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I'm aware of the standard methods of getting time deltas using CPU clock counters on various operating systems. My question is, how do such operating systems account for the change in CPU frequency for power saving purposes. I initially thought this could be explained based on the fact that OS's use specific calls to measure frequency to get the corrected frequency based on which core is being used, what frequency it's currently set to, etc. But then I realized, wouldn't that make any time delta inaccurate if the CPU frequency was lowered and raised back to it's original value in between two clock queries.

For example take the following scenario:

Query the CPU cycles. Operating system lowers CPU frequency for power saving. Some other code is run here. Operating system raises CPU frequency for performance. Query the CPU cycles. Calculate delta as cycle difference divided by frequency.

This would yield an inaccurate delta since the CPU frequency was not constant between the two queries. How is this worked around by the operating system or programs that have to work with time deltas using CPU cycles?

CRABOLO
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Flayneorange
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    I'm not sure but I think that most variable-speed systems use a fixed-frequency oscillator but change the divider to change CPU speed. A separate divider/counter is provided for the time clock. – Hot Licks Dec 08 '13 at 01:10

1 Answers1

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see this wrong clock cycle measurements with rdtsc

there are more ways how to deal with it

  1. set CPU clock to max

    read the link above to see how to do it?

  2. use PIT instead of RDTSC

    PIT is programmable interrupt timer (Intel 8253 if I remember correctly) it is present on all PC motherboards since x286 (and maybe even before) but the resolution is only ~119KHz and not all OS give you access to it.

  3. combine PIT and RDTSC

    just measure the CPU clock by PIT repeatedly when is stable enough start your measurement (and remain scanning for CPU clock change). If CPU clock changes during measurement then throw away the measurement and start again

Community
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Spektre
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