netdev - Re: Linux ECN Handling

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Message-ID: <CANn89iJScne8A_kme7sD54bbNXz4S=7GoECb_8q-8FEOyfL1VQ@mail.gmail.com>
Date:   Tue, 21 Nov 2017 08:52:03 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Steve Ibanez <sibanez@...nford.edu>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        Mohammad Alizadeh <alizadeh@...il.mit.edu>,
        Lawrence Brakmo <brakmo@...com>
Subject: Re: Linux ECN Handling

On Tue, Nov 21, 2017 at 8:20 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Tue, Nov 21, 2017 at 10:51 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
>> On Tue, Nov 21, 2017 at 7:01 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
>>>
>>> The original motivation for only allowing TLP in the CA_Open state was
>>> to be conservative and avoid having the TLP impose extra load on the
>>> bottleneck when it may be congested. Plus if there are any SACKed
>>> packets in the SACK scoreboard then there are other existing
>>> mechanisms to do speedy loss recovery.
>> Neal I like your idea of covering more states in TLP. but shouldn't we
>> also fix the tso_deferral_logic to work better w/ PRR in CWR state, b/c
>> it's a general transmission issue.
>
> Yes, I agree it's also worthwhile to see if we can make PRR and TSO
> deferral play well together. Sorry, I should have been more clear
> about that.

Yes, but tso auto defer is an heuristic, and since we do not have a
timer to 'send the partial packet'
after we understand the ACK that we were waiting for does not arrive in time,
we know that the heuristic is not perfect.

Adding a timer (and its overhead) for maybe a fraction of cases might
be overkill.

'Fixing' TSO autodefer has been on our plates for ever, we played some
games that proved to be too expensive.

Although I have not played re-using the new hr timer we added for TCP pacing.