CODEWITHSUNDEEP
clinuxkernel
ASBai
ASBai

Reputation: 762

linux: times or clock_gettime(CLOCK_MONOTONIC) ? which one is faster?

I've tested times() and clock_gettime(CLOCK_MONOTONIC) on several machines, and the results is confused (run each api 10M times with single thread):

Thinkpad P50 Xeon E3-1505Mv5 [Skylake 14nm]:

times(NULL)                   : 450ms
clock_gettime(CLOCK_MONOTONIC): 325ms

You can see on Skylake clock_gettime is faster than times.

Here is the result on a Xeon E5-2430 [Sandy Bridge 32nm]:

times(NULL)                   : 600ms
clock_gettime(CLOCK_MONOTONIC): 1420ms

times(NULL) is faster now.

I also doing the same test on an old Thinkpad W510 I7-720QM [Clarksfield 45nm]:

times(NULL)                   : 1.73s
clock_gettime(CLOCK_MONOTONIC): 20.4s

Seems like there a some new featrues implemented by newer hardware, which boosted the clock_gettime performance?

Upvotes: 1

Views: 792

Answers (1)

Anon
Anon

Reputation: 7154

Don't times(2)) and clock_gettime(2) measure different things? times() measure CPU used running your process whereas clock_gettime() measures wall clock time regardless of whether your process is "on-CPU" or not.

If the clock used by clock_gettime() is supported by the vDSO version then I would have thought the overhead of calling it would typically be comparable to times() but obviously they could return different results depending on whether your process was descheduled at some point.

Upvotes: 1

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