netdev - Re: Increasing TCP initial cwnd

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Date:	Tue, 27 Oct 2009 09:38:55 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Yair Gottdenker <yairgott@...il.com>
cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: Increasing TCP initial cwnd

On Mon, 26 Oct 2009, Yair Gottdenker wrote:

> I already had a look in it. Their changes that are related to changing
> the initial cwnd  (snd_cwnd) are in several places in the patch:

Right. I know what I've modified in that patch... ;-)

> 1. tcp_ipv4.c in function tcp_init_cwnd. since currently I am working
> with no metric this is irrelevant.

You managed to mix up this filename twice in a row in a different way :-). 
...That function resides in tcp_input.c instead. And changes in 
tcp_init_metrics are not completely irrelevant, even with no metrics it 
is executed as "no metrics" works a bit differenctly than you seem to 
think.

> 2. tcp_ipv4.c: in function tcp_v4_init_sock, which I applied
> 3. tcp_minisocks.c - which I applied
> . They patched kernel 2.6.18.8 which is very old and there were a lot
> of changes since that in the kernel tcp layer. I didn't test their
> patch but applying the same logic to 2.6.31.3 doesn't seems to change
> initial window size.
> 
> It is clear that in order to correctly make the change to the initial
> cwnd, one should apply changes to many more scenarios like setting the
> initial cwnd after idle time and more. I want to proceed step by step,
> first the naive changes to make it work.

Hmm, it now rings a bell... You cannot directly set initial window to a 
large value (or you can but that doesn't do anything on the wire) because 
you hit first the limit of receiver advertised window that applies 
auto-tuning (with a starting value was four IIRC).

-- 
 i.
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