Re: [PATCH] - sched_clock() broken for ia64 SN platform

[Rich Altmaier <richa@sgi.com>]

Just to add a small note to Jack's comments, the reason for the
existance of this "globally synchronized hi-res" counter is exactly
for scheduling.  On IRIX we use this in the frame-scheduler, to
achieve order(tens of microsecond) simultaneous launch of processes on
multiple CPUs for hard realtime.

The counter is replicated in each memory controller, where the
NUMAflex interconnect provides a broadcast clock signal to drive them.
It's a fairly cool feature, and the realtime people love it...

FYI, Rich


Jack Steiner wrote:

> 
> 
> This is slightly off-topic, but the shared platform counter on the SGI 
> platform isnt a single counter. The counter is replicated in each chipset
> It is synchronized thruout the system so that all cpus will see
> the same value - ie., no drift. Reading the counter does not required any 
> off-node references. There shouldnt be any scaling issues. However,
> reading the ITC is faster & preferred if intercpu drift is not an issue.
> 
>