netdev - Re: resend: tcp: performance issue with fastopen connections (mss

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Date:   Fri, 13 Jan 2017 20:07:12 +0300
From:   Alexey Kodanev <alexey.kodanev@...cle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Vasily Isaenko <vasily.isaenko@...cle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: resend: tcp: performance issue with fastopen connections (mss >
 window)

Hi Eric,
On 13.01.2017 18:35, Eric Dumazet wrote:
> On Fri, 2017-01-13 at 18:01 +0300, Alexey Kodanev wrote:
>> Hi,
>>
>> Got the issue when running LTP/netstress test on localhost with mss
>> greater than the send window advertised by client (right after 3WHS).
>> Here is the testscenario that can reproduce this:
> Hi Alexey
>
> So this is a combination of Fastopen + small window + large MSS ?

Yeah, this happens only in the beginning, after first ack from client.
Later window gets
lager than mss and it doesn't happen.

>
> I would rather not force burning tons of coal or other fossil fuel,
> by making each tcp_sendmsg() done by billions of linux devices more
> expensive, only to accommodate for some LTP test doing something not
> sensible ;)
>
> Fact that you removed one condition in the BUG_ON() might hide another
> issue later in the path.
>
> I would suggest to clamp MSS to half the initial window, but I guess
> this is impractical since window in SYN/SYNACK are not scaled.
> Care to send a packetdrill test so that we have a clear picture of what
> is going on ?

Is it capable of making two connections in the single test, one after
another?

Thanks,
Alexey