Re: Where is the performance bottleneck?

[Ming Zhang]

join the party. ;)

8 400GB SATA disk on same Marvel 8 port PCIX-133 card. P4 CPU.
Supermicro SCT board.

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6]
[raid10] [faulty]
md0 : active raid0 sdh[7] sdg[6] sdf[5] sde[4] sdd[3] sdc[2] sdb[1] sda
[0]
      3125690368 blocks 64k chunks

8 DISK RAID0 from same slot and card. Stripe size is 512KB.

run oread

# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- ----
cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy
id wa
 1  1      0 533216 330424  11004    0    0  7128  1610 1069    77  0  2
95  3
 1  0      0 298464 560828  11004    0    0 230404     0 2595  1389  1
23  0 76
 0  1      0  64736 792248  11004    0    0 231420     0 2648  1342  0
26  0 74
 1  0      0   8948 848416   9696    0    0 229376     0 2638  1337  0
29  0 71
 0  0      0 868896    768   9696    0    0 29696    48 1224   162  0 19
73  8

# time ./oread /dev/md0

real    0m6.595s
user    0m0.004s
sys     0m0.151s

run dd

# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- ----
cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy
id wa
 2  2      0 854008   2932  17108    0    0  7355  1606 1071    80  0  2
95  3
 0  2      0 848888   3112  21388    0    0 164332     0 2985  3564  2
7  0 91
 0  2      0 844024   3260  25664    0    0 164040     0 2990  3665  1
7  0 92
 0  2      0 840328   3380  28920    0    0 164272     0 2932  3791  1
9  0 90
 0  2      0 836360   3500  32232    0    0 163688   100 3001  5045  2
7  0 91
 0  2      0 831432   3644  36612    0    0 164120   568 2977  3843  0
9  0 91
 0  1      0 826056   3752  41688    0    0  7872     0 1267  1474  1  3
0 96

# time dd if=/dev/md0 of=/dev/null bs=131072 count=8192
8192+0 records in
8192+0 records out

real    0m4.771s
user    0m0.005s
sys     0m0.973s

so the reasonable thing here is because of O_DIRECT, the sys time
reduced a lot.

but the time is longer! the reason i found is...

i attached a new oread.c which allow to set block size of each read and
total read count. so i read full strip once a time,

# time ./oread /dev/md0 524288 2048

real    0m4.950s
user    0m0.000s
sys     0m0.131s

compared to 

# time ./oread /dev/md0 131072 8192

real    0m6.633s
user    0m0.002s
sys     0m0.191s


but still, I can get linear speed at 4 DISKS, then no speed gain when
adding more disk into the RAID.

Ming

#include <stdio.h>
#include <stdlib.h>
#define __USE_GNU
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

#define ALIGN(buf)	(char *) (((unsigned long) (buf) + 4095) & ~(4095))

int main(int argc, char *argv[])
{
	char *p;
	int fd, i;
	int BS, BLOCKS;

	if (argc < 4) {
		printf("%s: <dev> bs cnt\n", argv[0]);
		return 1;
	}

	BS = atoi(argv[2]);
	BLOCKS = atoi(argv[3]);
	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;
}