[Haskell-cafe] Blitting one IOUArray into another

[Don Stewart]

You can of course memcpy unboxed arrays fairly easily.

bulat.ziganshin:
> Hello Denis,
> 
> Wednesday, January 7, 2009, 6:56:36 PM, you wrote:
> 
> memory allocated for i :)))
> 
> each new copy of i needs one word. the situation was much worse with
> Int64, of course :)
> 
> 
> > 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've attached two versions of the code, just for comparison.  The only
> > difference is the way the copying is done.  One calls `getBounds' to figure
> > out the bounds, and one is given the bounds for copying.  They're both about
> > the same speed and allocation (originally I was using IOUArray Int64 Int64,
> > and there was a much greater allocation difference between the two versions;
> > but that went away.  Oh well).
> 
> > So, does anyone know why copying takes so much allocation?  I expect there
> > is _some_ way to just move memory from one array to another, like a memcpy
> > -- how can I do that?
> >                               Denis
> 
> 
> 
> 
> -- 
> Best regards,
>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
> 
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