netdev - Re: [PATCH next resend] tcp: use zero-window when free

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Date:	Thu, 13 Feb 2014 16:34:41 +0100
From:	Florian Westphal <fw@...len.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH next resend] tcp: use zero-window when free_space is low

Eric Dumazet <eric.dumazet@...il.com> wrote:
> Do we have one SNMP counter tracking number of time we took the decision
> to send a 0 window ?

No.

> Would you mind waiting we run our packetdrill tests before acknowledging
> this patch, because I suspect this might have some impact ?

Of course not.  I am very happy that you folks have these kinds of tests
and are willing to double-check.  Take all time you need, there is no
need to haste.

Many Thanks Eric.

Do you think it makes sense to add counters for this?

The caveat is that decision to send 0 window doesn't mean we end
up sending one, since we cannot shrink already offered window.

static u16 tcp_select_window(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    u32 cur_win = tcp_receive_window(tp);
    u32 new_win = __tcp_select_window(sk);

    /* Never shrink the offered window */
    if (new_win < cur_win) {

Would you add SNMP counter for '__tcp_select_window() wants 0 window'
or for 'tcp_select_window() does pick 0 window' ?

[ or even different counters for both ? ]

Cheers,
Florian
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