Re: [PATCH] tcp_cubic: enable TCP timestamps

[David Miller <davem@davemloft.net>]

From: Stephen Hemminger <shemminger@vyatta.com>
Date: Tue, 8 Mar 2011 10:42:11 -0800

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