Subject: Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

From: Willy Tarreau <[email-masked]>
Date: Mon, 26 Jun 2006 00:22:43 +0200

Hi Andi,

On Mon, Jun 19, 2006 at 05:24:31PM +0200, Andi Kleen wrote:
> 
> > If you use "pmtmr" try to reboot with kernel option "clock=tsc".
> 
> That's dangerous advice - when the system choses not to use
> TSC it often has a reason.
> 
> > 
> > On my Opteron AMD system i normally can route 400 kpps, but with 
> > timesource "pmtmr" i could only route around 83 kpps.  (I found the timer 
> > to be the issue by using oprofile).
> 
> Unless you're using packet sniffing or any other application
> that requests time stamps on a socket then the timer shouldn't 
> make much difference. Incoming packets are only time stamped
> when someone asks for the timestamps.

I encountered the same problem on a dual core opteron equipped with a
broadcom NIC (tg3) under 2.4. It could receive 1 Mpps when using TSC
as the clock source, but the time jumped back and forth, so I changed
it to 'notsc', then the performance dropped dramatically to around the
same value as above with one CPU saturated. I suspect that the clock
precision is needed by the tg3 driver to correctly decide to switch to
polling mode, but unfortunately, the performance drop rendered the
solution so much unusable that I finally decided to use it only in
uniprocessor with TSC enabled.

> -Andi

Regards,
Willy