Date:   Fri, 13 Dec 2019 14:40:00 +0530
From:   Krishna Chaitanya <chaitanya.mgit@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: debugging TCP stalls on high-speed wifi

On Fri, Dec 13, 2019 at 2:43 AM Johannes Berg <johannes@...solutions.net> 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.

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