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=-7.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 1DE7BC433DF for ; Fri, 7 Aug 2020 03:36:09 +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 EADFD2086A for ; Fri, 7 Aug 2020 03:36:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="GewCrEWj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EADFD2086A 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]:45880 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1k3tAu-00037C-21 for qemu-devel@archiver.kernel.org; Thu, 06 Aug 2020 23:36:08 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51916) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k3tAH-0002gx-Az for qemu-devel@nongnu.org; Thu, 06 Aug 2020 23:35:29 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:23213 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1k3tAE-0002sv-Hy for qemu-devel@nongnu.org; Thu, 06 Aug 2020 23:35:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1596771324; 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=QzD1aZW27As4sQOSEoFwffFyztx83cRaX7sXgNmwGsk=; b=GewCrEWjqXVqaIlC30RhiYefP5LZqY/YPLbDA3zTQB+5KICAXsJ3G9rg3nDNQISoBfMEoP hNCRibjlYXmH9RDTdEL0E7sPpbtyoG38V4lknQilxEGPj/zVHzRpPgXNzLZNq0enlSXmOp 7iMUmVPLBxSn5X6/tsNRbKIA/Kb61Ao= 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-371-JKeQ_umPNoyS3t8eZwqa7w-1; Thu, 06 Aug 2020 23:35:20 -0400 X-MC-Unique: JKeQ_umPNoyS3t8eZwqa7w-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3526710059A9; Fri, 7 Aug 2020 03:35:19 +0000 (UTC) Received: from [10.72.13.215] (ovpn-13-215.pek2.redhat.com [10.72.13.215]) by smtp.corp.redhat.com (Postfix) with ESMTP id 76F5D65C8C; Fri, 7 Aug 2020 03:35:10 +0000 (UTC) Subject: Re: Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit? To: Stefan Hajnoczi , "Michael S. Tsirkin" References: <20200805121107.GG361702@stefanha-x1.localdomain> <20200805081144-mutt-send-email-mst@kernel.org> <20200806123708.GC379937@stefanha-x1.localdomain> From: Jason Wang Message-ID: Date: Fri, 7 Aug 2020 11:35:08 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200806123708.GC379937@stefanha-x1.localdomain> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.81; envelope-from=jasowang@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/06 23:35:24 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: Yajun Wu , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020/8/6 下午8:37, Stefan Hajnoczi wrote: > On Wed, Aug 05, 2020 at 08:13:29AM -0400, Michael S. Tsirkin wrote: >> On Wed, Aug 05, 2020 at 01:11:07PM +0100, Stefan Hajnoczi wrote: >>> On Thu, Jul 30, 2020 at 07:46:09AM +0000, Yajun Wu wrote: >>>> I'm doing iperf test on VIRTIO net through vhost-user(HW VDPA). >>>> Find maximal acceptable tx_queue_size/rx_queue_size is 1024. >>>> Basically increase queue size can get better RX rate for my case. >>>> >>>> Can we increase the limit(VIRTQUEUE_MAX_SIZE) to 8192 to possibly gain better performance? >>> Hi, >>> The VIRTIO 1.1 specification says the maximum number of descriptors is >>> 32768 for both split and packed virtqueues. >>> >>> The vhost kernel code seems to support 32768. >>> >>> The 1024 limit is an implementation limit in QEMU. Increasing it would >>> require QEMU code changes. For example, VIRTQUEUE_MAX_SIZE is used as >>> the size of arrays. >>> >>> I can't think of a fundamental reason why QEMU needs to limit itself to >>> 1024 descriptors. Raising the limit would require fixing up the code and >>> ensuring that live migration remains compatible with older versions of >>> QEMU. >>> >>> Stefan >> There's actually a reason for a limit: in theory the vq size >> also sets a limit on the number of scatter/gather entries. >> both QEMU and vhost can't handle a packet split over > 1k chunks. >> >> We could add an extra limit for s/g size like block and scsi do, >> this will need spec, guest and host side work. > Interesting, thanks for explaining! This could be made explicit by > changing the QEMU code to: > > include/hw/virtio/virtio.h:#define VIRTQUEUE_MAX_SIZE IOV_MAX > > Looking more closely at the vhost kernel code I see that UIO_MAXIOV is > used in some places but not in vhost_vring_set_num() (ioctl > VHOST_SET_VRING_NUM). Is there a reason why UIO_MAXIOV isn't enforced > when the application sets the queue size? Actually three things: 1) queue size 2) #descriptors in a list 3) IOV size Spec limit the 2) to 1) but 2) may not equal to 3). So enforcing UIO_MAXIOV can not solve the problem completely. For vhost-net, it depends on socket to build skb which requires an iov array to work. We need remove this limitation by: - build skb by vhost-net itself - do piecewise copying Then we're not limited with #iov and more and support up to what spec supports. Thanks > > Stefan