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

[Jakob Oestergaard]

On Sun, Feb 09, 2003 at 08:33:43PM -0800, Andrew Morton wrote:
> David Lang <[e-mail]> 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).

> 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.

Just a quirk; NFS file servers.

The client does read-ahead - the nfsd on the server-side will wait for
the read request, thus the read is synchronous...  But it's not.

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