Getting access to perf events on GitHub-hosted ubuntu-latest

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#1

Hi!

I am looking to adding benchmarks to my project. As wall-clock time has a high-variance, I would like to use perf-events to measure cpu-instructions executed instead. This should be much more stable.

However, my testing shows that perf-event are not available for the ubuntu-latest runner:

$ sudo sh -c 'echo 0 >/proc/sys/kernel/perf_event_paranoid'
$ perf stat ls || true
Cargo.lock

 Performance counter stats for 'ls':

              1.05 msec task-clock                #    0.618 CPUs utilized          
                 0      context-switches          #    0.000 K/sec                  
                 0      cpu-migrations            #    0.000 K/sec                  
               107      page-faults               #    0.102 M/sec                  
   <not supported>      cycles                                                      
   <not supported>      instructions  # sad octocat                                                
   <not supported>      branches                                                    
   <not supported>      branch-misses                                               

       0.001694797 seconds time elapsed

       0.001523000 seconds user
       0.000000000 seconds sys

(the above repo contains couple of tries to poke at perf-events)

Is there perhaps some magic config somewhere which I can tweak to fix this? Alternatively, is there perhaps a non-perf based way to measure how many CPU cycles my program spends?

#2

Not sure about perf, but one thing you can do to get CPU cycles, or at least simulated CPU cycles, is to use Cachegrind; it runs a simulated CPU and tells you the instruction count. SQLite uses this technique: https://sqlite.org/cpu.html#performance_measurement

I wrote an article about the technique with a lot more detail here: https://pythonspeed.com/articles/consistent-benchmarking-in-ci/

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#3

Oh :wave:! Your post was exactly the thing that caused me to look into this closer, thanks for writing it up :heart: Cachegrind is indeed a cool solution, but it unfortunately is way to slow for my use-case :frowning:

That’s why I am looking at perf: like Cachegrind, it counts instructions, so it is much less noisy than wall-clock time. At the same time, it incurs almost no slow-down.

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