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

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Date:	Tue, 8 Mar 2011 14:15:00 -0500
From:	Sangtae Ha <sangtae.ha@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, lucas.nussbaum@...ia.fr,
	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp_cubic: enable TCP timestamps

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.

Sangtae

On Tue, Mar 8, 2011 at 1:55 PM, David Miller <davem@...emloft.net> wrote:
>
> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Tue, 8 Mar 2011 10:42:11 -0800
>
> > 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.
>
> This also means that whatever testing was originally done on Hystart
> is dependent upon whatever value CONFIG_HZ had in the test kernel.
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