Messages in this thread | | | Date | Fri, 8 May 2015 09:05:24 -0700 | Subject | Re: [PATCHv3] ftrace: Provide trace clock monotonic raw | From | John Stultz <> |
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On Fri, May 8, 2015 at 7:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote: > On Fri, 8 May 2015 07:30:39 -0700 > Drew Richardson <drew.richardson@arm.com> wrote: > >> Expose the NMI safe accessor to the monotonic raw clock to the >> tracer. The mono clock was added with commit >> 1b3e5c0936046e7e023149ddc8946d21c2ea20eb. The advantage of the >> monotonic raw clock is that it will advance more constantly than the >> monotonic clock. >> >> Imagine someone is trying to optimize a particular program to reduce >> instructions executed for a given workload while minimizing the effect >> on runtime. Also suppose that NTP is running and potentially making >> larger adjustments to the monotonic clock. If NTP is adjusting the >> monotonic clock to advance more rapidly, the program will appear to >> use fewer instructions per second but run longer than if the monotonic >> raw clock had been used. The total number of instructions observed >> would be the same regardless of the clock source used, but how it's >> attributed to time would be affected. >> >> Conversely if NTP is adjusting the monotonic clock to advance more >> slowly, the program will appear to use more instructions per second >> but run more quickly. Of course there are many sources that can cause >> jitter in performance measurements on modern processors, but let's >> remove NTP from the list. >> >> The monotonic raw clock can also be useful for tracing early boot, >> e.g. when debugging issues with NTP. >> > > Peter, Thomas, John, you OK with this?
Yea. No objections from me wrt the functionality of the patch.
I'd just again maybe caution folks to not go too far in assuming CLOCK_MONOTONIC_RAW doesn't have frequency changes, as the underlying hardware will drift due to varying thermal conditions.
thanks -john
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