[GHC] #7850: Strangely high memory usage on optimized Ackermann function

[GHC]

#7850: Strangely high memory usage on optimized Ackermann function
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Reporter:  dolio                    |          Owner:                  
    Type:  bug                      |         Status:  new             
Priority:  normal                   |      Component:  Compiler        
 Version:  7.6.2                    |       Keywords:                  
      Os:  Unknown/Multiple         |   Architecture:  Unknown/Multiple
 Failure:  Runtime performance bug  |      Blockedby:                  
Blocking:                           |        Related:                  
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Comment(by dolio):

 The benchmarks game used to have an entry called 'recursive' that ran fib,
 ack and tak.* But it's since disappeared, and isn't one of the entries
 that was copied into nofib.

 ack is the only one that displays the pathological behavior by default. I
 thought tak might, but it didn't. This seems to be related to a shallower
 stack usage. For instance, tak 60000 30 15 runs comfortably (for a very
 long time) with -K32K, so it will never overflow the default chunk size.

 However, one can get tak to eat memory in a similar way by making the
 stack chunking small enough. For (an extreme) instance -kb25 -kc256,
 setting the chunk size to 256 bytes, will cause the same memory exhausting
 behavior.

 This meshes with nomeata's observations above.

 Also for reference, I just built 7.6.3, and whatever fixed this in HEAD
 has not been incorporated into it. It still displays the pathological
 behavior.

 ----

 [*] tak is defined as follows:

 {{{
 tak x y z
   | y < x     = tak (tak (x-1) y z) (tak (y-1) z x) (tak (z-1) x y)
   | otherwise = z
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7850#comment:12>
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