wiki:Debugging/LowLevelProfiling/oprofile

oprofile

OProfile does a system-wide profile using performance counters. It can also generate annotated source/assembly just like VTune. Unfortunately, because GHC can't export source-level debugging symbols, OProfile cannot do source-level annotation of GHC programs.

It can be installed from a package on Ubuntu, which is cool.

There seems to be a problem with the counters just stopping arbitrarily, requiring a restart of the daemon.

Recipe for profiling something:

$ sudo opcontrol --init
$ sudo opcontrol --reset
$ sudo opcontrol --setup --event=<event>
$ sudo opcontrol --start
$ ... run program...
$ sudo opcontrol --stop
$ sudo opreport

check that the program you ran is at the top of the profile somewhere. Sometimes for me it doesn't appear, and have to run it again. To see a list of possible <event>s, use $ opcontrol --list-events.

It may be necessary to use opcontrol --dump; opcontrol --shutdown prior to sudo opreport above. We are not sure.

Then to annotate the source:

$ sudo opannotate --source <program> >&! log

lists the contents of source files in order of importance. Alternatively use --output-dir to put the source files in a directory, but then you don't get to find out which are the important ones.

Things to be aware of

  • You might think that you could use the -fvia-C -optc-g options to force GHC to export C-source-level debugging symbols. The Evil Mangler script that post-processes the .s file output by gcc does not understand the debug annotations added by gcc's -g flag, and so -optc-g will not work. You can, however, get GHC to output assembly-level debugging symbols by using -opta-g. You'll probably also want to use the -keep-s-files option so that opannotate can get at the relevant .s files.
  • Suppose that you do
    $ sudo opcontrol --setup --event=<eventname>:<count>
    
    and receive an error message that "Count <count> for event <eventname> is below the minimum." If you check the minimum value for <eventname> using the --list-events option and see that <count> is in fact not below the purported minimum count, then you may have passed in a non-zero value for --callgraph option. When doing callgraph profiling, OProfile requires that the sample count be at least 15 times the minimum.

    opcontrol --status will show you if callgraph is currently set. To reset it, make sure the oprofile daemon is stopped and reset the callgraph value:
    $ sudo opcontrol --shutdown
    $ sudo opcontrol --callgraph=0
    
    Now you can re-run your profiling and the original <count> should work.
Last modified 3 years ago Last modified on May 9, 2011 9:28:08 PM