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[76.14.218.44]) by smtp.gmail.com with ESMTPSA id r5sm374912pjd.2.2021.05.17.16.00.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 May 2021 16:00:44 -0700 (PDT) Date: Mon, 17 May 2021 16:00:36 -0700 From: Stephen Hemminger To: Dave Taht Cc: Willem de Bruijn , "Michael S. Tsirkin" , Xianting Tian , Linux Kernel Network Developers , LKML , virtualization , bloat , Jakub Kicinski , "David S. Miller" Message-ID: <20210517160036.4093d3f2@hermes.local> In-Reply-To: References: <56270996-33a6-d71b-d935-452dad121df7@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Bloat] virtio_net: BQL? X-BeenThere: bloat@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: General list for discussing Bufferbloat List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 17 May 2021 23:00:46 -0000 On Mon, 17 May 2021 14:48:46 -0700 Dave Taht wrote: > On Mon, May 17, 2021 at 1:23 PM Willem de Bruijn > wrote: > > > > On Mon, May 17, 2021 at 2:44 PM Dave Taht wrote: > > > > > > Not really related to this patch, but is there some reason why virtio > > > has no support for BQL? > > > > There have been a few attempts to add it over the years. > > > > Most recently, https://lore.kernel.org/lkml/20181205225323.12555-2-mst@redhat.com/ > > > > That thread has a long discussion. I think the key open issue remains > > > > "The tricky part is the mode switching between napi and no napi." > > Oy, vey. > > I didn't pay any attention to that discussion, sadly enough. > > It's been about that long (2018) since I paid any attention to > bufferbloat in the cloud and my cloudy provider (linode) switched to > using virtio when I wasn't looking. For over a year now, I'd been > getting reports saying that comcast's pie rollout wasn't working as > well as expected, that evenroute's implementation of sch_cake and sqm > on inbound wasn't working right, nor pf_sense's and numerous other > issues at Internet scale. > > Last week I ran a string of benchmarks against starlink's new services > and was really aghast at what I found there, too. but the problem > seemed deeper than in just the dishy... > > Without BQL, there's no backpressure for fq_codel to do its thing. > None. My measurement servers aren't FQ-codeling > no matter how much load I put on them. Since that qdisc is the default > now in most linux distributions, I imagine that the bulk of the cloud > is now behaving as erratically as linux was in 2011 with enormous > swings in throughput and latency from GSO/TSO hitting overlarge rx/tx > rings, [1], breaking various rate estimators in codel, pie and the tcp > stack itself. > > See: > > http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq_codel.png > > See the swings in latency there? that's symptomatic of tx/rx rings > filling and emptying. > > it wasn't until I switched my measurement server temporarily over to > sch_fq that I got a rrul result that was close to the results we used > to get from the virtualized e1000e drivers we were using in 2014. > > http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq.png > > While I have long supported the use of sch_fq for tcp-heavy workloads, > it still behaves better with bql in place, and fq_codel is better for > generic workloads... but needs bql based backpressure to kick in. > > [1] I really hope I'm overreacting but, um, er, could someone(s) spin > up a new patch that does bql in some way even half right for this > driver and help test it? I haven't built a kernel in a while. > The Azure network driver (netvsc) also does not have BQL. Several years ago I tried adding it but it benchmarked worse and there is the added complexity of handling the accelerated networking VF path.