Re: do_gettimeofday vs. rdtsc in the scheduler

[john stultz <johnstul@us.ibm.com>]

On Tue, 2002-09-17 at 16:12, David S. Miller wrote:
>    From: James Cleverdon <jamesclv@us.ibm.com>
>    Date: Tue, 17 Sep 2002 15:55:52 -0700
>    
>    The initial sync was easy, even with variable latencies on cache lines.  A 
>    much simplified NTP-ish algorithm works fine.  The painful thing was bus 
>    clock drift and programs that foolishly relied on the TSC being the same 
>    between CPUs and between nodes.
> 
> This is why the gettimeofday implementation should use the system tick
> thing and also any profiling support in the C library should avoid
> TSC as well.

I think the point James is making is that on very large systems, you
will get system tick skew as well. On one system I know of, the bus
frequency is intensionally skewed slightly between nodes. This is what
causes the TSCs to skew, and I believe would also cause this "system
tick" to skew as well.

Additionally, where is this system tick thing? You make it sound like
its a register in the cpu, and while the Ultra-III may have one, I'm
unaware of a system/bus tick register on intel chips. Is it in some
semi-documented MSR?

I apologize for being confused, I'm just not sure if your criticizing
the code or the hardware. 

thanks
-john