netdev - Re: [PATCH] tcp_cubic: enable TCP timestamps

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Date:	Tue, 8 Mar 2011 20:36:02 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	Sangtae Ha <sangtae.ha@...il.com>
Cc:	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp_cubic: enable TCP timestamps

On 08/03/11 at 14:15 -0500, Sangtae Ha wrote:
> Yes. I remember that CONFIG_HZ was 1000 at that time and the value of
> CONFIG_HZ could affect the algorithm.
> I don't think HyStart needs this extra RTT_STAMP since we only need a
> rough delay estimate.
> Let me check HyStart with the latest git and with different CONFIG_HZ values.

One of the problems I discovered was that ca->delay_min was completely
off if RTT_STAMP is disabled. For example, on a link with a 11ms RTT, I
could get delay_min = 4ms. But even with RTT_STAMP, there's the problem
that the code rounds the computed RTT value to a jiffie (in
bictcp_acked()).

My other patch mitigates the performance problem by making it harder for
Hystart to abort slow start. But after a few days working on this issue,
I'm wondering whether Hystart shouldn't be completely rewritten to work
on the usec values, or disabled by default.
-- 
| Lucas Nussbaum             MCF Université Nancy 2 |
| lucas.nussbaum@...ia.fr         LORIA / AlGorille |
| http://www.loria.fr/~lnussbau/  +33 3 54 95 86 19 |
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