Messages in this thread | | | From | Aaron Carroll <> | Date | Fri, 22 Aug 2008 21:02:54 +1000 | Subject | Re: [PATCH] cfq-iosched: fix queue depth detection |
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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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