Messages in this thread | | | Date | Fri, 27 Nov 2009 12:48:47 +0100 | From | Jens Axboe <> | Subject | Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula |
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On Fri, Nov 27 2009, Corrado Zoccolo wrote: > Hi Jens, > let me explain why my improved formula should work better. > > The original problem was that, even if an async queue had a slice of 40ms, > it could take much more to complete since it could have up to 31 > requests dispatched at the moment of expiry. > In total, it could take up to 40 + 16 * 8 = 168 ms (worst case) to > complete all dispatched requests, if they were seeky (I'm taking 8ms > average service time of a seeky request). > > With your patch, within the first 200ms from last sync, the max depth > will be 1, so a slice will take at most 48ms. > My patch still ensures that a slice will take at most 48ms within the > first 200ms from last sync, but lifts the restriction that depth will > be 1 at all time. > In fact, after the first 100ms, a new async slice will start allowing > 5 requests (async_slice/slice_idle). Then, whenever a request > completes, we compute remaining_slice / slice_idle, and compare this > with the number of dispatched requests. If it is greater, it means we > were lucky, and the requests were sequential, so we can allow more > requests to be dispatched. The number of requests dispatched will > decrease when reaching the end of the slice, and at the end we will > allow only depth 1. > For next 100ms, you will allow just depth 2, and my patch will allow > depth 2 at the end of the slice (but larger at the beginning), and so > on. > > I think the numbers by Mel show that this idea can give better and > more stable timings, and they were just with a single NCQ rotational > disk. I wonder how much improvement we can get on a raid, where > keeping the depth at 1 hits performance really hard. > Probably, waiting until memory reclaiming is noticeably active (since > in CFQ we will be sampling) may be too late.
I'm not saying it's a no-go, just that it invalidates the low latency testing done through the 2.6.32 cycle and we should re-run those tests before committing and submitting anything.
If the 'check for reclaim' hack isn't good enough, then that's probably what we have to do.
-- Jens Axboe
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