[Haskell-cafe] Blitting one IOUArray into another
daniel.is.fischer at web.de
Wed Jan 7 11:59:00 EST 2009
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.
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