Re: [patch] new fifo I/O elevator that really does nothing at all

[Bill Davidsen <davidsen@tmr.com>]

Jens Axboe wrote:
> On Mon, Mar 28 2005, Chen, Kenneth W wrote:
>
> > The noop elevator is still too fat for db transaction processing
> > workload.  Since the db application already merged all blocks before
> > sending it down, the I/O presented to the elevator are actually not
> > merge-able anymore. Since I/O are also random, we don't want to sort
> > them either.  However the noop elevator is still doing a linear search
> > on the entire list of requests in the queue.  A noop elevator after
> > all isn't really noop.
> >
> > We are proposing a true no-op elevator algorithm, no merge, no
> > nothing. Just do first in and first out list management for the I/O
> > request.  The best name I can come up with is "FIFO".  I also piggy
> > backed the code onto noop-iosched.c.  I can easily pull those code
> > into a separate file if people object.  Though, I hope Jens is OK with
> > it.
>
>
> It's not quite ok, because you don't honor the insertion point in
> fifo_add_request. The only 'fat' part of the noop io scheduler is the
> merge stuff, the original plan was to move that to a hash table lookup
> instead like the other io schedulers do. So I would suggest just
> changing noop to hash the request on the end point for back merges and
> forget about front merges, since they are rare anyways. Hmm actually,
> the last merge hint should catch most of the merges at almost zero cost.

Making the noop faster is clearly a good thing, but some database 
software may depend on transaction order as provided by a true fifo 
process. It would be nice to have both.

--
   -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me
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