netdev - Re: [PATCH] tcp: sysctl for initial receive window

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Date:	Mon, 1 Oct 2012 15:36:06 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, brouer@...hat.com,
	netdev@...r.kernel.org, nanditad@...gle.com
Subject: Re: [PATCH] tcp: sysctl for initial receive window

On Wed, Sep 26, 2012 at 4:53 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2012-09-21 at 14:48 -0400, David Miller wrote:
>> From: Jesper Dangaard Brouer <brouer@...hat.com>
>> Date: Fri, 21 Sep 2012 20:32:06 +0200
>> > The would defeat the purpose of the patch.  Perhaps we could, allow a
>> > sensible max... (but this max is already being controlled as described).
>>
>> Any new max which is truly sensible, could be the new default, and we
>> would apply the same amount of vetting for such a thing.
>
>
> We have in linux a very conservative and complex rwin control at the
> beginning of a TCP session, only for the very first packets,
> if applications are reasonably fast at draining their receive queue.
> (They mostly are)
>
> Last time I had to take a look (after truesize changes), I was kind of
> worried to not find a good reason why we were doing this.
>
> We now have :
>
> - rcvbuf autotuning, letting rwin growing up to 3MB or so
> - Better truesize tracking
> - global/cgroup tcp mem accounting/pressure
> - TCP coalescing to minimize the effect of bad citizen packets
>     (very low len/truesize ratio)
> - People tracking TCP stack inefficiencies and working on new CCs...
>    (An example is Joe Touch I-D
> http://tools.ietf.org/html/draft-touch-tcpm-automatic-iw-03 that
> proposes increasing IW over a longer period of time (as opposed to
> revisiting constants every few years).
> - ...
>
> TCP congestion control is controlled by the sender, driven by the ACK
> coming back from receiver, and initial rwin should not change CC at all,
> unless we deliberately constrain rwin to a too small value.
>
> We did the 3 -> 10 change only two years ago.
> And 3 was really too small even 5 years ago.
>
> Browsers had to open simultaneous sessions to the same server only to
> workaround this limit, and they still do.
>
> I would just remove the 10 'hard constant', (but not so hard, since it
> was 3 only 2 years ago), and let tcp_rmem[1]/SO_RCVBUF decide of the
> initial receive window.
I like this idea a lot. Got a patch for us to try?

>
>
>
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