From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk Date: Wed, 04 Jul 2012 16:10:40 +0200 Message-ID: <4FF44EE0.9040200@redhat.com> References: <1340002390-3950-1-git-send-email-asias@redhat.com> <1340002390-3950-4-git-send-email-asias@redhat.com> <87hau9yse7.fsf@rustcorp.com.au> <4FDEE0CB.1030505@redhat.com> <87zk81x7dp.fsf@rustcorp.com.au> <4FDF0DA7.40604@redhat.com> <4FF3001C.9020706@redhat.com> <4FF3017C.4020605@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4FF3017C.4020605@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: dlaor@redhat.com Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Ronen Hod , Christoph Hellwig List-Id: virtualization@lists.linuxfoundation.org Il 03/07/2012 16:28, Dor Laor ha scritto: >>>> Users using a spinning disk still get IO scheduling in the host though. >>>> What benefit is there in doing it in the guest as well? >>> >>> The io scheduler waits for requests to merge and thus batch IOs >>> together. It's not important w.r.t spinning disks since the host can >>> do it but it causes much less vmexits which is the key issue for VMs. >> >> Does it make sense to use the guest's I/O scheduler at all? > > That's the reason we have a noop io scheduler. But is performance really better with noop? We had to revert usage of QUEUE_FLAG_NONROT in the guests because it caused performance degradation (commit f8b12e513b953aebf30f8ff7d2de9be7e024dbbe). The bio-based path is really QUEUE_FLAG_NONROT++, so it should really be a special case for people who know what they're doing. (Better would be to improve QEMU, there's definitely room for 20% improvement...). Paolo