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SubjectRe: Where is the performance bottleneck?
From linux-kernel mailing list.....

Don't do this. BLKDEV_MIN_RQ sets the size of the mempool reserved
requests and will only get slightly used in low memory conditions, so
most memory will probably be wasted.....

Change /sys/block/xxx/queue/nr_requests

Tom Callahan
TESSCO Technologies
(443)-506-6216
callahant@tessco.com



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 */
>
>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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>
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