Linux perf events: cpu-clock and task-clock - what is the difference
linux-kernel, perf, scheduler
Solution
1) By default, `perf stat` shows `task-clock`, and does not show `cpu-clock`. Therefore we can tell `task-clock` was expected to be much more useful.
2) `cpu-clock` was simply broken, and has not been fixed for many years. It is best to ignore it.
It was intended that `cpu-clock` of `sleep 1` would show about 1 second. In contrast, `task-clock` would show close to zero. It would have made sense to use `cpu-clock` to read wall clock time. You could then look at the ratio between `cpu-clock` and `task-clock`.
But in the current implementation, `cpu-clock` is equivalent to `task-clock`. It is even possible that "fixing" the existing counter might break some userspace program. If there is such a program, Linux might not be able to "fix" this counter. Linux might need to define a new counter instead.
Exception: starting with v4.7-rc1, when profiling a CPU or CPUs - as opposed to a specific task - e.g. `perf stat -a`. `perf stat -a` shows `cpu-clock` instead of `task-clock`. In this specific case, the two counters were intended to be equivalent. The original intention for `cpu-clock` makes more sense in this case. So for `perf stat -a`, you could just ignore this difference, and interpret it as `task-clock`.
If you write your own code which profiles a CPU or CPUs - as opposed to a specific task - perhaps it would be clearest to follow the implementation of `perf stat -a`. But you might link to this question, to explain what your code is doing :-).
Subject: Re: perf: some questions about perf software events From: Peter Zijlstra
On Sat, 2010-11-27 at 14:28 +0100, Franck Bui-Huu wrote:
Peter Zijlstra writes:
On Wed, 2010-11-24 at 12:35 +0100, Franck Bui-Huu wrote:
[...]
Also I'm currently not seeing any real differences between cpu-clock and task-clock events. They both seem to count the time elapsed when the task is running on a CPU. Am I wrong ?
No, Francis already noticed that, I probably wrecked it when I added the multi-pmu stuff, its on my todo list to look at (Francis also handed me a little patchlet), but I keep getting distracted with other stuff :/
OK.
Does it make sense to adjust the period for both of them ?
Also, when creating a task clock event, passing 'pid=-1' to sys_perf_event_open() doesn't really make sense, does it ?
Same with cpu clock and 'pid=n': whatever value, the event measure the cpu wall time clock.
Perhaps proposing only one clock in the API and internally bind this clock to the cpu or task clock depending on pid or cpu parameters would have been better ?
No, it actually makes sense to count both cpu and task clock on a task (cpu clock basically being wall-time).
On a more superficial level, `perf stat` output for `cpu-clock` can be slightly different from that of `task-clock` in perf earlier than v4.7-rc1. For example, it may print "CPUs utilized" for `task-clock` but not for `cpu-clock`.
Problem
Linux `perf` tools (some time ago named `perf_events`) has several builtin universal software events. Two most basic of them are: `task-clock` and `cpu_clock` (internally called `PERF_COUNT_SW_CPU_CLOCK` and `PERF_COUNT_SW_TASK_CLOCK`). But what is wrong with them is lack of description. ysdx user reports that `man perf_event_open` has short description: ``` PERF_COUNT_SW_CPU_CLOCK This reports the CPU clock, a high-resolution per- CPU timer. PERF_COUNT_SW_TASK_CLOCK This reports a clock count specific to the task that is running. ``` But the description is hard to understand. Can somebody give authoritative answer about how and when the `task-clock` and `cpu-clock` events are accounted? How does they relate to the linux kernel scheduler? When `task-clock` and `cpu-clock` will give different values? Which one should I use?