linux-kernel - Re: [PATCH 20/22] atari_scsi: Set a reasonable default for cmd_per

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Date:	Tue, 15 Mar 2016 07:53:01 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Finn Thain <fthain@...egraphics.com.au>
Cc:	"James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Michael Schmitz <schmitzmic@...il.com>,
	linux-m68k@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Ondrej Zary <linux@...nbow-software.org>,
	Sam Creasey <sammy@...my.net>
Subject: Re: [PATCH 20/22] atari_scsi: Set a reasonable default for
 cmd_per_lun

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 <fthain@...egraphics.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
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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