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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 2201BC33CAC for ; Mon, 3 Feb 2020 10:26:34 +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 D749B20721 for ; Mon, 3 Feb 2020 10:26:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="IGIk9dCn" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D749B20721 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]:37552 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iyYw5-0002BA-0k for qemu-devel@archiver.kernel.org; Mon, 03 Feb 2020 05:26:33 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:43762) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iyYvH-0001fs-KD for qemu-devel@nongnu.org; Mon, 03 Feb 2020 05:25:44 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iyYvF-0003er-Sg for qemu-devel@nongnu.org; Mon, 03 Feb 2020 05:25:42 -0500 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:21347 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iyYvF-0003du-N3 for qemu-devel@nongnu.org; Mon, 03 Feb 2020 05:25:41 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1580725541; 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: in-reply-to:in-reply-to:references:references; bh=1y0SfidCOnKkvwGnx9nIwZtGis/d1AbO10+Aa0SwBpQ=; b=IGIk9dCnioN/nOPb28QPyh+u0WcDROCpAxCYGXmT3g+yCYb1d67eZLB8zVpcw7G6Yk07Yo +ssKiZlHt0DzJQHFHjxJo+Z9D+Y7MKI7YLVeH7hfy84iLz4DeYQCLRSViqIFgM0RO9xgRa 8alkLCCvFd+VuiN8Q0iEHt5pffsk67E= 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-137-FX8zvH9zMtu6locaSHAnog-1; Mon, 03 Feb 2020 05:25:37 -0500 X-MC-Unique: FX8zvH9zMtu6locaSHAnog-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E5DA6800D41; Mon, 3 Feb 2020 10:25:35 +0000 (UTC) Received: from localhost (unknown [10.33.36.133]) by smtp.corp.redhat.com (Postfix) with ESMTP id DEED289A9C; Mon, 3 Feb 2020 10:25:30 +0000 (UTC) Date: Mon, 3 Feb 2020 11:25:29 +0100 From: Sergio Lopez To: Stefan Hajnoczi Subject: Re: [PATCH v2 2/4] virtio-scsi: default num_queues to -smp N Message-ID: <20200203102529.3op54zggtquoguuo@dritchie> References: <20200124100159.736209-1-stefanha@redhat.com> <20200124100159.736209-3-stefanha@redhat.com> <20200127141031.6e108839.cohuck@redhat.com> <20200129154438.GC157595@stefanha-x1.localdomain> <20200130105235.GC176651@stefanha-x1.localdomain> MIME-Version: 1.0 In-Reply-To: <20200130105235.GC176651@stefanha-x1.localdomain> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="jecw2dyy5cznd624" Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: Kevin Wolf , Fam Zheng , Eduardo Habkost , qemu-block@nongnu.org, "Michael S. Tsirkin" , Cornelia Huck , qemu-devel@nongnu.org, Max Reitz , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --jecw2dyy5cznd624 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Jan 30, 2020 at 10:52:35AM +0000, Stefan Hajnoczi wrote: > On Thu, Jan 30, 2020 at 01:29:16AM +0100, Paolo Bonzini wrote: > > On 29/01/20 16:44, Stefan Hajnoczi wrote: > > > On Mon, Jan 27, 2020 at 02:10:31PM +0100, Cornelia Huck wrote: > > >> On Fri, 24 Jan 2020 10:01:57 +0000 > > >> Stefan Hajnoczi wrote: > > >>> @@ -47,10 +48,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIPro= xy *vpci_dev, Error **errp) > > >>> { > > >>> VHostSCSIPCI *dev =3D VHOST_SCSI_PCI(vpci_dev); > > >>> DeviceState *vdev =3D DEVICE(&dev->vdev); > > >>> - VirtIOSCSICommon *vs =3D VIRTIO_SCSI_COMMON(vdev); > > >>> + VirtIOSCSIConf *conf =3D &dev->vdev.parent_obj.parent_obj.conf= ; > > >>> + > > >>> + /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */ > > >>> + if (conf->num_queues =3D=3D VIRTIO_SCSI_AUTO_NUM_QUEUES) { > > >>> + conf->num_queues =3D current_machine->smp.cpus; > > >> This now maps the request vqs 1:1 to the vcpus. What about the fixed > > >> vqs? If they don't really matter, amend the comment to explain that? > > > The fixed vqs don't matter. They are typically not involved in the d= ata > > > path, only the control path where performance doesn't matter. > >=20 > > Should we put a limit on the number of vCPUs? For anything above ~128 > > the guest is probably not going to be disk or network bound. >=20 > Michael Tsirkin pointed out there's a hard limit of VIRTIO_QUEUE_MAX > (1024). We need to at least stay under that limit. >=20 > Should the guest have >128 virtqueues? Each virtqueue requires guest > RAM and 2 host eventfds. Eventually these resource requirements will > become a scalability problem, but how do we choose a hard limit and what > happens to guest performance above that limit? >From the UX perspective, I think it's safer to use a rather low upper limit for the automatic configuration. Users of large VMs (>=3D32 vCPUs) aiming for the optimal performance are already facing the need of manually tuning (or relying on a software to do that for them) other aspects of it, like vNUMA, IOThreads and CPU pinning, so I don't think we should focus on this group. On the other hand, the increase in host resource requirements may have unforeseen in some environments, specially to virtio-blk users with multiple disks. All in all, I don't have data that would justify setting the limit to one value or the other. The only argument I can put on the table is that, so far, we only had one VQ per device, so perhaps a conservative value (4? 8?) would make sense from a safety and compatibility point of view. 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