On 21/12/2011 22:36, Roman Cheplyaka wrote:
* Ian Lynagh<ig...@earth.li> [2011-12-21 18:29:21+0000]
* The profiling and hpc implementations have been merged and overhauled.
Visible changes include renaming of profiling flags:
http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/flag-reference.html#id589412
and the cost-centre stacks have a new semantics, which should in most
cases result in more useful and intuitive profiles. The +RTS -xc flag
now also gives a stack trace.
Where can we learn more about the new semantics?
I haven't writtne down the semantics formally yet, I'm afraid, and it
may yet change. However, you should find that the changes give more
intuitive results, and profiling is now more robust to compiler
optimisations.
There are a few visible changes. One is that -auto-all will label
nested functions by default now (but you can revert to the previous
behaviour of labelling only top-level functions with -fprof-auto-top).
Another visible change is that higher-order functions are now pushed on
the stack when they are called, not when they are referenced. For
example, in "map f xs" you will see that map calls f, whereas previously
f would be shown as a child of the function containing the call to map
(sometimes!).
This also means that the costs of a calling a higher-order function are
always part of the aggregate costs of the caller, rather than being
attributed to the higher-order function itself. For example, if you have
f xs ys = {-# SCC "map1" #-} map g xs ++ {-# SCC "map2" #-} map g ys
where g = ...
then you'll see that map1 calls g and map2 calls g, and the costs of
calling g on the elements of xs are recorded separately from the costs
of calling g on the elements of ys. Previously all the costs of g would
be attributed to g itself, which is much less useful.
I'd be interested in hearing feedback, particularly if you find a case
where costs are attributed somewhere that you didn't expect, or the
stack looks wrong.
Cheers,
Simon
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