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linux: use CLOCK_MONOTONIC_COARSE if available #975
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On some systems, clock_gettime(CLOCK_MONOTONIC) is only serviced from the vDSO when the __vdso_clock_gettime() wrapper is confident enough that the vDSO timestamp is highly accurate. When in doubt, it falls back to making a traditional SYS_clock_gettime system call with all the overhead that entails. While a commendable approach, it's overkill for our purposes because we don't usually need high precision time. That's why this commit changes the default clock to CLOCK_MONOTONIC_COARSE, provided it supports a granularity of one millisecond or better. This change should eliminate the system call on almost all systems, including virtualized ones, provided the kernel is >= 2.6.32 and your glibc is new enough to find and parse the vDSO.
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fst stands for first or fast?
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Whatever works for you. :-) It stands for 'first' in my head but that's because I'm a closet ML programmer.
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LGTM |
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Landed in 21c37a7 for great justice. |
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clock_gettime(CLOCK_MONOTONIC_COARSE) eliminates the SYS_clock_gettime
system call entirely. It just reads from the vDSO timestamp, which is
the kernel's idea of "now" and is good enough for our purposes.
Suggested reviewer: @indutny