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Date: Sun, 9 Feb 2003 20:33:43 -0800
From: Andrew Morton <akpm@digeo.com>
To: David Lang <david.lang@digitalinsight.com>
Cc: riel@conectiva.com.br, andrea@suse.de, ckolivas@yahoo.com.au,
       linux-kernel@vger.kernel.org, axboe@suse.de
Subject: Re: stochastic fair queueing in the elevator [Re: [BENCHMARK]
 2.4.20-ck3 / aa / rmap with contest]
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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.

It's still approximate.  An exact solution would involve only marking I/O as
synchronous when some process actually waits on its completion.  I do not
believe that all the extra lookup and locking infrastructure and storage
which this would require is justified.  Certainly not in a first iteration.

The Rice Uni researchers did implement controls which attempted to learn IO
submission patterns on a per-process basis, and I believe these were also
used to avoid undesirable starvations.  We have only briefly played with
process-aware logic.

The first thing to do is to get the anticipatory scheduler working properly.
Nick has been tied down for a week chasing generic bugs in the request layer.
He seems to have nailed the important ones now.


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