multithreading - 为什么 `-threaded` 让它变慢？

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

为速度而构建:

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

为了更好的衡量:

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

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

显然，这是一个单线程事件。不过，我想知道为什么它会这么慢。

另外，你有什么建议，我如何使它与 wc 竞争？

Answer 1

这是 GC。用 +RTS -s 执行你的程序，结果说明一切。

-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

-N4

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 开关打开时，ghc 会在运行时尽可能地平衡线程之间的任何工作。您的整个程序是一个顺序进程，因此唯一可以将工作转移到其他线程的是 GC，而您的程序实际上不能并行 GC，因此这些线程相互等待完成它们的工作，导致大量时间浪费在同步上.

如果您通过 +RTS -qm 告诉运行时不要在线程之间进行平衡，那么有时 -N4 与 -N1 一样快。