Re: RFC on io-stalls patch

From: Jens Axboe <axboe@suse.de>
To: Andrea Arcangeli <andrea@suse.de>
Cc: Marcelo Tosatti <marcelo@conectiva.com.br>,
	Chris Mason <mason@suse.com>, lkml <linux-kernel@vger.kernel.org>,
	"Stephen C. Tweedie" <sct@redhat.com>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Jeff Garzik <jgarzik@pobox.com>, Andrew Morton <akpm@digeo.com>,
	Alexander Viro <viro@math.psu.edu>
Subject: Re: RFC on io-stalls patch
Date: Tue, 15 Jul 2003 07:26:40 +0200	[thread overview]
Message-ID: <20030715052640.GY833@suse.de> (raw)
In-Reply-To: <20030714201637.GQ16313@dualathlon.random>

On Mon, Jul 14 2003, Andrea Arcangeli wrote:
> On Mon, Jul 14, 2003 at 09:51:39PM +0200, Jens Axboe wrote:
> > -	rl = &q->rq;
> > -	if (!list_empty(&rl->free) && !blk_oversized_queue(q)) {
> > +	if ((rw == WRITE) && (blk_oversized_queue(q) || (rl->count < 4)))
> 
> did you disable the oversized queue check completely for reads? This

Yes

> looks unsafe, you can end with loads of ram locked up this way, the
> request queue cannot be limited in requests anymore. this isn't the
> "request reservation", this a "nearly unlimited amount of ram locked in
> for reads".

Sorry, but I think that is nonsense. This is the way we have always
worked. You just have to maintain a decent queue length still (like we
always have in 2.4) and there are no problems.

> Of course, the more reads can be in the queue, the less the background
> write loads will hurt parallel apps like a kernel compile as shown in
> xtar_load.
> 
> This is very different from the schedule advantage provided by the old
> queue model. If you allow an unlimited I/O queue for reads, that means
> the I/O queues will be filled by an huge amount of reads and a few
> writes (no matter how fast the xtar_load is writing to disk).
> 
> In the past (2.4.22pre4) the I/O queue would been at most 50/50, with
> your patch it can be 90/10, hence it can generate an huge performance
> difference, that can penealize tremendously the writers in server loads
> using fsync plus it can hurt the VM badly if all ram is locked up by
> parallel reads. Of course contest mostly cares about reads, not writes.
> 
> Overall I think your patch is unsafe and shouldn't be applied.

It is _not_ unsafe, stop spewing nonsense like that. The patch should
not be applied, it's just the first few things I did to see if it would
make a difference like I described. And it had a big effect, so I posted
results and went to bed. Know we have a grounds for further discussion,
and I'll bench the changes seperately too as well. It's about getting
data points you can use, you have to try extremese as well.

> Still if you want to allow 50/50, go ahead, that logic in pre4 was an
> order of magnitude more fair and generic than this patch.

Sigh... No I don't want 90/10 distribution of course, that would be
silly.

-- 
Jens Axboe