netdev - Re: Linux ECN Handling

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Date:   Tue, 21 Nov 2017 19:02:49 -0800
From:   Steve Ibanez <sibanez@...nford.edu>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        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

Hi Neal,

I just tried out your fix for enabling TLPs in the CWR state (while
leaving tcp_tso_should_defer() unchanged), but I'm still seeing the
host enter long timeouts. Feel free to let me know if there is
something else you'd like me to try.

Best,
-Steve

On Tue, Nov 21, 2017 at 8:52 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> 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.