import Data.List
test :: Int -> Int
test n = foldl' (+) 0 [1..n]
main :: IO ()
main = do
print $ test $ 10^8
GHC优化了上述代码,使得垃圾回收器甚至不必做任何工作:
$ ghc -rtsopts -O2 testInt && ./testInt +RTS -s
[1 of 1] Compiling Main ( testInt.hs, testInt.o )
Linking testInt ...
5000000050000000
51,752 bytes allocated in the heap
3,480 bytes copied during GC
44,384 bytes maximum residency (1 sample(s))
17,056 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max pause
Gen 0 0 colls, 0 par 0.000s 0.000s 0.0000s 0.0000s
Gen 1 1 colls, 0 par 0.000s 0.000s 0.0001s 0.0001s
INIT time 0.000s ( 0.000s elapsed)
MUT time 0.101s ( 0.101s elapsed)
GC time 0.000s ( 0.000s elapsed)
EXIT time 0.000s ( 0.000s elapsed)
Total time 0.103s ( 0.102s elapsed)
%GC time 0.1% (0.1% elapsed)
Alloc rate 511,162 bytes per MUT second
Productivity 99.8% of total user, 100.9% of total elapsed
然而,如果我将test
的类型更改为test :: Word -> Word
,则会产生大量垃圾,并且代码运行速度变慢40倍。
ghc -rtsopts -O2 testWord && ./testWord +RTS -s
[1 of 1] Compiling Main ( testWord.hs, testWord.o )
Linking testWord ...
5000000050000000
11,200,051,784 bytes allocated in the heap
1,055,520 bytes copied during GC
44,384 bytes maximum residency (2 sample(s))
21,152 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max pause
Gen 0 21700 colls, 0 par 0.077s 0.073s 0.0000s 0.0000s
Gen 1 2 colls, 0 par 0.000s 0.000s 0.0001s 0.0001s
INIT time 0.000s ( 0.000s elapsed)
MUT time 4.551s ( 4.556s elapsed)
GC time 0.077s ( 0.073s elapsed)
EXIT time 0.000s ( 0.000s elapsed)
Total time 4.630s ( 4.630s elapsed)
%GC time 1.7% (1.6% elapsed)
Alloc rate 2,460,957,186 bytes per MUT second
Productivity 98.3% of total user, 98.3% of total elapsed
为什么会出现这种情况?我本以为它们的性能几乎相同?
(我正在使用x86_64 GNU/Linux上的 GHC 版本 8.0.1)
编辑:我提交了一个错误报告:https://ghc.haskell.org/trac/ghc/ticket/12354#ticket