Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]

[Nick Piggin <piggin@cyberone.com.au>]

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