Re: 2.5.2-pre performance degradation on an old 486

[Jens Axboe <axboe@suse.de>]

On Sat, Jan 05 2002, Davide Libenzi wrote:
> > > (*) 100MHz 486DX4, 28MB ram, no L2 cache, two old and slow IDE disks,
> > > small custom no-nonsense RedHat 7.2, kernels compiled with gcc 2.95.3.
> >
> > Is this ISA (maybe it has something to do with ISA bouncing)? Mine is:
> >
> > 486 DX/2 ISA, Adaptec 1542, two slow scsi disks and a self-made
> > slackware-based system.
> >
> > Can you also backout the scheduler changes to verify this? I have a
> > backout patch for 2.5.2-pre6, if you don't want to do this for yourself.
> 
> There should be some part of the kernel that assume a certain scheduler
> behavior. There was a guy that reported a bad  hdparm  performance and i
> tried it. By running  hdparm -t  my system has a context switch of 20-30
> and an irq load of about 100-110.
> The scheduler itself, even if you code it in visual basic, cannot make
> this with such loads.
> Did you try to profile the kernel ?

Davide,

If this is caused by ISA bounce problems, then you should be able to
reproduce by doing something ala

[ drivers/ide/ide-dma.c ]

ide_toggle_bounce()
{
	...

+	addr = BLK_BOUNCE_ISA;
	blk_queue_bounce_limit(&drive->queue, addr);
}

pseudo-diff, just add the addr = line. Now compare performance with and
without your scheduler changes.

-- 
Jens Axboe