI_MAX_PHYS_SEGMENTS to
256.

I first tried direct I/O to the block device and found that the max
I/O size increased to 4MB from 2MB.  I then built an XFS filesystem and
tried direct I/O to it, and found the same results.

Believe it or not, there are some h/w RAIDs in which you can get a
significant performance benefit by increasing I/O size to 4MB from
2MB.  I don't have one immediately available to post MB/s numbers,
but you can trust me ('cause I can trust those who told me) :-)

So I heartily approve of this change.

Are you thinking of making SCSI_MAX_PHYS_SEGMENTS a config variable?
Or would you increase it to 256 by default?

Thanks for doing this,

jeremy