Hi all. I wrote a very simple program to try out parallel Haskel and check how it would look like to make use of more than one core in this language. When I tried the program with RTS option -N1, total time shows it took 2.48 seconds to complete and around 65% of that time was taken by GC. Then I tried the same program with RTS options -N2 and total time decreased to 1.15 seconds as I expected a gain here. But what I didn't expect is the GC time to drop to 0%. I guess I'm having trouble to understand the output of the RTS option -s. Can you enlighten me? The source for the testing program:
module Main where
import Data.List (foldl1') import Control.Parallel (par, pseq) import Control.Arrow ((&&&))
f `parApp` (a, b) = a `par` (b `pseq` (f a b)) seqApp = uncurry
main = print result where result = (+) `parApp` minMax list minMax = minlist &&& maxlist minlist = foldl1' min maxlist = foldl1' max list = [1..19999999]
The results on a Windows 7 64bits with an Intel Core 2 Duo, compiled with GHC from Haskell Platform: c:\tmp\hs>par +RTS -s -N1 par +RTS -s -N1 20000000 803,186,152 bytes allocated in the heap 859,916,960 bytes copied during GC 233,465,740 bytes maximum residency (10 sample(s)) 30,065,860 bytes maximum slop 483 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1523 collections, 0 parallel, 0.80s, 0.75s elapsed Generation 1: 10 collections, 0 parallel, 0.83s, 0.99s elapsed Parallel GC work balance: nan (0 / 0, ideal 1) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.90s) 0.00s ( 0.06s) Task 1 (worker) : 0.00s ( 0.90s) 0.00s ( 0.00s) Task 2 (bound) : 0.86s ( 0.90s) 1.62s ( 1.69s) SPARKS: 1 (0 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 0.86s ( 0.90s elapsed) GC time 1.62s ( 1.74s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.48s ( 2.65s elapsed) %GC time 65.4% (65.9% elapsed) Alloc rate 936,110,032 bytes per MUT second Productivity 34.6% of total user, 32.4% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0 c:\tmp\hs>par +RTS -s -N2 par +RTS -s -N2 20000000 1,606,279,644 bytes allocated in the heap 74,924 bytes copied during GC 28,340 bytes maximum residency (1 sample(s)) 29,004 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1566 collections, 1565 parallel, 0.00s, 0.01s elapsed Generation 1: 1 collections, 1 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: 1.78 (15495 / 8703, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.59s) 0.00s ( 0.00s) Task 1 (worker) : 0.58s ( 0.59s) 0.00s ( 0.01s) Task 2 (bound) : 0.58s ( 0.59s) 0.00s ( 0.00s) Task 3 (worker) : 0.00s ( 0.59s) 0.00s ( 0.00s) SPARKS: 1 (1 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 1.15s ( 0.59s elapsed) GC time 0.00s ( 0.01s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.15s ( 0.61s elapsed) %GC time 0.0% (2.4% elapsed) Alloc rate 1,391,432,695 bytes per MUT second Productivity 100.0% of total user, 190.3% of total elapsed gc_alloc_block_sync: 90 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0