Re: Build time measurements

[Jukka Ruohonen <jruohonen%iki.fi@localhost>]

On Thu, Mar 26, 2020 at 08:44:13PM +0000, Andrew Doran wrote:
> It's a software problem right now.  The ACPI idle loop doesn't currenly
> enter a low power sleep state because there are issues with interrupts to
> solve first.  Nevertheless it's very heavy on I/O port access, takes locks
> and under certain conditions flushes the entire L1/L2/L3 cache (I haven't
> verified that yet but the cache miss rate observed with tprof(8) is very
> high).

None of above is really true.  Well, true, it takes a lock, but I doubt
whether locking explains the numbers you are seeing.

Because C3 states are not currently used, the other things do not happen. 
Flushing the caches (when entering/leaving C3) happens only with old CPUs,
which certainly is not the case with these benchmarks.  The same goes with
I/O access.

To gain more insights, I'd just commment out acpi_cpu::acpicpu_cstate_attach. 
It could be, for instance, that you are getting interrupts because of
thermal issues (a.k.a. T-states), which, in turn, is a sign that you
shouldn't be hitting your CPU so hard.

- Jukka