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

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Message-ID: <1348660432.5093.353.camel@edumazet-glaptop>
Date:	Wed, 26 Sep 2012 13:53:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	brouer@...hat.com, netdev@...r.kernel.org, nanditad@...gle.com
Subject: Re: [PATCH] tcp: sysctl for initial receive window

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.

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