Re: [patch] new fifo I/O elevator that really does nothing at all
From: Bill Davidsen
Date: Tue Mar 29 2005 - 17:19:58 EST
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@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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