On 12/12/19 1:11 PM, Johannes Berg wrote: > Hi Eric, > > Thanks for looking :) > >>> I'm not sure how to do headers-only, but I guess -s100 will work. >>> >>> https://johannes.sipsolutions.net/files/he-tcp.pcap.xz >>> >> >> Lack of GRO on receiver is probably what is killing performance, >> both for receiver (generating gazillions of acks) and sender >> (to process all these acks) > Yes, I'm aware of this, to some extent. And I'm not saying we should see > even close to 1800 Mbps like we have with UDP... > > Mind you, the biggest thing that kills performance with many ACKs isn't > the load on the system - the sender system is only moderately loaded at > ~20-25% of a single core with TSO, and around double that without TSO. > The thing that kills performance is eating up all the medium time with > small non-aggregated packets, due to the the half-duplex nature of WiFi. > I know you know, but in case somebody else is reading along :-) > > But unless somehow you think processing the (many) ACKs on the sender > will cause it to stop transmitting, or something like that, I don't > think I should be seeing what I described earlier: we sometimes (have > to?) reclaim the entire transmit queue before TCP starts pushing data > again. That's less than 2MB split across at least two TCP streams, I > don't see why we should have to get to 0 (which takes about 7ms) until > more packets come in from TCP? > > Or put another way - if I free say 400kB worth of SKBs, what could be > the reason we don't see more packets be sent out of the TCP stack within > the few ms or so? I guess I have to correlate this somehow with the ACKs > so I know how much data is outstanding for ACKs. (*) > > The sk_pacing_shift is set to 7, btw, which should give us 8ms of > outstanding data. For now in this setup that's enough(**), and indeed > bumping the limit up (setting sk_pacing_shift to say 5) doesn't change > anything. So I think this part we actually solved - I get basically the > same performance and behaviour with two streams (needed due to GBit LAN > on the other side) as with 20 streams. > > >> I had a plan about enabling compressing ACK as I did for SACK >> in commit >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d9f4262b7ea41ca9981cc790e37cca6e37c789e >> >> But I have not done it yet. >> It is a pity because this would tremendously help wifi I am sure. > > Nice :-) > > But that is something the *receiver* would have to do. Yes, this is the plan. Eventually the receiver gets smarter. > > The dirty secret here is that we're getting close to 1700 Mbps TCP with > Windows in place of Linux in the setup, with the same receiver on the > other end (which is actually a single Linux machine with two GBit > network connections to the AP). So if we had this I'm sure it'd increase > performance, but it still wouldn't explain why we're so much slower than > Windows :-) > I presume you could hack TCP to no longer care about bufferbloat and you'll probably match Windows 'performance' on a single flow and a lossless network. Ie always send ~64KB TSO packets and fill the queues, inflating RTT. Then, in presence of losses, you get a problem because the retransmit packets can only be sent _after_ the huge queue that has been put on the sender. If only TCP could predict the future ;)
