Note: further work is being done on the project. Ideas/progress/etc is on EventLog/LiveMonitoring


EventLog is a fast, extensible event logging framework in the GHC run-time system (RTS) to support profiling of GHC run-time events. The GHC User's Guide describes how to enable event logging, after the program is linked with eventlog.

An example:

$ ghc -O -eventlog -rtsopts A.hs --make
$ ./A +RTS -l

.. produces A.eventlog ...

$ ghc-events show A.eventlog 
Event Types:
   0: Create thread (size 4)
   1: Run thread (size 4)
   2: Stop thread (size 6)
   3: Thread runnable (size 4)
   4: Migrate thread (size 6)

Log events in binary format to the file program.eventlog, where flags is a sequence of zero or more characters indicating which kinds of events to log. Currently there is only one type supported: -ls, for scheduler events.

The format of the log file is described by the header EventLogFormat.h that comes with GHC, and it can be parsed in Haskell using the ghc-events library. To dump the contents of a .eventlog file as text, use the tool ghc-events that comes with the ghc-events package.


Event logging can produce "observer effects". In particular, programs can pause while writing to the .eventlog file. These display in Threadscope as periods of 100% CPU use with no obvious cause. This effect is exacerbated by heavy use of debug tools which expand the eventlog file, such as Trace.traceMarkerIO. (This effect was at least seen on OS X. In an extreme case, a program doing nothing but writing 200,000 trace markers, which took 120 ms to run, showed a single 10ms pause in the middle to write them to the .eventlog file. These periods were shown to be due to writing the .eventlog file by using dtruss -a, which shows long periods of write_nocancel syscalls, writing to the .eventlog file opened earlier with open_nocancel.)


  • EventLog framework is located in ghc_root/rts/eventlog/
  • A ghc-events library to parse EventLog files for any visualizer, which relies upon the Data.Binary library.
  • Visualizer: ThreadScope

Code repository



  • Satnam Singh
  • Simon Marlow
  • Donnie Jones <donnie@…>
Last modified 2 years ago Last modified on Feb 6, 2018 5:19:27 PM