[Haskell-cafe] oddity with profiling

Matthias Fischmann fis at wiwi.hu-berlin.de
Mon Oct 2 17:01:59 EDT 2006


i have been staring at a strange profile for a while now, and was
wondering whether anybody could give me a hint.  (using ghc.6.4.  i
know i should upgrade, but has the profiler changed after that in

almost all time / memory costs occurs in a CAF cost center.  as far as
i understand, this means that the evaluation only takes place once
each time i run the program, and doesn't depend on the input.

   Q1. why should one distinguish time spent in top-level constants
   from time spent in functions?  (my program is basically a top-level
   constant that calls a function i am trying to understand with
   constant toy arguments.)

curiously, the costs inherited by the CAF cost center and caused
elsewhere do not show anywhere else, although i am pretty sure the
call stack doesn't leave my code for another few levels.

   Q2. how can i learn which parts of my code causes the CAF cost
   centers to bloat?  and what does it mean if the 100% inherited
   costs do not show in any other item in the cost center stack in my

user manual yields further confusion:

| The cost of evaluating any expression in your program is attributed to
| a cost-centre stack using the following rules:
|     * If the expression is part of the one-off costs of evaluating the
| enclosing top-level definition, then costs are attributed to the stack
| of lexically enclosing SCC annotations on top of the special CAF
| cost-centre.

how can an expression be part of the one-off cost of anything?  does
the second part mean that the SCC annotations are on top of the
special CAF cost-center?  which is the special CAF cost-center?  what
does being on top of that mean?

sorry, maybe i am just tired...  :-/

|     * Otherwise, costs are attributed to the stack of
| lexically-enclosing SCC annotations, appended to the cost-centre stack
| in effect at the call site of the current top-level
| definition[7]. Notice that this is a recursive definition.
|     * Time spent in foreign code (see Chapter 8, Foreign function
| interface (FFI) ) is always attributed to the cost centre in force at
| the Haskell call-site of the foreign function.

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