From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52640) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btqs7-0007vR-3R for qemu-devel@nongnu.org; Tue, 11 Oct 2016 02:49:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1btqs3-0007jz-US for qemu-devel@nongnu.org; Tue, 11 Oct 2016 02:49:07 -0400 Received: from mga07.intel.com ([134.134.136.100]:17307) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btqs3-0007jC-KJ for qemu-devel@nongnu.org; Tue, 11 Oct 2016 02:49:03 -0400 Date: Tue, 11 Oct 2016 14:49:54 +0800 From: Yuanhan Liu Message-ID: <20161011064954.GN1597@yliu-dev.sh.intel.com> References: <2b458818-01ef-0533-4366-1c35a8452e4a@redhat.com> <20160930221241-mutt-send-email-mst@kernel.org> <20161010040531.GZ1597@yliu-dev.sh.intel.com> <20161010070800-mutt-send-email-mst@kernel.org> <20161010042209.GB1597@yliu-dev.sh.intel.com> <3b71f113-26af-711c-4060-ca576260ec72@redhat.com> <20161010144209.GI1597@yliu-dev.sh.intel.com> <18372cc2-19d3-f455-728d-2f2ed405d800@redhat.com> <20161011060456.GM1597@yliu-dev.sh.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Maxime Coquelin Cc: "Michael S. Tsirkin" , Stephen Hemminger , dev@dpdk.org, qemu-devel@nongnu.org On Tue, Oct 11, 2016 at 08:39:54AM +0200, Maxime Coquelin wrote: > > > On 10/11/2016 08:04 AM, Yuanhan Liu wrote: > >On Mon, Oct 10, 2016 at 04:54:39PM +0200, Maxime Coquelin wrote: > >> > >> > >>On 10/10/2016 04:42 PM, Yuanhan Liu wrote: > >>>On Mon, Oct 10, 2016 at 02:40:44PM +0200, Maxime Coquelin wrote: > >>>>>>>At that time, a packet always use 2 descs. Since indirect desc is > >>>>>>>enabled (by default) now, the assumption is not true then. What's > >>>>>>>worse, it might even slow things a bit down. That should also be > >>>>>>>part of the reason why performance is slightly worse than before. > >>>>>>> > >>>>>>> --yliu > >>>>>> > >>>>>>I'm not sure I get what you are saying > >>>>>> > >>>>>>>commit 1d41d77cf81c448c1b09e1e859bfd300e2054a98 > >>>>>>>Author: Yuanhan Liu > >>>>>>>Date: Mon May 2 17:46:17 2016 -0700 > >>>>>>> > >>>>>>> vhost: optimize dequeue for small packets > >>>>>>> > >>>>>>> A virtio driver normally uses at least 2 desc buffers for Tx: the > >>>>>>> first for storing the header, and the others for storing the data. > >>>>>>> > >>>>>>> Therefore, we could fetch the first data desc buf before the main > >>>>>>> loop, and do the copy first before the check of "are we done yet?". > >>>>>>> This could save one check for small packets that just have one data > >>>>>>> desc buffer and need one mbuf to store it. > >>>>>>> > >>>>>>> Signed-off-by: Yuanhan Liu > >>>>>>> Acked-by: Huawei Xie > >>>>>>> Tested-by: Rich Lane > >>>>>> > >>>>>>This fast-paths the 2-descriptors format but it's not active > >>>>>>for indirect descriptors. Is this what you mean? > >>>>> > >>>>>Yes. It's also not active when ANY_LAYOUT is actually turned on. > >>>>>>Should be a simple matter to apply this optimization for indirect. > >>>>> > >>>>>Might be. > >>>> > >>>>If I understand the code correctly, indirect descs also benefit from this > >>>>optimization, or am I missing something? > >>> > >>>Aha..., you are right! > >> > >>The interesting thing is that the patch I send on Thursday that removes > >>header access when no offload has been negotiated[0] seems to reduce > >>almost to zero the performance seen with indirect descriptors enabled. > > > >Didn't follow that. > > > >>I see this with 64 bytes packets using testpmd on both ends. > >> > >>When I did the patch, I would have expected the same gain with both > >>modes, whereas I measured +1% for direct and +4% for indirect. > > > >IIRC, I did a test before (remove those offload code piece), and the > >performance was basically the same before and after that. Well, there > >might be some small difference, say 1% as you said. But the result has > >never been steady. > > > >Anyway, I think your patch is good to have: I just didn't see v2. > > I waited to gather some comments/feedback before sending the v2. > I'll send it today or tomorrow. Interesting, I saw a deadlock then: I haven't looked at the code carefully once you said there is a v2, thus I'm waiting for it. However, you are waitting for my review. :) Anyway, I will take time to look at it shortly. --yliu