PEP 418: Add a table with performances of TSC, ACPI PM and HPET clocks

pep-0418.txt

@@ -299,7 +299,7 @@ Hardware clocks
   frequency. Multiple cores having different TSC values. Hibernation of system
   will reset TSC value. The RDTSC instruction can be used to read this counter.
   CPU frequency scaling for power saving.
-* ACPI PMTMR (power management timer): ACPI 24-bit timer with a frequency
+* ACPI PMTMR (Power Management TiMeR): ACPI 24-bit timer with a frequency
   of 3.5 MHz (3,579,545 Hz). HPET can cause around 3 seconds of drift per day.
 * Cyclone: The Cyclone timer uses a 32-bit counter on IBM Extended
   X-Architecture (EXA) chipsets which include computers that use the
@@ -310,6 +310,30 @@ Hardware clocks
 * RTC (Real-time clock). Most RTCs use a crystal oscillator with a frequency of
   32,768 Hz
 
+Reading an hardware clock has a cost. The following table compares the
+performance of different hardware clocks on Linux 3.3 with Intel Core i7-2600
+at 3.40GHz (8 cores).
+
+========================  ======  =======  ======
+Function                  TSC     ACPI PM  HPET
+========================  ======  =======  ======
+time()                      2 ns     2 ns    2 ns
+CLOCK_REALTIME_COARSE      10 ns    10 ns   10 ns
+CLOCK_MONOTONIC_COARSE     12 ns    13 ns   12 ns
+CLOCK_THREAD_CPUTIME_ID   134 ns   135 ns  135 ns
+CLOCK_PROCESS_CPUTIME_ID  127 ns   129 ns  129 ns
+clock()                   146 ns   146 ns  143 ns
+gettimeofday()             23 ns   726 ns  637 ns
+CLOCK_MONOTONIC_RAW        31 ns   716 ns  607 ns
+CLOCK_REALTIME             27 ns   707 ns  629 ns
+CLOCK_MONOTONIC            27 ns   723 ns  635 ns
+========================  ======  =======  ======
+
+Each function was called 10,000,000 times and CLOCK_MONOTONIC was used to get
+the time before and after. The benchmark was run 5 times to keep the minimum
+time.
+
+
 
 NTP adjustment
 ==============