Re: [PATCH] cfq-iosched: fix queue depth detection

[Jens Axboe]

On Fri, Aug 22 2008, Aaron Carroll wrote:
> Jens Axboe wrote:
> >On Fri, Aug 22 2008, Aaron Carroll wrote:
> >>Hi Jens,
> >>
> >>This patch fixes a bug in the hw_tag detection logic causing a huge 
> >>performance
> >>hit under certain workloads on real queuing devices.  For example, an FIO 
> >>load
> >>of 16k direct random reads on an 8-disk hardware RAID yields about 2 
> >>MiB/s on
> >>default CFQ, while noop achieves over 20 MiB/s.
> >>
> >>While the solution is pretty ugly, it does have the advantage of adapting 
> >>to
> >>queue depth changes.  Such a situation might occur if the queue depth is
> >>configured in userspace late in the boot process.
> >
> >I don't think it's that ugly, and I prefer this logic to the existing
> >one in fact. Since it's a static property of the device, why did you
> >change it to toggle the flag back and forth instead of just setting it
> >once?
> 
> Because it is possible (albeit uncommon) that the queue depth can change
> at run time, like the example I gave.  However, there should be no false
> positives; the flag should only be toggled if the queue depth does change.
> So even if it doesn't occur often, we can handle this corner case for very
> little cost.

Good point, the user could fiddle with queue_depth to turn it on or off.
So the patch is fine from that stand point.

> >doesn't do queueing. So the interesting window is the one where we have
> >more requests pending yet the driver doesn't ask for it. I'd prefer a
> >patch that took that more into account, instead of just looking at the
> >past 50 samples and then toggle the hw_tag flag depending on the
> >behaviour in that time frame. You could easily have a depth of 1 there
> >always if it's a sync workload, even if hardware can do tagged queuing.
> 
> That's exactly what the lines
> 
> 	if (cfqd->rq_queued <= CFQ_HW_QUEUE_MIN &&
> 	    cfqd->rq_in_driver <= CFQ_HW_QUEUE_MIN)
> 		return;
> 
> are for.  It's not just the past 50 samples, but rather 50 samples with
> sufficient load to see whether the device could be queuing.

Alright, that answers that concern. And you still use the same magic
depth of 4, I think that still makes sense.

Thanks, I'll queue up the patch.

-- 
Jens Axboe

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