Question

一个简单的计划：

import qualified Data.ByteString.Lazy.Char8 as BS

main = do
  wc <- length . BS.words <$> BS.getContents
  print wc

为速度而建：

ghc -fllvm -O2 -threaded -rtsopts Words.hs

更多CPU意味着更慢？

$ time ./Words +RTS -qa -N1 < big.txt
331041862

real  0m25.963s
user  0m21.747s
sys 0m1.528s
$ time ./Words +RTS -qa -N2 < big.txt
331041862

real  0m36.410s
user  0m34.910s
sys 0m6.892s
$ time ./Words +RTS -qa -N4 < big.txt
331041862

real  0m42.150s
user  0m55.393s
sys 0m16.227s

好的方法：

$time wc -w big.txt
 331041862 big.txt

real    0m8.277s
user    0m7.553s
sys 0m0.529s

显然，这是一个单线程活动。不过，我想知道它为什么会这么慢。

另外，你有什么提示，我怎样才能让它与wc竞争？

Answer 1

它是GC。使用+RTS -s执行您的程序，结果告诉所有内容。

-N1

D:\>a +RTS -qa -N1 -s < lorem.txt
15470835
   4,558,095,152 bytes allocated in the heap
       1,746,720 bytes copied during GC
          77,936 bytes maximum residency (118 sample(s))
         131,856 bytes maximum slop
               2 MB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      8519 colls,     0 par    0.016s   0.021s     0.0000s    0.0001s
  Gen  1       118 colls,     0 par    0.000s   0.004s     0.0000s    0.0001s

  TASKS: 3 (1 bound, 2 peak workers (2 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.000s  (  0.001s elapsed)
  MUT     time    0.842s  (  0.855s elapsed)
  GC      time    0.016s  (  0.025s elapsed)
  EXIT    time    0.016s  (  0.000s elapsed)
  Total   time    0.874s  (  0.881s elapsed)

  Alloc rate    5,410,809,512 bytes per MUT second

  Productivity  98.2% of total user, 97.4% of total elapsed

gc_alloc_block_sync: 0
whitehole_spin: 0
gen[0].sync: 0
gen[1].sync: 0

-N 4

D:\>a +RTS -qa -N4 -s < lorem.txt
15470835
   4,558,093,352 bytes allocated in the heap
       1,720,232 bytes copied during GC
          77,936 bytes maximum residency (113 sample(s))
         160,432 bytes maximum slop
               4 MB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      8524 colls,  8524 par    4.742s   1.678s     0.0002s    0.0499s
  Gen  1       113 colls,   112 par    0.031s   0.027s     0.0002s    0.0099s

  Parallel GC work balance: 1.40% (serial 0%, perfect 100%)

  TASKS: 6 (1 bound, 5 peak workers (5 total), using -N4)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.000s  (  0.001s elapsed)
  MUT     time    1.950s  (  1.415s elapsed)
  GC      time    4.774s  (  1.705s elapsed)
  EXIT    time    0.000s  (  0.000s elapsed)
  Total   time    6.724s  (  3.121s elapsed)

  Alloc rate    2,337,468,786 bytes per MUT second

  Productivity  29.0% of total user, 62.5% of total elapsed

gc_alloc_block_sync: 21082
whitehole_spin: 0
gen[0].sync: 0
gen[1].sync: 0

最重要的部分是

                                   Tot time (elapsed)  Avg pause  Max pause
Gen  0      8524 colls,  8524 par    4.742s   1.678s     0.0002s    0.0499s

和

Parallel GC work balance: 1.40% (serial 0%, perfect 100%)

当-threaded开启时，在运行时ghc将尽可能地平衡线程之间的任何工作。你的整个程序是一个顺序的过程所以唯一的工作可以移动到其他线程是GC，而你的程序实际上不能并行GC，所以这些线程等待彼此完成他们的工作，导致大量的时间浪费在同步上。

如果告诉运行时不要在+RTS -qm之间平衡线程，那么有时-N4和-N1一样快。

为什么`-threaded`会让它变慢？

1 个答案: