Messages in this thread | | | Date | Mon, 10 Feb 2003 16:22:26 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH] SFQ disk scheduler |
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On Mon, Feb 10 2003, Andrea Arcangeli wrote: > On Mon, Feb 10, 2003 at 03:50:01PM +0100, Jens Axboe wrote: > > Hi, > > > > Here's a simple stochastic fairness queueing disk scheduler, for current > > 2.5.59-BK. It has known limitations right now, mainly because I didn't > > bother making it complete. But it should suffice for some rudimentary > > testing, at least. > > Cool, that was fast! ;)
It's pretty easy to do in 2.5 :-). A 2.4 backport is of course feasible, but requires a bit more work (and possibly abstracting some of the elevator stuff there).
> > I'm not going to go into great detail about how it works, see Andrea's > > initial post of the paper referenced. This version may not be completely > > true to the SFQ concept, but should be close enough I think. It divides > > traffic into a fixed number of buckets (64 per default), and perturbs > > the hash every 5 seconds (hash shamelessly borrowed from networking atm, > > see comment). > > I tend to think 5 seconds is too small, 30 sec would be better IMHO (it > should be tested at bit).
It probably is too small, testing will show. I don't see too many collisions from a dbench 32 or 64, so...
> > To avoid too many disk seeks, when it's time to dispatch requests to the > > driver, we round robin all non-empty buckets and grab a single request > > from each. These requests are sorted into the dispatch queue. > > > > For performance reasons, io scheduler request merging is still a > > per-queue function (and not per-bucket). > > Unsure if it worth, but it probably it won't make that much difference, > likely different workloads are working on different part of the disk > anyways.
The rate of unrelated merging is typically quite low, so no it probably doesn't provide much of a performance benefit. However, it also keeps the code simpler to simply have a single merge hash per queue.
> > In closing, let me stress that this version has not really been tested > > all that much. It passes simple SCSI and IDE testing, should work on any > > hardware basically. > > How does it feel?
I don't know yet, haven't booted it on my work station yet. Will do so soon :)
-- Jens Axboe
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