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SubjectRE: [PATCH 1/1] cciss per disk queue for 2.6
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> -----Original Message-----
> From: Jens Axboe [mailto:axboe@suse.de]
> Sent: Friday, July 01, 2005 2:02 AM
> To: Miller, Mike (OS Dev)
> Cc: akpm@osdl.org; linux-kernel@vger.kernel.org;
> linux-scsi@vger.kernel.org
> Subject: Re: [PATCH 1/1] cciss per disk queue for 2.6
>
> On Thu, Jun 30 2005, mike.miller@hp.com wrote:
> > This patch adds per disk queue functionality to cciss.
> Sometime back I
> > submitted a patch but it looks like only part of what I
> needed. In the
> > 2.6 kernel if we have more than one logical volume the driver will
> > Oops during rmmod. It seems all of the queues actually
> point back to
> > the same queue. So after deleting the first volume you hit a null
> > pointer on the second one.
> >
> > This has been tested in our labs. There is no difference in
> > performance, it just fixes the Oops. Please consider this patch for
> > inclusion.
>
> Going to per-disk queues is undoubtedly a good idea,
> performance will be much better this way. So far, so good.
>
> But you need to do something about the queueing for this to
> be acceptable, imho. You have a per-controller queueing limit
> in place, you need to enforce some fairness to ensure an
> equal distribution of commands between the disks.

Sometime back I submitted what we called the "fair enough" algorithm. I
don't know how I managed to separate the two. :( But it was accepted and
is in the mainstream kernel. Here's a snippet:
------------------------------------------------------------------------
-
- /*
- * See if we can queue up some more IO
- * check every disk that exists on this controller
- * and start it's IO
- */
- for(j=0; j < NWD; j++){
- /* make sure the disk has been added and the drive is
real */
- /* because this can be called from the middle of
init_one */
- if(!(h->gendisk[j]->queue) || !(h->drv[j].heads))
+ /* check to see if we have maxed out the number of commands that
can
+ * be placed on the queue. If so then exit. We do this check
here
+ * in case the interrupt we serviced was from an ioctl and did
not
+ * free any new commands.
+ */
+ if ((find_first_zero_bit(h->cmd_pool_bits, NR_CMDS)) == NR_CMDS)
+ goto cleanup;
+
+ /* We have room on the queue for more commands. Now we need to
queue
+ * them up. We will also keep track of the next queue to run so
+ * that every queue gets a chance to be started first.
+ */
+ for (j=0; j < NWD; j++){
+ int curr_queue = (start_queue + j) % NWD;
+ /* make sure the disk has been added and the drive is
real
+ * because this can be called from the middle of
init_one.
+ */
+ if(!(h->gendisk[curr_queue]->queue) ||
+ !(h->drv[curr_queue].heads))
------------------------------------------------------------------------
-
> Perhaps just limit the per-queue depth to something sane,
> instead of the huuuge 384 commands you have now. I've had
> several people complain to me, that ciss is doing some nasty
> starvation with that many commands in flight and we've
> effectively had to limit the queueing depth to something
> really low to get adequate read performance in presence of writes.

The combination of per disk queues and this "fairness" prevents
starvation on different LV's. Ex: if volumes 0 & 1 are being written and
volume 3 is being read all will get _almost_ equal time. We believe the
elevator algoritm(s) may be causing writes to starve reads on the same
logical volume. We continue to investigate our other performance issues.

Thanks,
mikem


>
>
> --
> Jens Axboe
>
>
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