From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: [PATCH for-2.6.31] virtio_blk: revert QUEUE_FLAG_VIRT addition Date: Fri, 4 Sep 2009 22:44:42 +0200 Message-ID: <20090904204442.GA30941@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Mark McLoughlin To: Rusty Russell Return-path: Received: from verein.lst.de ([213.95.11.210]:53730 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933957AbZIDUo6 (ORCPT ); Fri, 4 Sep 2009 16:44:58 -0400 Content-Disposition: inline Sender: kvm-owner@vger.kernel.org List-ID: It seems like the addition of QUEUE_FLAG_VIRT caueses major performance regressions for Fedora users: https://bugzilla.redhat.com/show_bug.cgi?id=509383 https://bugzilla.redhat.com/show_bug.cgi?id=505695 while I can't reproduce those extreme regressions myself I think the flag is wrong. Rationale: QUEUE_FLAG_VIRT expands to QUEUE_FLAG_NONROT which casus the queue unplugged immediately. This is not a good behaviour for at least qemu and kvm where we do have significant overhead for every I/O operations. Even with all the latested speeups (native AIO, MSI support, zero copy) we can only get native speed for up to 128kb I/O requests we already are down to 66% of native performance for 4kb requests even on my laptop running the Intel X25-M SSD for which the QUEUE_FLAG_NONROT was designed. If we ever get virtio-blk overhead low enough that this flag makes sense it should only be set based on a feature flag set by the host. Signed-off-by: Christoph Hellwig Index: linux-2.6/drivers/block/virtio_blk.c =================================================================== --- linux-2.6.orig/drivers/block/virtio_blk.c 2009-09-04 17:33:48.802523987 -0300 +++ linux-2.6/drivers/block/virtio_blk.c 2009-09-04 17:33:56.186522158 -0300 @@ -314,7 +314,6 @@ static int __devinit virtblk_probe(struc } vblk->disk->queue->queuedata = vblk; - queue_flag_set_unlocked(QUEUE_FLAG_VIRT, vblk->disk->queue); if (index < 26) { sprintf(vblk->disk->disk_name, "vd%c", 'a' + index % 26);