The following sequence
int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
for (i = 0; i < 100000; i++)
write(fd, buf, 4096);
iperforms 5% better if buf is aligned to 4096 bytes rather then to
512 bytes.
I have used the following program to test
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <malloc.h>
#include <string.h>
int main(int argc, char *argv[])
{
int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
void *buf;
int i = 0, align = atoi(argv[2]);
do {
buf = memalign(align, 4096);
if (align >= 4096)
break;
if ((unsigned long)buf & 4095)
break;
i++;
} while (1);
printf("%d %p\n", i, buf);
memset(buf, 0x11, 4096);
for (i = 0; i < 100000; i++) {
lseek(fd, SEEK_CUR, 4096);
write(fd, buf, 4096);
}
close(fd);
return 0;
}
for in in `seq 1 30` ; do a.out aa ; done
The file was placed into 8 GB partition on HDD below to avoid speed
change due to different offset on disk. Results are reliable:
- 189 vs 180 seconds on Linux 3.16
The following setups have been tested:
1) ext4 with block size equals to 1024 over 512/512 physical/logical
sector size SSD disk
2) ext4 with block size equals to 4096 over 512/512 physical/logical
sector size SSD disk
3) ext4 with block size equals to 4096 over 512/4096 physical/logical
sector size rotational disk (WDC WD20EZRX)
4) xfs with block size equals to 4096 over 512/512 physical/logical
sector size SSD disk
The difference is quite reliable and the same 5%.
qemu-io -n -c 'write -P 0xaa 0 1G' 1.img
for image in qcow2 format is 1% faster.
Changes from v3:
- portable way to calculate system page size used
- 512/4096 values are replaced with proper macros/values
Changes from v2:
- opt_mem_alignment is split to opt_mem_alignment for bounce buffering
and min_mem_alignment to check buffers coming from guest.
Changes from v1:
- enforces 4096 alignment in qemu_(try_)blockalign, avoid touching of
bdrv_qiov_is_aligned path not to enforce additional bounce buffering
as suggested by Paolo
- reduces 10% to 5% in patch description to better fit 180 vs 189
difference
Signed-off-by: Denis V. Lunev <address@hidden>
CC: Paolo Bonzini <address@hidden>
CC: Kevin Wolf <address@hidden>