HPET vs other timers

[Nate Lawson]

Dag-Erling Smørgrav wrote:
> "Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:
>> Dag-Erling Smørgrav <des at des.no> writes:
>>> "Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:
>>>> I can't rememember who raised the quality of it recently, CVS will
>>>> know.  I was sceptical, because I also have systems where HPET is
>>>> slow.
>>> I did, with your approval, almost a year ago.
>> Yes, I said "try it" or something of the sort.
> 
> For the record, I ran with HPET timers the entire time from HPET support
> was first committed until I finally committed that patch - about ten
> months - so I did test it to the best of my ability.
> 
> DES

Let's keep this technical.  I'm fine with bumping HPET to below ACPI
timer if the hw turns out to be this much slower.

Anyone able to speculate why though?  HPET only reads 32 bits from a
memory mapped region.  No locking or other requirements.  ACPI_timer
does multiple IO ops, which according to bde@ are much slower than
memory reads.  So unless something from the chipset is stopping the
processor (SMI?) when it reads from this region, I have a hard time
seeing why it's slower.

-- 
Nate