Re: 2.4.22pre6aa1

From: Andrea Arcangeli (
Date: Tue Jul 22 2003 - 08:59:57 EST

On Tue, Jul 22, 2003 at 03:34:16PM +0200, Marc-Christian Petersen wrote:
> On Friday 18 July 2003 00:50, Andrea Arcangeli wrote:
> Hi Andrea,
> > Can you try to change include/linux/blkdev.h like this:
> > -#define MAX_QUEUE_SECTORS (4 << (20 - 9)) /* 4 mbytes when full sized */
> > +#define MAX_QUEUE_SECTORS (16 << (20 - 9)) /* 4 mbytes when full sized */
> > This will raise the queue from 4 to 16M. That is the first(/only) thing
> > that can explain a drop in performnace while doing contigous I/O.
> > However I didn't expect it to make a difference, or at least not so
> > relevant.
> > If this doesn't help at all, it might not be an elevator/blkdev thing.
> > At least on my machines the contigous I/O still at the same speed.
> well, it doesn't help at all. I/O gets more worse with that change. (8mb/s
> less). How can this happen? *wondering*
> > You also where the only one reporting a loss of performance with
> > elevator-lowlatency, it could be still the same problem that you've
> > seen at that time.
> The only one? Surely not. Also Con tested your elevator-lowlatency and we both
> saw performance degration :)

performance degradation when? note that we're only talking about
contigous I/O here, not contest. I can't measure any performance
degradation during contigous I/O and if something it could be explained
by the now shorter queue, but you tried enlarging it and it went even
slower (this was good btw, confirming a larger queue was completely
worthless and it only hurts the VM without providing any I/O bandwidth
pipelining benefit). The elevator-lowlatency should have no other effect
other than a shorter queue during pure contigous I/O.

> > can you try with data=writeback (or ext2) or hdparm -W1 and see if you
> > can still see the same delta between the two kernels? (careful with -W1
> > as it invalidates journaling)
> Yes, I'll do it later this day.

please try plain ext2, this sounds like some fs effect of some sort. The
fs must throttle on the shorter queue or seek differently somehow.

> Sorry for my late reply. I've been very busy.

No problem ;)

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