Re: [PATCH] block: Reserve only one queue tag for sync IO if only 3 tags are available

[Jan Kara <jack@suse.cz>]

On Fri 28-06-13 21:31:52, Jens Axboe wrote:
> On Fri, Jun 28 2013, Jan Kara wrote:
> > In case a device has three tags available we still reserve two of them
> > for sync IO. That leaves only a single tag for async IO such as
> > writeback from flusher thread which results in poor performance.
> > 
> > Allow async IO to consume two tags in case queue has three tag availabe
> > to get a decent async write performance.
> > 
> > This patch improves streaming write performance on a machine with such disk
> > from ~21 MB/s to ~52 MB/s. Also postmark throughput in presence of
> > streaming writer improves from 8 to 12 transactions per second so sync
> > IO doesn't seem to be harmed in presence of heavy async writer.
> 
> That's pretty crazy! Never seen a device like that. But yes, it
> obviously exists, and the 2 reserved tags doesn't work well for that. So
> it's an improvement.
  Yeah, I agree it's a weird hardware. It's a ppc64 machine where storage
identifies as:
scsi2 : IBM 573E Storage Adapter
scsi 2:0:4:0: Direct-Access     IBM      ST373453LC       C51C PQ: 0 ANSI: 3
scsi 2:0:5:0: Direct-Access     IBM   H0 ST373207LC       C70D PQ: 0 ANSI: 4
scsi 2:0:8:0: Direct-Access     IBM   H0 ST373207LC       C70D PQ: 0 ANSI: 4

  It took me a while to figure out why this machine observes performance
characteristics noone else sees.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR