From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-2538-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [66.179.20.138]) by lists.oasis-open.org (Postfix) with ESMTP id E4CE258191EF for ; Tue, 12 Sep 2017 18:26:41 -0700 (PDT) References: <20160915223915.qjlnlvf2w7u37bu3@redhat.com> <0f0e1b94-2a46-689c-dbb3-0d578cc8df33@redhat.com> From: Jason Wang Message-ID: <092b643b-cfa1-f965-cbf1-0799df1685c7@redhat.com> Date: Wed, 13 Sep 2017 09:26:32 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Transfer-Encoding: 8bit Content-Language: en-US Subject: Re: [virtio-dev] Re: packed ring layout proposal v3 To: Willem de Bruijn Cc: "Michael S. Tsirkin" , virtio-dev@lists.oasis-open.org, virtualization@lists.linux-foundation.org List-ID: On 2017年09月13日 00:23, Willem de Bruijn wrote: > On Mon, Sep 11, 2017 at 3:47 AM, Jason Wang wrote: >> >> On 2017年09月10日 13:06, Michael S. Tsirkin wrote: >>> This is an update from v2 version. >>> Changes: >>> - update event suppression mechanism >>> - add wrap counter: DESC_WRAP flag in addition to >>> DESC_DRIVER flag used for validity so device does not have to >>> write out all used descriptors. >> >> Do we have benchmark result to show the advantage of DESC_DRIVER over e.g >> avail/used index? > The KVM forum presentation has some numbers. Yes. My question may be not accurate. I meant maybe we should benchmark packed ring layout without DESC_DRIVER but something like queue tail/head or producer/consumer (or whatever it called). Looks like most more nic does not use a flag inside descriptor to exam the descriptor ownership. > > I'm not sure that synthetic benchmarks will provide much value, as we > understand the trade-off quite well. > > The benefit of this model is improved best case performance, by having > a single cacheline read instead of two for the indirect used/avail ring model. > > The drawback is worse worst case, as scanning the ring of descriptors > introduces more cacheline misses than scanning the compressed > used/avail ring. Like I've replied, looks like the scanning is not friendly to batching or prefetching and can cause extra overheads. > > This model is easier to implement in hardware and the common case is > likely close to the best case, so I think it makes sense. Maybe, but we probably need inputs from hardware vendor guys. Thanks --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org