Messages in this thread | | | Date | Mon, 10 Feb 2003 15:58:26 +1100 | From | Nick Piggin <> | Subject | Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest] |
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Jakob Oestergaard wrote:
>On Sun, Feb 09, 2003 at 08:33:43PM -0800, Andrew Morton wrote: > >>David Lang <david.lang@digitalinsight.com> wrote: >> >>>note that issuing a fsync should change all pending writes to 'syncronous' >>>as should writes to any partition mounted with the sync option, or writes >>>to a directory with the S flag set. >>> >>We know, at I/O submission time, whether a write is to be waited upon. >>That's in writeback_control.sync_mode. >> >>That, combined with an assumption that "all reads are synchronous" would >>allow the outgoing BIOs to be appropriately tagged. >> > >This may be a terribly stupid question, if so pls. just tell me :) > >I assume read-ahead requests go elsewhere? Or do we assume that someone >is waiting for them as well? > >If we assume they are synchronous, that would be rather unfair >especially on multi-user systems - and the 90% accuracy that Rik >suggested would seem exaggerated to say the least (accuracy would be >more like 10% on a good day). > Remember that readahead gets scaled down quickly if it isn't getting hits. It is also likely to be sequential and in the track buffer, so it is a small cost.
Huge readahead is a problem however anticipatory scheduling will hopefully allow good throughput for multiple read streams without requiring much readahead.
Nick
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