[GHC] #8231: Haskell's Run Time System doesn't save electricity

[GHC]

#8231: Haskell's Run Time System doesn't save electricity
--------------------------+------------------------------------------------
       Reporter:          |             Owner:
  UnixJunkie              |            Status:  new
           Type:  bug     |         Milestone:
       Priority:  normal  |           Version:  7.4.1
      Component:          |  Operating System:  Linux
  Runtime System          |   Type of failure:  None/Unknown
       Keywords:  energy  |         Test Case:  provided in the description
  saving                  |          Blocking:
   Architecture:  x86_64  |
  (amd64)                 |
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
--------------------------+------------------------------------------------
 Someone provided me with a sequential implementation of the fibonacci
 function.
 Out of curiosity, it was compiled with -threaded and we played with the -N
 option of the runtime.

 That same someone noticed some suspicious behavior.
 I notice the same and so report about it: all cores specified by $np
 are fully used while there is no change in the wallclock time
 spent by the program.

 The code:
 ---

 {{{
 import Text.Printf

 fib :: Int -> Int
 fib 0 = 0
 fib 1 = 1
 fib n = fib (n - 1) + fib (n - 2)

 i = 44
 main = printf "n=%d => %d\n" i (fib i)
 }}}

 ---

 The build:
 ---

 {{{
 ghc -threaded -O2 sequential_fib.hs
 }}}

 ---

 The run (on an eight cores machine), shell is Bash:
 ---

 {{{
 for np in `seq 1 8` ; do time ./sequential_fib +RTS -N$np ; done
 }}}

 ---

 The output:
 ---

 {{{
 n=44 => 701408733

 real    0m10.838s
 user    0m10.817s
 sys     0m0.000s
 n=44 => 701408733

 real    0m10.762s
 user    0m12.009s
 sys     0m9.165s
 n=44 => 701408733

 real    0m10.774s
 user    0m24.846s
 sys     0m6.800s
 n=44 => 701408733

 real    0m10.769s
 user    0m33.654s
 sys     0m8.413s
 n=44 => 701408733

 real    0m11.222s
 user    0m47.427s
 sys     0m7.336s
 n=44 => 701408733

 real    0m11.217s
 user    0m57.872s
 sys     0m7.752s
 n=44 => 701408733

 real    0m11.208s
 user    1m9.172s
 sys     0m7.112s
 n=44 => 701408733

 real    0m11.204s
 user    1m18.909s
 sys     0m7.768s
 }}}

 ---

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8231>
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