From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35761) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YGYzP-0003eV-P2 for qemu-devel@nongnu.org; Wed, 28 Jan 2015 15:13:32 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YGYzL-0006fm-OG for qemu-devel@nongnu.org; Wed, 28 Jan 2015 15:13:27 -0500 Received: from mx2.parallels.com ([199.115.105.18]:53238) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YGYzL-0006fT-I1 for qemu-devel@nongnu.org; Wed, 28 Jan 2015 15:13:23 -0500 Message-ID: <54C942D7.4060908@openvz.org> Date: Wed, 28 Jan 2015 23:13:11 +0300 From: "Denis V. Lunev" MIME-Version: 1.0 References: <1422470996-20820-1-git-send-email-den@openvz.org> <1422470996-20820-2-git-send-email-den@openvz.org> <54C94172.6050004@redhat.com> In-Reply-To: <54C94172.6050004@redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 1/1] block: change default memory alignment for block requests to 4096 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Kevin Wolf , qemu-devel@nongnu.org, Stefan Hajnoczi On 28/01/15 23:07, Paolo Bonzini wrote: > > On 28/01/2015 19:49, 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); >> performs 10% better if buf is aligned to 4096 bytes rather then to >> 512 bytes on HDD with 512/4096 logical/physical sector size. >> >> The difference is quite reliable. > The 10% difference, however, is probably not enough to cover the cost of > providing a bounce buffer if a guest is (rightfully) using a 512-byte > aligned buffer: bs->bl.opt_mem_alignment is in fact badly named and it > should be bs->bl.min_mem_alignment instead. > > Instead, you probably should patch bdrv_opt_mem_align to return at least > 4096, and leave the detection logic intact. This will let > qemu_blockalign return a properly aligned buffer to qemu-img and other > in-process allocations, without negatively affecting the guest. > > Thanks, > > Paolo ok, this looks good to me :) >> Signed-off-by: Denis V. Lunev >> CC: Kevin Wolf >> CC: Stefan Hajnoczi >> --- >> block.c | 4 ++-- >> block/raw-posix.c | 4 ++-- >> 2 files changed, 4 insertions(+), 4 deletions(-) >> >> diff --git a/block.c b/block.c >> index d45e4dd..bc5d1e7 100644 >> --- a/block.c >> +++ b/block.c >> @@ -543,7 +543,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp) >> bs->bl.max_transfer_length = bs->file->bl.max_transfer_length; >> bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment; >> } else { >> - bs->bl.opt_mem_alignment = 512; >> + bs->bl.opt_mem_alignment = 4096; >> } >> >> if (bs->backing_hd) { >> @@ -966,7 +966,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file, >> >> bs->open_flags = flags; >> bs->guest_block_size = 512; >> - bs->request_alignment = 512; >> + bs->request_alignment = 4096; >> bs->zero_beyond_eof = true; >> open_flags = bdrv_open_flags(bs, flags); >> bs->read_only = !(open_flags & BDRV_O_RDWR); >> diff --git a/block/raw-posix.c b/block/raw-posix.c >> index ec38fee..d1b3388 100644 >> --- a/block/raw-posix.c >> +++ b/block/raw-posix.c >> @@ -266,7 +266,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp) >> if (!s->buf_align) { >> size_t align; >> buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE); >> - for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) { >> + for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) { >> if (pread(fd, buf + align, MAX_BLOCKSIZE, 0) >= 0) { >> s->buf_align = align; >> break; >> @@ -278,7 +278,7 @@ 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) { >> + for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) { >> if (pread(fd, buf, align, 0) >= 0) { >> bs->request_alignment = align; >> break; >>