Re: Playing with SATA NCQ

[Jens Axboe <axboe@suse.de>]

On Thu, May 26 2005, Mark Lord wrote:
> I also saw a good boost from NCQ on the qstor driver (full version,
> not the libata subset) last year.  Very good for busy servers
> and RAID arrays.

It does seem to work amazingly well, from a performance point of view.

> Jens Axboe wrote:
> +	do {
> +		/*
> +		 * we rely on the FIFO order of the exclusive waitqueues
> +		 */
> +		prepare_to_wait_exclusive(&ap->cmd_wait_queue, &wait,
> +					  TASK_UNINTERRUPTIBLE);
> +
> +		if (!ata_qc_issue_ok(ap, qc, 1)) {
> +			spin_unlock_irq(&ap->host_set->lock);
> +			schedule();
> +			spin_lock_irq(&ap->host_set->lock);
> +		}
> +
> +		finish_wait(&ap->cmd_wait_queue, &wait);
> +
> +	} while (!ata_qc_issue_ok(ap, qc, 1));
> 
> In this bit (above), is it possible for this code to ever
> be invoked from a SCHED_RR or SCHED_FIFO context?
> 
> If so, it will lock out all lower-priority processes
> for the duration of the polling interval.

Yeah, I'm not a huge fan of the need for the above code... If every qc
was tied to a SCSI command, we could just ask for a later requeue of the
request as is currently done with NCQ commands. Alternatively, we could
add an internal libata qc queue for postponing these commands. That
would take a little effort, as the sync errors reported by
ata_qc_issue() would then be need to signalled async through the
completion callback instead.

Jeff, what do you think?

-- 
Jens Axboe