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

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Date:	Tue, 8 Mar 2011 10:42:11 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
Cc:	netdev@...r.kernel.org, Sangtae Ha <sha2@...u.edu>
Subject: Re: [PATCH] tcp_cubic: enable TCP timestamps

On Tue, 8 Mar 2011 09:09:26 +0100
Lucas Nussbaum <lucas.nussbaum@...ia.fr> wrote:

> The Hystart slow start algorithm requires precise RTT delay measurements
> to decide when to leave slow start. However, currently, CUBIC doesn't
> enable TCP timestamps. This can cause Hystart to mis-estimate the RTT,
> and to leave slow start too early, generating bad performance since
> convergence to the optimal cwnd is slower.
> 
> Timestamps are already used by TCP Illinois, LP, Vegas, Veno and Yeah.
> 
> Signed-off-by: Lucas Nussbaum <lucas.nussbaum@...ia.fr>

Just to explain what RTT_STAMP does. It causes the tcp receive code
to compute the rtt using high resolution clocks rather than just
jiffies. It requires access to ktime_get_real which means accessing
clock source. This is cheap for TSC, a little expensive for HPET but
expensive for PIT. I worry that enabling it may hurt regular users
on old desktops.  But without it enabling RTT_STAMP, packets that
get acked in less than a jiffie (1 - 10 ms) will 

Also I should have used ktime_get rather than ktime_get_real
because real time is altered by NTP and other actions.
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