Re: [Qemu-devel] [PATCH v4 0/1] block: enforce minimal 4096 alignment in qemu_blockalign

["Denis V. Lunev" <den@openvz.org>]

On 06/02/15 20:37, Denis V. Lunev wrote:
> 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 <den@openvz.org>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
>
ping