On 03/15/2016 04:27 AM, Finn Thain wrote: > > On Mon, 14 Mar 2016, Hannes Reinecke wrote: > >> On 03/14/2016 05:27 AM, Finn Thain wrote: >>> This setting does not need to be conditional on Atari ST or TT. >>> >>> Without TCQ support, cmd_per_lun == 2 is probably reasonable... >>> >>> Signed-off-by: Finn Thain <fth...@telegraphics.com.au> >>> >>> --- >>> drivers/scsi/atari_scsi.c | 3 +-- >>> 1 file changed, 1 insertion(+), 2 deletions(-) >>> >>> Index: linux/drivers/scsi/atari_scsi.c >>> =================================================================== >>> --- linux.orig/drivers/scsi/atari_scsi.c 2016-03-14 15:26:45.000000000 >>> +1100 >>> +++ linux/drivers/scsi/atari_scsi.c 2016-03-14 15:26:55.000000000 +1100 >>> @@ -750,6 +750,7 @@ static struct scsi_host_template atari_s >>> .eh_abort_handler = atari_scsi_abort, >>> .eh_bus_reset_handler = atari_scsi_bus_reset, >>> .this_id = 7, >>> + .cmd_per_lun = 2, >>> .use_clustering = DISABLE_CLUSTERING, >>> .cmd_size = NCR5380_CMD_SIZE, >>> }; >> _2_ ? Are you being overly cheeky here? >> I sincerely doubt the driver is capable of submitting two >> simultaneous commands ... > > Right. The LLD has LU busy flags to prevent a LU from being issued more > than one command. > >> Care to explain? > > It seemed harmless and it is consistent with the all of the other 5380 > drivers. > > I don't know why it was done that way. Perhaps it was done to create a > pipeline. That is, to keep a small number of commands in the LLD issue > queue so that the NCR5380_main() work item does not have to terminate and > then get requeued needlessly. > Like I suspected. While I'm aware of the reasoning, I sincerely doubt whether it makes any difference in real life. After all, a 'BUSY' return value still relies on someone kicking the queue so that the next command can be submitted. So it's not much different from using a queuedepth of '1' and use the 'official' way.
Have you done any benchmarking here? Would be very interesting to check if it makes a difference in real life ... Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking h...@suse.de +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)
