Re: 2.4.22pre6aa1

[Chris Mason <mason@suse.com>]

On Thu, 2003-07-17 at 18:50, Andrea Arcangeli wrote:
> On Fri, Jul 18, 2003 at 12:30:45AM +0200, Marc-Christian Petersen wrote:
> > 2.4.22-pre[6|6aa1]:
> > -------------------
> > root@codeman:[/] # dd if=/dev/zero of=/home/largefile bs=16384 count=131072
> > 131072+0 records in
> > 131072+0 records out
> > 2147483648 bytes transferred in 128.765686 seconds (16677453 bytes/sec)
> > 
> > 2.4.22-pre2:
> > ------------
> > root@codeman:[/] # dd if=/dev/zero of=/home/largefile bs=16384 count=131072
> > 131072+0 records in
> > 131072+0 records out
> > 2147483648 bytes transferred in 98.489331 seconds (21804226 bytes/sec)
> > 
> > both kernels freshly rebooted.
> 
> this explains it.
> 
> 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.
> 
Especially with just one writer, you really shouldn't be able to see a
difference in pre6.  Did you measure this change on both pre6 and
pre6aa1.  Your message indicated that but I wanted to double check to
make sure.

-chris