Does clock_gettime function use tsc for timing?

rainman1985_2010 · 07-02-2015, 09:19 AM

Hi, everyone.

I need to measure time precisely and intend to use clock_gettime function. It's said that clock_gettime function uses TSC for timing, and TSC increments is constant relative to CPU clock. However, the CPU clock may be scaled back for power saving, which may leads to a relative change in the TSC increments. Does this mean that the time clock_gettime function gives may not be precise?

Thanks

genss · 07-02-2015, 10:03 AM

it uses the actual clock, not tsc

it may be inaccurate due to the time to do the system call and whatnot
CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW are vdso-s and dont do some special conversions so they are more accurate

metaschima · 07-03-2015, 05:10 PM

Quote:

Originally Posted by genss

it uses the actual clock, not tsc

it may be inaccurate due to the time to do the system call and whatnot
CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW are vdso-s and dont do some special conversions so they are more accurate

I agree, clock_gettime with CLOCK_MONOTONIC (or CLOCK_MONOTONIC_RAW) option is the most accurate.