Messages in this thread | | | Date | Tue, 12 May 2015 16:26:27 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCHv3] ftrace: Provide trace clock monotonic raw |
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On Fri, 8 May 2015, Steven Rostedt 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?
Yup.
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