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SubjectRe: Where is the performance bottleneck?


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 */
Jeff



Jens Axboe wrote:

>On Wed, Aug 31 2005, Holger Kiehl wrote:
>
>
>>On Wed, 31 Aug 2005, Jens Axboe wrote:
>>
>>
>>
>>>Nothing sticks out here either. There's plenty of idle time. It smells
>>>like a driver issue. Can you try the same dd test, but read from the
>>>drives instead? Use a bigger blocksize here, 128 or 256k.
>>>
>>>
>>>
>>I used the following command reading from all 8 disks in parallel:
>>
>> dd if=/dev/sd?1 of=/dev/null bs=256k count=78125
>>
>>Here vmstat output (I just cut something out in the middle):
>>
>>procs -----------memory---------- ---swap-- -----io---- --system--
>>----cpu----^M
>> r b swpd free buff cache si so bi bo in cs us sy id
>> wa^M
>> 3 7 4348 42640 7799984 9612 0 0 322816 0 3532 4987 0 22
>> 0 78
>> 1 7 4348 42136 7800624 9584 0 0 322176 0 3526 4987 0 23
>> 4 74
>> 0 8 4348 39912 7802648 9668 0 0 322176 0 3525 4955 0 22
>> 12 66
>> 1 7 4348 38912 7803700 9636 0 0 322432 0 3526 5078 0 23
>>
>>
>
>Ok, so that's somewhat better than the writes but still off from what
>the individual drives can do in total.
>
>
>
>>>You might want to try the same with direct io, just to eliminate the
>>>costly user copy. I don't expect it to make much of a difference though,
>>>feels like the problem is elsewhere (driver, most likely).
>>>
>>>
>>>
>>Sorry, I don't know how to do this. Do you mean using a C program
>>that sets some flag to do direct io, or how can I do that?
>>
>>
>
>I've attached a little sample for you, just run ala
>
># ./oread /dev/sdX
>
>and it will read 128k chunks direct from that device. Run on the same
>drives as above, reply with the vmstat info again.
>
>
>
>------------------------------------------------------------------------
>
>#include <stdio.h>
>#include <stdlib.h>
>#define __USE_GNU
>#include <fcntl.h>
>#include <stdlib.h>
>#include <unistd.h>
>
>#define BS (131072)
>#define ALIGN(buf) (char *) (((unsigned long) (buf) + 4095) & ~(4095))
>#define BLOCKS (8192)
>
>int main(int argc, char *argv[])
>{
> char *p;
> int fd, i;
>
> if (argc < 2) {
> printf("%s: <dev>\n", argv[0]);
> return 1;
> }
>
> fd = open(argv[1], O_RDONLY | O_DIRECT);
> if (fd == -1) {
> perror("open");
> return 1;
> }
>
> p = ALIGN(malloc(BS + 4095));
> for (i = 0; i < BLOCKS; i++) {
> int r = read(fd, p, BS);
>
> if (r == BS)
> continue;
> else {
> if (r == -1)
> perror("read");
>
> break;
> }
> }
>
> return 0;
>}
>
>

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