#7850: Strangely high memory usage on optimized Ackermann function ---------------------------------+------------------------------------------ Reporter: dolio | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime performance bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Comment(by dolio): Here's the simplest test case I can think of to demonstrate the behavior: {{{ -- main = print $ stk 4051 1000000 main = print $ stk 4052 1000000 stk :: Int -> Int -> Int stk _ 0 = 0 stk depth n = loop depth `seq` stk depth (n-1) loop :: Int -> Int loop 0 = 0 loop n = loop (n-1) `seq` 0 }}} With -O0, ~0% memory usage. With -O2, memory usage grows about linearly with time. If the alternate main is used, -O2 also uses ~0% memory. The boundary can be found using +RTS -K32K, which will stack overflow after one chunk is used. Choices of depth that do not overflow under such a setting will run in little memory, and choices that do will use large amounts of memory. The boundary number will probably be higher when running with 4-byte ints, for instance. Some +RTS -s stats look like: 4052 -O2: 32259 MB total memory in use (504 MB lost due to fragmentation) 4052 -O0: 1 MB total memory in use (0 MB lost due to fragmentation) 4051 -O2: 1 MB total memory in use (0 MB lost due to fragmentation) -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7850#comment:22 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler