Re: Playing with SATA NCQ

Mark Lord
2005-05-26T21:50:39Z

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.

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.

Cheers