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SubjectRe: io performance...
On Mon, Jan 16 2006, Jeff V. Merkey wrote:
> Max Waterman wrote:
>
> >Hi,
> >
> >I've been referred to this list from the linux-raid list.
> >
> >I've been playing with a RAID system, trying to obtain best bandwidth
> >from it.
> >
> >I've noticed that I consistently get better (read) numbers from kernel
> >2.6.8
> >than from later kernels.
>
>
> To open the bottlenecks, the following works well. Jens will shoot me
> for recommending this,
> but it works well. 2.6.9 so far has the highest numbers with this fix.
> You can manually putz
> around with these numbers, but they are an artificial constraint if you
> are using RAID technology
> that caches ad elevators requests and consolidates them.
>
>
> Jeff
>
>

>
> diff -Naur ./include/linux/blkdev.h ../linux-2.6.9/./include/linux/blkdev.h
> --- ./include/linux/blkdev.h 2004-10-18 15:53:43.000000000 -0600
> +++ ../linux-2.6.9/./include/linux/blkdev.h 2005-12-06 09:54:46.000000000 -0700
> @@ -23,8 +23,10 @@
> typedef struct elevator_s elevator_t;
> struct request_pm_state;
>
> -#define BLKDEV_MIN_RQ 4
> -#define BLKDEV_MAX_RQ 128 /* Default maximum */
> +//#define BLKDEV_MIN_RQ 4
> +//#define BLKDEV_MAX_RQ 128 /* Default maximum */
> +#define BLKDEV_MIN_RQ 4096
> +#define BLKDEV_MAX_RQ 8192 /* Default maximum */

Yeah I could shoot you. However I'm more interested in why this is
necessary, eg I'd like to see some numbers from you comparing:

- The stock settings
- Doing
# echo 8192 > /sys/block/<dev>/queue/nr_requests
for each drive you are accessing.
- The kernel with your patch.
If #2 and #3 don't provide very similar profiles/scores, then we have
something to look at.

The BLKDEV_MIN_RQ increase is just silly and wastes a huge amount of
memory for no good reason.

--
Jens Axboe

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