netdev - Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP

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Date:   Wed, 11 Sep 2019 20:49:56 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Thomas Higdon <tph@...com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>, Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP_INFO

On Wed, Sep 11, 2019 at 6:32 PM Thomas Higdon <tph@...com> wrote:
>
> Neal Cardwell mentioned that rcv_wnd would be useful for helping
> diagnose whether a flow is receive-window-limited at a given instant.
>
> This serves the purpose of adding an additional __u32 to avoid the
> would-be hole caused by the addition of the tcpi_rcvi_ooopack field.
>
> Signed-off-by: Thomas Higdon <tph@...com>
> ---

Thanks, Thomas.

I know that when I mentioned this before I mentioned the idea of both
tp->snd_wnd (send-side receive window) and tp->rcv_wnd (receive-side
receive window) in tcp_info, and did not express a preference between
the two. Now that we are faced with a decision between the two,
personally I think it would be a little more useful to start with
tp->snd_wnd. :-)

Two main reasons:

(1) Usually when we're diagnosing TCP performance problems, we do so
from the sender, since the sender makes most of the
performance-critical decisions (cwnd, pacing, TSO size, TSQ, etc).
>From the sender-side the thing that would be most useful is to see
tp->snd_wnd, the receive window that the receiver has advertised to
the sender.

(2) From the receiver side, "ss" can already show a fair amount of
info about receive-side buffer/window limits, like:
info->tcpi_rcv_ssthresh, info->tcpi_rcv_space,
skmeminfo[SK_MEMINFO_RMEM_ALLOC], skmeminfo[SK_MEMINFO_RCVBUF]. Often
the rwin can be approximated by combining those.

Hopefully Eric, Yuchung, and Soheil can weigh in on the question of
snd_wnd vs rcv_wnd. Or we can perhaps think of another field, and add
the tcpi_rcvi_ooopack, snd_wnd, rcv_wnd, and that final field, all
together.

thanks,
neal