From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maxime Coquelin Subject: Re: [RFC PATCH] net/virtio: Align Virtio-net header on cache line in receive path Date: Wed, 22 Feb 2017 10:39:42 +0100 Message-ID: <458456c8-57e7-94f3-a145-f73f66106418@redhat.com> References: <20170221173243.20779-1-maxime.coquelin@redhat.com> <20170222013734.GJ18844@yliu-dev.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: "Liang, Cunming" , "Tan, Jianfeng" , "dev@dpdk.org" To: "Yang, Zhiyong" , Yuanhan Liu Return-path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id CCE572B88 for ; Wed, 22 Feb 2017 10:39:47 +0100 (CET) In-Reply-To: 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 02/22/2017 03:49 AM, Yang, Zhiyong wrote: > >> -----Original Message----- >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Yuanhan Liu >> Sent: Wednesday, February 22, 2017 9:38 AM >> To: Maxime Coquelin >> Cc: Liang, Cunming ; Tan, Jianfeng >> ; dev@dpdk.org >> Subject: Re: [dpdk-dev] [RFC PATCH] net/virtio: Align Virtio-net header on >> cache line in receive path >> >> On Tue, Feb 21, 2017 at 06:32:43PM +0100, Maxime Coquelin wrote: >>> This patch aligns the Virtio-net header on a cache-line boundary to >>> optimize cache utilization, as it puts the Virtio-net header (which is >>> always accessed) on the same cache line as the packet header. >>> >>> For example with an application that forwards packets at L2 level, a >>> single cache-line will be accessed with this patch, instead of two >>> before. >> >> I'm assuming you were testing pkt size <= (64 - hdr_size)? >> >>> In case of multi-buffers packets, next segments will be aligned on a >>> cache-line boundary, instead of cache-line boundary minus size of vnet >>> header before. >> >> The another thing is, this patch always makes the pkt data cache unaligned >> for the first packet, which makes Zhihong's optimization on memcpy (for big >> packet) useless. > > Why not could we keep pkt data starting always on the cache-line boundary? > In case of multi-buffer, the first remains unchanged, next segments can do as > Maxime said that. This is not possible to have both the first and next buffers aligned on cache line boundary, as we don't know whether the deccriptor will be the first one, or a subsequent one on Virtio side when we refill the vq. Cheers, Maxime > > Thanks > Zhiyong >