Re: Where is the performance bottleneck?

[Jens Axboe]

On Wed, Aug 31 2005, jmerkey wrote:
> 
> 
> I have seen an 80GB/sec limitation in the kernel unless this value is 
> changed in the SCSI I/O layer
> for 3Ware and other controllers during testing of 2.6.X series kernels.
> 
> Change these values in include/linux/blkdev.h and performance goes from 
> 80MB/S to over 670MB/S on the 3Ware controller.
> 
> 
> //#define BLKDEV_MIN_RQ    4
> //#define BLKDEV_MAX_RQ    128    /* Default maximum */
> #define BLKDEV_MIN_RQ    4096
> #define BLKDEV_MAX_RQ    8192    /* Default maximum */

That's insane, you just wasted 1MiB of preallocated requests on each
queue in the system!

Please just do

# echo 512 > /sys/block/dev/queue/nr_requests

after boot for each device you want to increase the queue size too. 512
should be enough with the 3ware.

-- 
Jens Axboe

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