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

[Kevin Wolf <kwolf@redhat.com>]

Am 12.05.2015 um 16:20 hat Denis V. Lunev geschrieben:
> On 12/05/15 17:08, Kevin Wolf wrote:
> >Am 12.05.2015 um 15:41 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 <den@openvz.org>
> >>CC: Paolo Bonzini <pbonzini@redhat.com>
> >>CC: Kevin Wolf <kwolf@redhat.com>
> >>CC: Stefan Hajnoczi <stefanha@redhat.com>
> >>---
> >>  block.c           |  8 ++++----
> >>  block/io.c        |  2 +-
> >>  block/raw-posix.c | 15 +++++++++------
> >>  3 files changed, 14 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) {
> >>diff --git a/block/raw-posix.c b/block/raw-posix.c
> >>index 7083924..4659552 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 = MAX(s->buf_align, getpagesize());
> >>+    }
> >>  }
> >I think this should be unconditional now.
> >
> >Kevin
> frankly speaking I am not comfortable with that. With the 'if' in the code
> we are protected if for some reason
>   bs->bl.opt_mem_alignment
> is greater then page size, f.e. if this is a requirement
> of underlying backing storage.

The old value of bs->bl.opt_mem_alignment is meaningless here. It's not
initialised except for some more or less random default value from
block.c. By being explicit I meant that we should always overwrite it
with known good values. (Currently, that default is getpagesize(), so we
never overwrite it with something smaller anyway.)

Kevin