Messages in this thread | | | Date | Mon, 10 Feb 2003 18:43:47 +1100 | From | Nick Piggin <> | Subject | Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest] |
| |
Andrea Arcangeli wrote:
>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) > >Andrea > If you pass the information down, the IO scheduler can easily put a lower expiry time on sync writes for example, or a higher one on async reads. This isn't what the anticipatory scheduler patch is about.
The point is that it is very easy to get a huge backlog of writes for the purpose of disk head schedule optimising, but this is difficult with reads, anticipatory scheduling helps address this. Thats all really.
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |