From: Jens Axboe <axboe@suse.de>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: James Bottomley <James.Bottomley@SteelEye.com>,
Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BK PATCHES] add ata scsi driver
Date: Mon, 26 May 2003 22:57:25 +0200 [thread overview]
Message-ID: <20030526205725.GT845@suse.de> (raw)
In-Reply-To: <Pine.LNX.4.44.0305261345460.13489-100000@home.transmeta.com>
On Mon, May 26 2003, Linus Torvalds wrote:
>
> On Mon, 26 May 2003, Jens Axboe wrote:
> >
> > I know this is a pet peeve of yours (must be, I remember you bringing it
> > up at least 3 time before :), but I don't think that's necessarily true.
> > It shouldn't matter _one_ bit whether you leave the request there or
> > not, it's unmergeable.
>
> It's not the merging that I worry about. It's latency and starvation.
>
> Think of it this way: if you keep feeding a disk requests, and the disk
> always tries to do the closest one (which is a likely algorithm), you can
> easily have a situation where the disk _never_ actually schedules a
> request that is at one "end" of the platter.
Then you have a bad disk, period. If the disks always tries to
approximate SPTF internally, then it's a bad design. Apparently that
Other OS times read/write requests out after 3 seconds, we we at least
know we are getting service in that time frame. Not saying that is good
enough, just a data point.
But the situation you describe above can easily be fixed, you described
the solution yourself in the previous mail. The silly tag depth is a
problem in itself, it should not be done. Keeping a sane number of tags
just to keep the disk busy, and we can use the "don't queue more
requests before X finishes, because X has been waiting for Y ms" tactic.
In fact, considering folks want to make error handling for command
timeouts a block property (that I agree with, we are already going there
with the SG_IO stuff), we can soft timeout a command if need be and
handle the case from there. What do you think?
> Think of all the fairness issues we've had in the elevator code, and
> realize that the low-level disk probably implements _none_ of those
> fairness algorithms.
I think it does, to some extent at least.
> > As long as the io scheduler keeps track of this (and it does!) we are
> > golden.
>
> Hmm.. Where does it keep track of request latency for requests that have
> been removed from the queue?
Well, it doesn't...
--
Jens Axboe
next prev parent reply other threads:[~2003-05-26 20:44 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-26 18:12 [BK PATCHES] add ata scsi driver James Bottomley
2003-05-26 18:18 ` Jens Axboe
2003-05-26 18:47 ` James Bottomley
2003-05-26 19:07 ` Jens Axboe
2003-05-26 19:17 ` James Bottomley
2003-05-26 19:33 ` Jens Axboe
2003-05-27 12:39 ` Jens Axboe
2003-05-27 14:26 ` James Bottomley
2003-05-27 17:16 ` Jens Axboe
2003-05-27 18:09 ` James Bottomley
2003-05-27 18:21 ` Jens Axboe
2003-05-27 18:30 ` James Bottomley
2003-05-26 20:27 ` Linus Torvalds
2003-05-26 20:36 ` James Bottomley
2003-05-26 20:45 ` Linus Torvalds
2003-05-26 20:51 ` Jens Axboe
2003-05-26 20:56 ` James Bottomley
2003-05-26 20:38 ` Jens Axboe
2003-05-26 20:49 ` Linus Torvalds
2003-05-26 20:57 ` Jens Axboe [this message]
2003-05-26 21:34 ` Linus Torvalds
2003-05-26 23:58 ` Nick Piggin
2003-05-27 0:09 ` Linus Torvalds
2003-05-27 0:49 ` Nick Piggin
2003-05-27 0:16 ` Alan Cox
2003-05-27 6:54 ` Jens Axboe
2003-05-27 14:20 ` James Bottomley
2003-05-27 14:36 ` Linus Torvalds
2003-05-27 14:59 ` James Bottomley
2003-05-27 15:21 ` Jeff Garzik
2003-05-27 15:38 ` James Bottomley
2003-05-27 15:50 ` Jeff Garzik
2003-05-27 16:00 ` James Bottomley
2003-05-27 16:16 ` Jeff Garzik
2003-05-28 9:35 ` Christoph Hellwig
2003-05-28 10:50 ` Lincoln Dale
2003-05-27 19:43 ` Jens Axboe
-- strict thread matches above, loose matches on Subject: below --
2003-05-26 4:58 Jeff Garzik
2003-05-26 5:15 ` Linus Torvalds
2003-05-26 5:30 ` Jeff Garzik
2003-05-26 5:36 ` Jeff Garzik
2003-05-26 5:42 ` Linus Torvalds
2003-05-26 6:01 ` Jeff Garzik
2003-05-26 16:56 ` Linus Torvalds
2003-05-26 17:47 ` Jeff Garzik
2003-05-26 20:09 ` Linus Torvalds
2003-05-27 0:29 ` Alan Cox
2003-05-27 6:07 ` Jeff Garzik
2003-05-27 6:30 ` Linus Torvalds
2003-05-27 6:51 ` Linus Torvalds
2003-05-27 7:29 ` Jeff Garzik
2003-05-26 5:40 ` Linus Torvalds
2003-05-26 5:53 ` Jeff Garzik
2003-05-26 6:21 ` Jeff Garzik
2003-05-26 16:57 ` Linus Torvalds
2003-05-26 17:24 ` Jens Axboe
2003-05-26 17:54 ` Jeff Garzik
2003-05-26 17:59 ` Jeff Garzik
2003-05-26 18:11 ` Jens Axboe
2003-05-27 0:22 ` Alan Cox
2003-05-27 4:15 ` Linus Torvalds
2003-05-26 10:32 ` Bartlomiej Zolnierkiewicz
2003-05-26 11:13 ` Jeff Garzik
2003-05-26 11:37 ` Bartlomiej Zolnierkiewicz
2003-05-26 5:59 ` Benjamin Herrenschmidt
2003-05-26 6:03 ` Jeff Garzik
2003-06-02 9:46 ` Andre Hedrick
2003-06-02 13:56 ` Alan Cox
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