Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP_INFO

From: Dave Taht <dave.taht@gmail.com>
To: Neal Cardwell <ncardwell@google.com>
Cc: Thomas Higdon <tph@fb.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Dave Jones <dsj@fb.com>, Eric Dumazet <edumazet@google.com>,
	Yuchung Cheng <ycheng@google.com>,
	Soheil Hassas Yeganeh <soheil@google.com>
Subject: Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP_INFO
Date: Thu, 12 Sep 2019 10:14:33 +0100	[thread overview]
Message-ID: <CAA93jw7q71mpenRMD0dWiVNap1WKD6O4+GCBagcPa5OhHTMErw@mail.gmail.com> (raw)
In-Reply-To: <CADVnQynNiTEAmA-++JL7kMeht+dzfh2b==R_UJnEdnX3W=3k8g@mail.gmail.com>

On Thu, Sep 12, 2019 at 1:59 AM Neal Cardwell <ncardwell@google.com> wrote:
>
> On Wed, Sep 11, 2019 at 6:32 PM Thomas Higdon <tph@fb.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@fb.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.

I am under the impression, that particularly in the mobile space, that
network behavior
is often governed by rcv_wnd. At least, there's been so many papers on
this that I'd
tended to assume so.

Given a desire to do both vars, is there a *third* u32 we could add to
fill in the next hole? :)
ecn marks?

>
> (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

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740