From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Zhang Haoyu" Subject: Re: [Qemu-devel] [question] virtio-blk performancedegradationhappened with virito-serial Date: Mon, 1 Sep 2014 20:57:11 +0800 Message-ID: <201409012057085393668@sangfor.com> References: <201408291545282753855@sangfor.com>, <20140829143849.GA8909@grmbl.mre>, <201409012038178763909@sangfor.com>, <20140901124647.GC14171@grmbl.mre> Mime-Version: 1.0 Content-Type: text/plain; charset="gb2312" Content-Transfer-Encoding: 7bit Cc: "qemu-devel" , "kvm" To: "Amit Shah" Return-path: Received: from smtp.sanfor.com ([58.251.49.30]:50461 "EHLO mail.sangfor.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753282AbaIAM5N (ORCPT ); Mon, 1 Sep 2014 08:57:13 -0400 Sender: kvm-owner@vger.kernel.org List-ID: >> >> 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? > I suspect windows driver specific, too. I have not test linux guest, I'll test it later. Thanks, Zhang Haoyu > Amit