[Haskell-cafe] Blitting one IOUArray into another

[Daniel Fischer]

Am Mittwoch, 7. Januar 2009 16:56 schrieb Bueno, Denis:
> Hi all,
>
> I'm seeing a lot of unexpected memory allocation with some simple code that
> copies the contents of one vector (IOUArray Int Int64) into another of the
> same type and dimensions.  In particular, profiling reveals that `copyInto'
> is allocating oodles and oodles of memory.
>
> My small test case creates two 50000-element arrays and performs 10000
> copies from one into the other.  Since the elements are Int64s and the
> arrays are unboxed, each array should be
>
>     50000 elements * 8 bytes per element = 400,000 bytes
>
> and so the arrays should only take 800,000 bytes total.  I understand
> there's some overhead for thunks and whatnot, but the profiler reported
> allocation is around 40 billion bytes.  And almost all of that allocation
> is in my copying function, not in main (main allocates the arrays).

I think you've run into a profiling/optimising incopatibility.
Compiling the code just with -O2 --make and running with -sstderr, both report
    899,944 bytes allocated in the heap
      1,272 bytes copied during GC (scavenged)
          0 bytes copied during GC (not scavenged)
     16,384 bytes maximum residency (1 sample(s))

          2 collections in generation 0 (  0.00s)
          1 collections in generation 1 (  0.00s)

          2 Mb total memory in use

which looks reasonable, but it is dog slow on my box, 26.7/26.9 seconds :(
Compiled with -prof -auto-all -O2 --make, both allocate madly (~40G) and take 
nearly ten times as long.

It is known that profiling and optimising don't mix too well, but this is 
remarkable, maybe it's worth an investigation.

Cheers,
Daniel