From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50303) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XNNKR-00015j-Du for qemu-devel@nongnu.org; Fri, 29 Aug 2014 10:39:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XNNKL-00007G-3v for qemu-devel@nongnu.org; Fri, 29 Aug 2014 10:39:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36019) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XNNKK-00006p-TD for qemu-devel@nongnu.org; Fri, 29 Aug 2014 10:38:57 -0400 Date: Fri, 29 Aug 2014 20:08:49 +0530 From: Amit Shah Message-ID: <20140829143849.GA8909@grmbl.mre> References: <201408291545282753855@sangfor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201408291545282753855@sangfor.com> Subject: Re: [Qemu-devel] [question] virtio-blk performance degradation happened with virito-serial List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Zhang Haoyu Cc: qemu-devel , kvm On (Fri) 29 Aug 2014 [15:45:30], Zhang Haoyu wrote: > Hi, all > > I start a VM with virtio-serial (default ports number: 31), and found that virtio-blk performance degradation happened, about 25%, this problem can be reproduced 100%. > without virtio-serial: > 4k-read-random 1186 IOPS > with virtio-serial: > 4k-read-random 871 IOPS > > but if use max_ports=2 option to limit the max number of virio-serial ports, then the IO performance degradation is not so serious, about 5%. > > And, ide performance degradation does not happen with virtio-serial. Pretty sure it's related to MSI vectors in use. It's possible that the virtio-serial device takes up all the avl vectors in the guests, leaving old-style irqs for the virtio-blk device. If you restrict the number of vectors the virtio-serial device gets (using the -device virtio-serial-pci,vectors= param), does that make things better for you? Amit