On 9 November 2015 at 13:23, David Miller <davem@xxxxxxxxxxxxx> wrote: > From: Dexuan Cui <decui@xxxxxxxxxxxxx> > Date: Mon, 9 Nov 2015 03:11:35 +0000 > >>> -----Original Message----- >>> From: David Miller [mailto:davem@xxxxxxxxxxxxx] >>> Sent: Monday, November 9, 2015 10:53 >>> To: Dexuan Cui <decui@xxxxxxxxxxxxx> >>> Cc: eric.dumazet@xxxxxxxxx; dsa@xxxxxxxxxxxxxxxxxxx; Simon Xiao >>> <sixiao@xxxxxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; Haiyang Zhang >>> <haiyangz@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; >>> devel@xxxxxxxxxxxxxxxxxxxxxx >>> Subject: Re: linux-next network throughput performance regression >>> >>> From: Dexuan Cui <decui@xxxxxxxxxxxxx> >>> Date: Mon, 9 Nov 2015 02:39:24 +0000 >>> >>> >> Throughput on a single TCP flow for a 40G NIC can be tricky to tune. >>> > Why is a single TCP flow trickier than multiple TCP flows? >>> > IMO it should be easier to analyze the issue of a single TCP flow? >>> >>> Because a single TCP flow can only use one of the many TX queues >>> that such modern NICs have. >>> >>> The single TX queue becomes the bottleneck. >>> >>> Whereas if you have several TCP flows, all of them can use independant >>> TX queues on the NIC in parallel to fill the link with traffic. >>> >>> That's why. >> >> Thanks, David! >> I understand 1 TX queue is the bottleneck (however in Simon's >> test, TX=1 => 36.7Gb/s, TX=8 => 37.7 Gb/s, so it looks the TX=1 bottleneck >> is not so obvious). >> I'm just wondering how the bottleneck became much narrower with >> recent linux-next in Simon's result (36.7 Gb/s vs. 18.2 Gb/s). IMO there >> must be some latency somewhere. > > I think the whole thing here is that you misinterpreted what Eric said. > > He is not arguing that some regression did, or did not, happen. > > He instead was making the basic statement about the fact that due to > the lack of paralellness a single stream TCP case is harder to > optimize for high speed NICs. > > That is all. We recently had a regression tracked down in a similiar area that was because of link order. Dave. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
