how much can %alloc in profiling be trusted

[Hal Daume]

In my program, I get the following time/allocation information for a
call to my cosine function:

                             individual      inherited
COST CENTRE   no. entries  %time  %alloc  %time  %alloc
cosine               2721  43.1   74.3    43.1   74.3

this is a shocking amount of allocation, considering the definition of
the function:

cosine :: Vector -> Vector -> Double
cosine v1 v2 =3D=20
  case dot v1 v2 0 of
    n | n <=3D 0    -> 0
      | otherwise -> n / (size v1 * size v2)
  where=20
    dot [] _  n =3D n
    dot _  [] n =3D n
    dot ((x,xv):xs) ((y,yv):ys) n =3D
      case x `compare` y of
        EQ -> dot xs ys $! (xv * yv + n)
        LT -> dot xs ((y,yv):ys) n
        GT -> dot ((x,xv):xs) ys n
--    size =3D sqrt . sum . map (square . snd)
    size l =3D sqrt $! size' l 0
    size' [] n =3D n
    size' ((_,x):xs) n =3D size' xs $! n + x*x

where Vector =3D [(Int,Double)] is a sparse vector representation.  This
was even higher (moderately) until I switched from the old to the new
definition of size listed there.

You can't blame this on the fact that the two vectors are being passed
lazily either: they're being read strictly from a file and even seq'd
before being put into the list.  Specifically, we have (v0 is passed in
to the function from top-level):

  ... do
    v <- readList_H h
    return (cosine v0 v, w)

where readList_H is defined as:

readList_H h =3D do
  b <- FastIO.isEOF h
  c <- FastIO.fscanfChar h
  if c /=3D ' '=20
    then return []=20
    else do
      b <- FastIO.isEOF h
      i <- FastIO.fscanfInt h
      FastIO.fscanfChar h
      v <- FastIO.fscanfFloat h
      rest <- readList_H h
      return ((force (force i,force $ floatToDouble v)) : rest)
  where force x =3D x `seq` x

as far as i can tell, all the list allocation should be happening here.
(the forces were not there in the beginning -- I added them later but it
changed nothing.)

 - Hal

--
 Hal Daume III                                   | hdaume@isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume