From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yuanhan Liu Subject: Re: [PATCH] vhost: fix virtio_net cache sharing of broadcast_rarp Date: Thu, 16 Mar 2017 14:21:22 +0800 Message-ID: <20170316062122.GN18844@yliu-dev.sh.intel.com> References: <1489605049-18686-1-git-send-email-ktraynor@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: maxime.coquelin@redhat.com, dev@dpdk.org, stable@dpdk.org To: Kevin Traynor Return-path: Content-Disposition: inline In-Reply-To: <1489605049-18686-1-git-send-email-ktraynor@redhat.com> List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Wed, Mar 15, 2017 at 07:10:49PM +0000, Kevin Traynor wrote: > The virtio_net structure is used in both enqueue and dequeue datapaths. > broadcast_rarp is checked with cmpset in the dequeue datapath regardless > of whether descriptors are available or not. > > It is observed in some cases where dequeue and enqueue are performed by > different cores and no packets are available on the dequeue datapath > (i.e. uni-directional traffic), the frequent checking of broadcast_rarp > in dequeue causes performance degradation for the enqueue datapath. > > In OVS the issue can cause a uni-directional performance drop of up to 15%. > > Fix that by moving broadcast_rarp to a different cache line in > virtio_net struct. Thanks, but I'm a bit confused. The drop looks like being caused by cache false sharing, but I don't see anything would lead to a false sharing. I mean, there is no write in the same cache line where the broadcast_rarp belongs. Or, the "volatile" type is the culprit here? Talking about that, I had actually considered to turn "broadcast_rarp" to a simple "int" or "uint16_t" type, to make it more light weight. The reason I used atomic type is to exactly send one broadcast RARP packet once SEND_RARP request is recieved. Otherwise, we may send more than one RARP packet when MQ is invovled. But I think we don't have to be that accurate: it's tolerable when more RARP are sent. I saw 4 SEND_RARP requests (aka 4 RARP packets) in the last time I tried vhost-user live migration after all. I don't quite remember why it was 4 though. That said, I think it also would resolve the performance issue if you change "rte_atomic16_t" to "uint16_t", without moving the place? --yliu