From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1191CC4361B for ; Tue, 15 Dec 2020 08:30:17 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 58B6C222B3 for ; Tue, 15 Dec 2020 08:30:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 58B6C222B3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:37598 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kp5io-0004vI-Ok for qemu-devel@archiver.kernel.org; Tue, 15 Dec 2020 03:30:14 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:44094) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kp5g8-0003UA-JE for qemu-devel@nongnu.org; Tue, 15 Dec 2020 03:27:36 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:24149) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kp5g2-0006e6-Sp for qemu-devel@nongnu.org; Tue, 15 Dec 2020 03:27:27 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1608020842; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hFUqRgF+mlVd3aF0yY5lFKJdOEq8SaK9TJQHcJuy4wI=; b=GTIHD+/h3sbXVGL9PrjjbLZ2DBJ4LPgK87TQ+V5EUyjPrhHHDDROP582q79MS+cWgBhWpI 7GZDtS+l5n8yK2GBwH84URAihaqoNkR9bWepDhFJssLWmwzBWCw9gmGMRhaiKNRQXKrX5e mEcWahNbFUzaETrCZjMfPcjz3vGoTEQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-473-xwuUW-5AN2mgGizcuLiCvg-1; Tue, 15 Dec 2020 03:27:19 -0500 X-MC-Unique: xwuUW-5AN2mgGizcuLiCvg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A080B802B53; Tue, 15 Dec 2020 08:27:17 +0000 (UTC) Received: from gondolin (ovpn-114-220.ams2.redhat.com [10.36.114.220]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0E7E81042A95; Tue, 15 Dec 2020 08:26:58 +0000 (UTC) Date: Tue, 15 Dec 2020 09:26:56 +0100 From: Cornelia Huck To: Christian Borntraeger Subject: Re: [PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues Message-ID: <20201215092656.1b95e030.cohuck@redhat.com> In-Reply-To: References: <20201109154831.20779-1-pasic@linux.ibm.com> <20201109170616.6875f610.cohuck@redhat.com> <20201109195303.459f6fba.pasic@linux.ibm.com> <0a6d17ce-ed7f-98e8-2937-f266bb4f0f5a@de.ibm.com> <20201110114015.1ba4cdac.pasic@linux.ibm.com> Organization: Red Hat GmbH MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=cohuck@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, DKIM_INVALID=0.1, DKIM_SIGNED=0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Thomas Huth , David Hildenbrand , "Michael S. Tsirkin" , qemu-devel@nongnu.org, Halil Pasic , qemu-s390x@nongnu.org, Michael Mueller Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 10 Nov 2020 14:18:39 +0100 Christian Borntraeger wrote: > On 10.11.20 11:40, Halil Pasic wrote: > > On Tue, 10 Nov 2020 09:47:51 +0100 > > Christian Borntraeger wrote: > > > >> > >> > >> On 09.11.20 19:53, Halil Pasic wrote: > >>> On Mon, 9 Nov 2020 17:06:16 +0100 > >>> Cornelia Huck wrote: > >>> > >>>>> @@ -20,6 +21,11 @@ static void virtio_ccw_blk_realize(VirtioCcwDevice *ccw_dev, Error **errp) > >>>>> { > >>>>> VirtIOBlkCcw *dev = VIRTIO_BLK_CCW(ccw_dev); > >>>>> DeviceState *vdev = DEVICE(&dev->vdev); > >>>>> + VirtIOBlkConf *conf = &dev->vdev.conf; > >>>>> + > >>>>> + if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) { > >>>>> + conf->num_queues = MIN(4, current_machine->smp.cpus); > >>>>> + } > >>>> > >>>> I would like to have a comment explaining the numbers here, however. > >>>> > >>>> virtio-pci has a pretty good explanation (use 1:1 for vqs:vcpus if > >>>> possible, apply some other capping). 4 seems to be a bit arbitrary > >>>> without explanation, although I'm sure you did some measurements :) > >>> > >>> Frankly, I don't have any measurements yet. For the secure case, > >>> I think Mimu has assessed the impact of multiqueue, hence adding Mimu to > >>> the cc list. @Mimu can you help us out. > >>> > >>> Regarding the normal non-protected VMs I'm in a middle of producing some > >>> measurement data. This was admittedly a bit rushed because of where we > >>> are in the cycle. Sorry to disappoint you. > >>> > >>> The number 4 was suggested by Christian, maybe Christian does have some > >>> readily available measurement data for the normal VM case. @Christian: > >>> can you help me out? > >> My point was to find a balance between performance gain and memory usage. > >> As a matter of fact, virtqueue do consume memory. So 4 looked like a > >> reasonable default for me for large guests as long as we do not have directed > >> interrupts. > >> > >> Now, thinking about this again: If we want to change the default to something > >> else in the future (e.g. to num vcpus) then the compat handling will get > >> really complicated. > > > > Regarding compat handling, I believe we would need a new property for > > virtio-blk-ccw: something like def_num_queues_max. Then logic would > > morph to MIN(def_num_queues_max, current_machine->smp.cpus), and we could > > relatively freely do compat stuff on def_num_queues_max. > > > > IMHO not pretty but certainly doable. > > > >> > >> So we can > >> - go with num queues = num cpus. But this will consume memory > >> for guests with lots of CPUs. > > > > In absence of data that showcases the benefit outweighing the obvious > > detriment, I lean towards finding this option the least favorable. > > > >> - go with the proposed logic of min(4,vcpus) and accept that future compat handling > >> is harder > > > > IMHO not a bad option, but I think I would still feel better about a > > more informed decision. In the end, the end user can already specify the > > num_queues explicitly, so I don't think this is urgent. > > > >> - defer this change > > > > So I tend to lean towards deferring. > > Yes, I was pushing this for 5.2 to avoid compat handling. But maybe it is better > to wait and do it later. But we should certainly continue the discussion to have > something for the next release. Do we have a better idea now about which values would make sense here? > > > > > Another thought is, provided the load is about evenly spread on the > > different virtqueues, if the game is about vCPU locality, one could > > think of decreasing the size of each individual virtqueue while > > increasing their number, with the idea of not paying much more in terms > > of memory. The queue size however needs to be a power of 2, > > so there is a limit on the granularity. > > > > Regarding secure VMs, currently we have to cramp at least the swiotlb and > > the virtqueues into ZONE_DMA. So increasing the number of > > virtqueues heavily may get us into new trouble with exotic configs. > > > > Regards, > > Halil > > >