Re: [Qemu-devel] [PATCH 2/2] block: align bounce buffers to page

[Kevin Wolf]

Am 12.05.2015 um 07:47 hat Denis V. Lunev geschrieben:
> 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);
> performs 5% better if buf is aligned to 4096 bytes.
> 
> The difference is quite reliable.
> 
> On the other hand we do not want at the moment to enforce bounce
> buffering if guest request is aligned to 512 bytes.
> 
> The patch changes default bounce buffer optimal alignment to
> MAX(page size, 4k). 4k is chosen as maximal known sector size on real
> HDD.
> 
> The justification of the performance improve is quite interesting.
> From the kernel point of view each request to the disk was split
> by two. This could be seen by blktrace like this:
>   9,0   11  1     0.000000000 11151  Q  WS 312737792 + 1023 [qemu-img]
>   9,0   11  2     0.000007938 11151  Q  WS 312738815 + 8 [qemu-img]
>   9,0   11  3     0.000030735 11151  Q  WS 312738823 + 1016 [qemu-img]
>   9,0   11  4     0.000032482 11151  Q  WS 312739839 + 8 [qemu-img]
>   9,0   11  5     0.000041379 11151  Q  WS 312739847 + 1016 [qemu-img]
>   9,0   11  6     0.000042818 11151  Q  WS 312740863 + 8 [qemu-img]
>   9,0   11  7     0.000051236 11151  Q  WS 312740871 + 1017 [qemu-img]
>   9,0    5  1     0.169071519 11151  Q  WS 312741888 + 1023 [qemu-img]
> After the patch the pattern becomes normal:
>   9,0    6  1     0.000000000 12422  Q  WS 314834944 + 1024 [qemu-img]
>   9,0    6  2     0.000038527 12422  Q  WS 314835968 + 1024 [qemu-img]
>   9,0    6  3     0.000072849 12422  Q  WS 314836992 + 1024 [qemu-img]
>   9,0    6  4     0.000106276 12422  Q  WS 314838016 + 1024 [qemu-img]
> and the amount of requests sent to disk (could be calculated counting
> number of lines in the output of blktrace) is reduced about 2 times.
> 
> Both qemu-img and qemu-io are affected while qemu-kvm is not. The guest
> does his job well and real requests comes properly aligned (to page).
> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> ---
>  block.c           |  8 ++++----
>  block/io.c        |  2 +-
>  block/raw-posix.c | 14 ++++++++------
>  3 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/block.c b/block.c
> index e293907..325f727 100644
> --- a/block.c
> +++ b/block.c
> @@ -106,8 +106,8 @@ int is_windows_drive(const char *filename)
>  size_t bdrv_opt_mem_align(BlockDriverState *bs)
>  {
>      if (!bs || !bs->drv) {
> -        /* 4k should be on the safe side */
> -        return 4096;
> +        /* page size or 4k (hdd sector size) should be on the safe side */
> +        return MAX(4096, getpagesize());
>      }
>  
>      return bs->bl.opt_mem_alignment;
> @@ -116,8 +116,8 @@ size_t bdrv_opt_mem_align(BlockDriverState *bs)
>  size_t bdrv_min_mem_align(BlockDriverState *bs)
>  {
>      if (!bs || !bs->drv) {
> -        /* 4k should be on the safe side */
> -        return 4096;
> +        /* page size or 4k (hdd sector size) should be on the safe side */
> +        return MAX(4096, getpagesize());
>      }
>  
>      return bs->bl.min_mem_alignment;
> diff --git a/block/io.c b/block/io.c
> index 908a3d1..071652c 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -205,7 +205,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error 
> **errp)
>          bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
>      } else {
>          bs->bl.min_mem_alignment = 512;
> -        bs->bl.opt_mem_alignment = 512;
> +        bs->bl.opt_mem_alignment = getpagesize();
>      }
>  
>      if (bs->backing_hd) {

I think it would make more sense to keep this specific to the raw-posix
driver. After all, it's only the kernel page cache that we optimise
here. Other backends probably don't take advantage of page alignment.

> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 7083924..04f3d4e 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -301,6 +301,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int 
> fd, Error **errp)
>  {
>      BDRVRawState *s = bs->opaque;
>      char *buf;
> +    size_t max_align = MAX(MAX_BLOCKSIZE, getpagesize());
>  
>      /* For /dev/sg devices the alignment is not really used.
>         With buffered I/O, we don't have any restrictions. */
> @@ -330,9 +331,9 @@ static void raw_probe_alignment(BlockDriverState *bs, int 
> fd, Error **errp)
>      /* If we could not get the sizes so far, we can only guess them */
>      if (!s->buf_align) {
>          size_t align;
> -        buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE);
> -        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
> -            if (raw_is_io_aligned(fd, buf + align, MAX_BLOCKSIZE)) {
> +        buf = qemu_memalign(max_align, 2 * max_align);
> +        for (align = 512; align <= max_align; align <<= 1) {
> +            if (raw_is_io_aligned(fd, buf + align, max_align)) {
>                  s->buf_align = align;
>                  break;
>              }
> @@ -342,8 +343,8 @@ static void raw_probe_alignment(BlockDriverState *bs, int 
> fd, Error **errp)
>  
>      if (!bs->request_alignment) {
>          size_t align;
> -        buf = qemu_memalign(s->buf_align, MAX_BLOCKSIZE);
> -        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
> +        buf = qemu_memalign(s->buf_align, max_align);
> +        for (align = 512; align <= max_align; align <<= 1) {
>              if (raw_is_io_aligned(fd, buf, align)) {
>                  bs->request_alignment = align;
>                  break;
> @@ -726,7 +727,9 @@ static void raw_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>  
>      raw_probe_alignment(bs, s->fd, errp);
>      bs->bl.min_mem_alignment = s->buf_align;
> -    bs->bl.opt_mem_alignment = s->buf_align;
> +    if (bs->bl.min_mem_alignment > bs->bl.opt_mem_alignment) {
> +        bs->bl.opt_mem_alignment = bs->bl.min_mem_alignment;
> +    }

Or, if you want to keep the getpagesize() initialisation as a generic
fallback just in case, I would still suggest to be explicit here instead
of relying on the default, like this:

    bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize()).

Kevin