Re: Where is the performance bottleneck?

[Jens Axboe <axboe@xxxxxxx>]

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.

-- 
Jens Axboe

#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;
}