diff options
Diffstat (limited to 'docs/users_guide/profiling.rst')
| -rw-r--r-- | docs/users_guide/profiling.rst | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/docs/users_guide/profiling.rst b/docs/users_guide/profiling.rst index b1f85562be..977d02a895 100644 --- a/docs/users_guide/profiling.rst +++ b/docs/users_guide/profiling.rst @@ -310,6 +310,33 @@ and become CAFs. You will probably need to consult the Core .. index:: single: -fprof-cafs +Profiling and foreign calls +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The one sentence summary is, the profiler includes time spent in unsafe foreign +calls but ignores time taken in safe foreign calls. + +The profiler estimates CPU time and only CPU time for the Haskell portion of the +program. In particular, time "taken" by the program by blocking foreign calls +is not accounted for in time profiles. The runtime has the notion of a virtual +processor which is known as a "capability". Haskell code is run on capabilities, +and the profiler samples the capabilities in order to determine what is being +executed at a certain time. When a safe foreign call is executed it's run outside +the context of a capability and hence the sampling does not account for the time +taken. Whilst the safe call is executed, other +Haskell threads are free to run on the capability, and their cost will be attributed +to the profiler. When the safe call is finished, the blocked, descheduled thread can +be resumed and rescheduled. + +However, the time taken by blocking on unsafe foreign calls is accounted for in the profile. +This can be explained because an unsafe foreign call is executed directly in the +same capability as the Haskell code is being executed. Therefore, an unsafe foreign +call will block the entire capability whilst it is running, and any time that +capability is sampled then the "cost" of the foreign call will be attributed to the +calling cost-centre stack. + + + .. _prof-compiler-options: Compiler options for profiling |