On Sun, 29 Jul 2012, Alexander Motin wrote:

With ACPI timer gradually becoming one of slowest in the system, is there some reason to use it directly in acpi_cpu_idle()? I've made a patch:
http://people.freebsd.org/~mav/sleep_time.patch
to use binuptime() instead. Using even HPET from system time counter (not even speaking about TSC) that significantly improves performance on some workloads if this code is not covered by MWAIT optimization in cpu_idle().


Does it work with a perverse timecounter like the i8254 work?  The
user is permitted to switch to any supported timecounter.  There are
other perverse ones:
- ACPI.  This seems to be unavailable if the system thinks ACPI-fast
  works.  Bug.  The user should be able to downgrade to it if ACPI-fast
  in fact doesn't work.  Since it reads the hardware more than once,
  it is much slower than direct use of the hardware.
- ACPI-fast.  Even this is perverse.  It only reads the hardware once,
  but goes through many software layers.

binuptime() is more accurate than uncalibrated scaling.  Is accuracy
required?  If not, the CPU ticker might work, and is faster than HPET,
and and is not under user control for perverse settings.  It normally
reduces to readtsc() with no serializing instruction even in proposed
changes.  This is good enough for process times (not very good) and
depends on the CPU not changing.  Its calibration is very accurate
(similar to timecounters) modulo bugs, but not always up to date.

Bruce
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