Re: RFC on io-stalls patch

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Jul 14, 2003 at 05:17:16PM -0300, Marcelo Tosatti wrote:
> 
> 
> On Mon, 14 Jul 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
> > 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".
> >
> > 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.
> >
> > 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.
> 
> Well, I change my mind and wont apply it as-is in -pre6.

I would at least wait a clarification from Jens first. If I missed
something fundamental in my analysis of his patch, of course we could
apply it then, but at the moment it looks not safe and a bit cheating ;)

thanks,

Andrea