Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3

[Nick Piggin]

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.