From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44194) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5CYT-0004Vi-6w for qemu-devel@nongnu.org; Tue, 24 May 2016 09:39:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b5CYP-0003AC-GC for qemu-devel@nongnu.org; Tue, 24 May 2016 09:39:28 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:32991 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5CYP-00039m-6O for qemu-devel@nongnu.org; Tue, 24 May 2016 09:39:25 -0400 From: Peter Lieven Date: Tue, 24 May 2016 15:39:11 +0200 Message-Id: <1464097151-19479-1-git-send-email-pl@kamp.de> Subject: [Qemu-devel] [PATCH] block/io: optimize bdrv_co_pwritev for small requests List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, famz@redhat.com, kwolf@redhat.com, stefanha@redhat.com, mreitz@redhat.com, Peter Lieven in a read-modify-write cycle a small request might cause head and tail to fall into the same alignment. Currently QEMU reads the same block twice in this case which is not necessary. Signed-off-by: Peter Lieven --- block/io.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/block/io.c b/block/io.c index 60a6bd8..fa40121 100644 --- a/block/io.c +++ b/block/io.c @@ -1430,6 +1430,18 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs, bytes += offset & (align - 1); offset = offset & ~(align - 1); + + /* if head and tail fall into the same alignment + * we can omit the second read as it would read + * the same block again */ + if ((offset + bytes) & (align - 1) && + offset / align == (offset + bytes) / align) { + size_t tail_offs; + tail_offs = (offset + bytes) & (align - 1); + qemu_iovec_add(&local_qiov, head_buf + tail_offs, + align - tail_offs); + bytes += align - tail_offs; + } } if ((offset + bytes) & (align - 1)) { -- 1.9.1