From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amit Shah Subject: Re: [Qemu-devel] [question] virtio-blk performance degradationhappened with virito-serial Date: Mon, 1 Sep 2014 18:16:47 +0530 Message-ID: <20140901124647.GC14171@grmbl.mre> References: <201408291545282753855@sangfor.com> <20140829143849.GA8909@grmbl.mre> <201409012038178763909@sangfor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: qemu-devel , kvm To: Zhang Haoyu Return-path: Received: from mx1.redhat.com ([209.132.183.28]:31634 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752608AbaIAMq7 (ORCPT ); Mon, 1 Sep 2014 08:46:59 -0400 Content-Disposition: inline In-Reply-To: <201409012038178763909@sangfor.com> Sender: kvm-owner@vger.kernel.org List-ID: On (Mon) 01 Sep 2014 [20:38:20], 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. > > > I don't think so, > I use iometer to test 64k-read(or write)-sequence case, if I disable the virtio-serial dynamically via device manager->virtio-serial => disable, > then the performance get promotion about 25% immediately, then I re-enable the virtio-serial via device manager->virtio-serial => enable, > the performance got back again, very obvious. > So, I think it has no business with legacy interrupt mode, right? > > I am going to observe the difference of perf top data on qemu and perf kvm stat data when disable/enable virtio-serial in guest, > and the difference of perf top data on guest when disable/enable virtio-serial in guest, > any ideas? So it's a windows guest; it could be something windows driver specific, then? Do you see the same on Linux guests too? Amit