netdev - Re: [PATCH net-next 3/4] tcp: add SACK compression

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Date:   Thu, 17 May 2018 09:59:06 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Christoph Paasch <cpaasch@...le.com>
Subject: Re: [PATCH net-next 3/4] tcp: add SACK compression

On Thu, May 17, 2018 at 9:41 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Thu, May 17, 2018 at 11:40 AM Eric Dumazet <eric.dumazet@...il.com>
> wrote:
> > On 05/17/2018 08:14 AM, Neal Cardwell wrote:
> > > Is there a particular motivation for the cap of 127? IMHO 127 ACKs is
> quite
> > > a few to compress. Experience seems to show that it works well to have
> one
> > > GRO ACK for ~64KBytes that triggers a single TSO skb of ~64KBytes. It
> might
> > > be nice to try to match those dynamics in this SACK compression case,
> so it
> > > might be nice to cap the number of compressed ACKs at something like 44?
> > > (0xffff / 1448 - 1).  That way for high-speed paths we could try to keep
> > > the ACK clock going with ACKs for ~64KBytes that trigger a single TSO
> skb
> > > of ~64KBytes, no matter whether we are sending SACKs or cumulative ACKs.
>
> > 127 was chosen because the field is u8, and since skb allocation for the
> ACK
> > can fail, we could have cases were the field goes above 127.
>
> > Ultimately, I believe a followup patch would add a sysctl, so that we can
> fine-tune
> > this, and eventually disable ACK compression if this sysctl is set to 0
>
> OK, a sysctl sounds good. I would still vote for a default of 44.  :-)
>
>
> > >> +       if (hrtimer_is_queued(&tp->compressed_ack_timer))
> > >> +               return;
> > >> +
> > >> +       /* compress ack timer : 5 % of srtt, but no more than 2.5 ms */
> > >> +
> > >> +       delay = min_t(unsigned long, 2500 * NSEC_PER_USEC,
> > >> +                     tp->rcv_rtt_est.rtt_us * (NSEC_PER_USEC >>
> 3)/20);
> > >
> > > Any particular motivation for the 2.5ms here? It might be nice to match
> the
> > > existing TSO autosizing dynamics and use 1ms here instead of having a
> > > separate new constant of 2.5ms. Smaller time scales here should lead to
> > > less burstiness and queue pressure from data packets in the network,
> and we
> > > know from experience that the CPU overhead of 1ms chunks is acceptable.
>
> > This came from my tests on wifi really :)
>
> > I also had the idea to make this threshold adjustable for wifi, like we
> did for sk_pacing_shift.
>
> > (On wifi, we might want to increase the max delay between ACK)
>
> > So maybe use 1ms delay, when sk_pacing_shift == 10, but increase it if
> sk_pacing_shift has been lowered.
>
> Sounds good to me.
>
> Thanks for implementing this! Overall this patch seems nice to me.
>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
>
> BTW, I guess we should spread the word to maintainers of other major TCP
> stacks that they need to be prepared for what may be a much higher degree
> of compression/aggregation in the SACK stream. Linux stacks going back many
> years should be fine with this, but I'm not sure about the other major OSes
> (they may only allow sending one MSS per ACK-with-SACKs received).
Patch looks really good but Neal's comment just reminds me a potential
legacy issue.

I recall at least Apple and Windows TCP stacks still need 3+ DUPACKs
(!= a SACK covering 3+ packets) to trigger fast recovery. Will we have
an issue there interacting w/ these stacks?

>
> neal