Re: Where is the performance bottleneck?

[jmerkey <jmerkey@utah-nac.org>]

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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