Am 12.05.2015 um 12:36 hat Denis V. Lunev geschrieben: > On 12/05/15 13:27, Kevin Wolf wrote: > >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 <d...@openvz.org> > >>CC: Paolo Bonzini <pbonz...@redhat.com> > >>CC: Kevin Wolf <kw...@redhat.com> > >>CC: Stefan Hajnoczi <stefa...@redhat.com> > >>--- > >> 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 > > > definitely I can do this if this is a strict requirement and I have > not performed any real testing on Windows and other platforms > but from my point of view we will be on a safe side with this > alignment.
Yes, it certainly won't hurt as a default, so I'm okay with keeping it in block.c. I would only like to have it explicit in raw-posix, too, because the justification you use in the commit message is specific to raw-posix (or, to be more precise, specific to raw-posix on Linux). Paolo is right that I missed that the page cache isn't involved, but then it must be the Linux block layer that splits the requests as you reported. That's still raw-posix only. For other backends (like network protocols), defaulting to pagesize shouldn't hurt and possibly there are some effects that make it an improvement there as well, but for raw-posix we actually have a good reason to do so and to be explicit about it in the driver. > Pls note, that I do not make any new allocation and any new > alignment check. The patch just forces alignment of the > allocation which will be performed in any case. And this > approach just matches IO coming from guest with IO initiated > by the qemu-img/io. All guest operations (both Windows and > Linux) are really page aligned by address and offset > nowadays. > > This approach is safe. It does not bring any additional > (significant) overhead. Yes, I understand that. :-) Kevin