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

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

Andrea Arcangeli wrote:

>On Mon, Feb 10, 2003 at 03:58:26PM +1100, Nick Piggin wrote:
>
>>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.
>>
>
>the main purpose of readahead is to generate 512k scsi commands when you
>read a file with a 4k user buffer, anticipatory scheduling isn't very
>related to readahead.
>
You seem to be forgetting things like seek time.