Re: how to measure scheduler latency on powerpc? realfeel doesn't work due to /dev/rtc issues

mikpe
2003-05-09T08:11:07Z

William Lee Irwin III writes:
> On Thu, 8 May 2003, William Lee Irwin III wrote:
> >>>>> Why would you want to use an interrupt? Just count jiffies in sched.c
>
> On Thu, May 08, 2003 at 11:52:29PM -0400, Chris Friesen wrote:
> > I'm trying to get a feel for the maximum time from an interrupt coming in
> > until the userspace handler gets notified. On intel you can program the
> > hardware to generate interupts through /dev/rtc. The powerpc doesn't seem
> > to support this.
> > Jiffies are not accurate enough, I am expecting max latencies in the 1-2 ms
> > range.
> > Unfortunately no. USB/Firewire/Ethernet on the desktop, ethernet/serial
> > for compactPCI.
> > I want to find an additional programmable interrupt source. It bites that
> > cheap PCs have this, and the powerpc doesn't.
>
> Try the timebase instead.

On all Moto PPCs I've checked, the time-base runs at 1/4 the bus clock
rather than at the core clock like we're used to on x86.

I believe many PPCs allow you to program one of the performance counters
to count core clocks. Be advised though that many Moto PPCs in the 750-7410
range have an errata that prevents using performance counter interrupts,
so you'd have to run the counter in non-interrupting mode.

This isn't available to user-space (yet), so you'd also have to hack
the kernel to use this facility.