From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59823) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YsB2y-00043P-K4 for qemu-devel@nongnu.org; Tue, 12 May 2015 10:20:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YsB2r-0004Ot-Rr for qemu-devel@nongnu.org; Tue, 12 May 2015 10:20:36 -0400 Message-ID: <55520C23.2020705@openvz.org> Date: Tue, 12 May 2015 17:20:19 +0300 From: "Denis V. Lunev" MIME-Version: 1.0 References: <1431438060-23324-1-git-send-email-den@openvz.org> <1431438060-23324-3-git-send-email-den@openvz.org> <20150512140831.GC3524@noname.str.redhat.com> In-Reply-To: <20150512140831.GC3524@noname.str.redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 2/2] block: align bounce buffers to page List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Paolo Bonzini , Stefan Hajnoczi , qemu-devel@nongnu.org, qemu-block@nongnu.org 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 >> CC: Paolo Bonzini >> CC: Kevin Wolf >> CC: Stefan Hajnoczi >> --- >> 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. Keeping if here is safe IMHO. Den