Messages in this thread | | | Date | Mon, 10 Feb 2003 08:26:50 +0100 | From | Andrea Arcangeli <> | Subject | Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest] |
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On Mon, Feb 10, 2003 at 03:44:35PM +1100, Nick Piggin wrote: > Rik van Riel wrote: > > >On Mon, 10 Feb 2003, Nick Piggin wrote: > > > >>Andrea Arcangeli wrote: > >> > >> > >>>The only way to get the minimal possible latency and maximal fariness is > >>>my new stochastic fair queueing idea. > >>> > >>Sounds nice. I would like to see per process disk distribution. > >> > > > >Sounds like the easiest way to get that fair, indeed. Manage > >every disk as a separately scheduled resource... > > > I hope this option becomes available one day. > > > > > > >>However dependant reads can not merge with each other obviously so > >>you could end up say submitting 4K reads per process. > >> > > > >Considering that one medium/far disk seek counts for about 400 kB > >of data read/write, I suspect we'll just want to merge requests or > >put adjacant requests next to each other into the elevator up to > >a fairly large size. Probably about 1 MB for a hard disk or a cdrom, > >but much less for floppies, opticals, etc... > > > Yes, but the point is _dependant reads_. This is why Andrea's approach > alone can't solve dependant read vs write or nondependant read - while > maintaining a good throughput.
note that for soem workloads the _dependant reads_ can have _dependant writes_ in between. I know it's not the most common case though, but I don't love too much to make reads that much special. But again, it makes perfect sense to have it as a feature, but I wouldn't like it to be only option, I mean there must always be a way to disable anticipatory scheduling like there must be a way to disable SFQ (infact for SFQ it makes sense to leave it disabled by default, it's only a few people that definitely only cares to have the lowest possible latency for mplayer or xmms, or for multiuser system doing lots of I/O, but those guys can't live without it)
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