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On a Skylake with 1500MHz base frequency,
the TSC runs at 1512MHz.
This is because the TSC is no longer in the n*100 MHz BCLK domain,
but is now in the m*24MHz crystal clock domain. (24 MHz * 63 = 1512 MHz)
This adds error to several calculations in turbostat,
unless the TSC sample sizes are adjusted for this difference.
Note that calculations in the time domain are immune
from this issue, as the timing sub-system has already
calibrated the TSC against a known wall clock.
AVG_MHz = APERF_delta/measurement_interval
need no adjustment. APERF_delta is in the BCLK domain,
and measurement_interval is in the time domain.
TSC_MHz = TSC_delta/measurement_interval
needs no adjustment -- as we really do want to report
the actual measured TSC delta here, and measurement_interval
is in the accurate time domain.
%Busy = MPERF_delta/TSC_delta
needs adjustment to use TSC_BCLK_DOMAIN_delta.
TSC_BCLK_DOMAIN_delta = TSC_delta * base_hz / tsc_hz
Bzy_MHz = TSC_delta/APERF_delta/MPERF_delta/measurement_interval
need adjustment as above.
No other metrics in turbostat need to be adjusted.
Before:
CPU Avg_MHz %Busy Bzy_MHz TSC_MHz
- 550 24.84 2216 1512
0 2191 98.73 2219 1514
2 0 0.01 2130 1512
1 9 0.43 2016 1512
3 2 0.08 2016 1512
After:
CPU Avg_MHz %Busy Bzy_MHz TSC_MHz
- 550 25.05 2198 1512
0 2190 99.62 2199 1512
2 0 0.01 2152 1512
1 9 0.46 2000 1512
3 2 0.10 2000 1512
Note that in this example, the "Before" Bzy_MHz
was reported as exceeding the 2200 max turbo rate.
Also, even a pinned spin loop would not be reported
as over 99% busy.
Signed-off-by: Len Brown <len.brown@intel.com>
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