From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55324) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5DLu-0004HQ-8K for qemu-devel@nongnu.org; Tue, 24 May 2016 10:30:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b5DLn-0007xw-Fj for qemu-devel@nongnu.org; Tue, 24 May 2016 10:30:33 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:33400 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5DLn-0007xB-5x for qemu-devel@nongnu.org; Tue, 24 May 2016 10:30:27 -0400 From: Peter Lieven Date: Tue, 24 May 2016 16:30:20 +0200 Message-Id: <1464100220-21317-1-git-send-email-pl@kamp.de> Subject: [Qemu-devel] [PATCH V2] 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 aligned block. Currently QEMU reads the same block twice in this case which is not necessary. Signed-off-by: Peter Lieven --- v1->v2: following Paolos suggestions to simplify the if condition and adjusting the comment block/io.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/block/io.c b/block/io.c index 60a6bd8..7459dfb 100644 --- a/block/io.c +++ b/block/io.c @@ -1430,6 +1430,14 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs, bytes += offset & (align - 1); offset = offset & ~(align - 1); + + /* We have read the tail already if the request is smaller + * than one aligned block. + */ + if (bytes < align) { + qemu_iovec_add(&local_qiov, head_buf + bytes, align - bytes); + bytes = align; + } } if ((offset + bytes) & (align - 1)) { -- 1.9.1