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

[Aaron Carroll]

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.

> 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.


Thanks,
  -- Aaron

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