On Fri, Dec 13, 2019 at 2:43 AM Johannes Berg <johannes@xxxxxxxxxxxxxxxx> 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. (*) Maybe try 'reno' instead of 'cubic' to see if congestion control is being too careful?I n my experiments a while ago reno was a bit more aggressive esp. in less lossy environments. > > > 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. As you have said CPU util is low, maybe try disabling RSS (as we are using 2 streams) and see if that is causing any concurrency issues? > > > > 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. > > 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 :-) > > Now, I'm certainly not saying that TCP behaviour is the only reason for > the difference, we already found an issue for example where due to a > small Windows driver bug some packet extension was always used, and the > AP is also buggy in that it needs the extension but didn't request it > ... so the two bugs cancelled each other out and things worked well, but > our Linux driver believed the AP ... :) Certainly there can be more > things like that still, I just started on the TCP side and ran into the > queueing behaviour that I cannot explain. > > > In any case, I'll try to dig deeper into the TCP stack to understand the > reason for this transmit behaviour. > > Thanks, > johannes > > > (*) Hmm. Now I have another idea. Maybe we have some kind of problem > with the medium access configuration, and we transmit all this data > without the AP having a chance to send back all the ACKs? Too bad I > can't put an air sniffer into the setup - it's a conductive setup. > > > (**) As another aside to this, the next generation HW after this will > have 256 frames in a block-ack, so that means instead of up to 64 (we > only use 63 for internal reasons) frames aggregated together we'll be > able to aggregate 256 (or maybe we again only 255?). Each one of those > frames may be an A-MSDU with ~11k content though (only 8k in the setup I > have here right now), which means we can get a LOT of data into a single > PPDU ... we'll probably have to bump the sk_pacing_shift to be able to > fill that with a single TCP stream, though since we run all our > performance numbers with many streams, maybe we should just leave it :) > > -- Thanks, Regards, Chaitanya T K.
