频繁的GC可防止火花并行运行

Question

我尝试在这里运行第一个例子:http://chimera.labs.oreilly.com/books/1230000000929/ch03.html

代码:https://github.com/simonmar/parconc-examples/blob/master/strat.hs

import Control.Parallel
import Control.Parallel.Strategies (rpar, Strategy, using)
import Text.Printf
import System.Environment

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

main = print pair
 where
  pair =
-- <<pair
   (fib 35, fib 36) `using` parPair
-- >>

-- <<parPair
parPair :: Strategy (a,b)
parPair (a,b) = do
  a' <- rpar a
  b' <- rpar b
  return (a',b')
-- >>

我使用ghc 7.10.2(在OSX上,使用多核机器)使用以下命令构建:

ghc -O2 strat.hs -threaded -rtsopts -eventlog

并运行使用:

./strat +RTS -N2 -l -s

我预计2个fibs计算将并行运行(前面的章节示例按预期工作,因此没有设置问题),而且我根本没有获得任何加速,如下所示:

  % ./strat +RTS -N2 -l -s
(14930352,24157817)
   3,127,178,800 bytes allocated in the heap
       6,323,360 bytes copied during GC
          70,000 bytes maximum residency (2 sample(s))
          31,576 bytes maximum slop
               2 MB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      5963 colls,  5963 par    0.179s   0.074s     0.0000s    0.0001s
  Gen  1         2 colls,     1 par    0.000s   0.000s     0.0001s    0.0001s

  Parallel GC work balance: 2.34% (serial 0%, perfect 100%)

  TASKS: 6 (1 bound, 5 peak workers (5 total), using -N2)

  SPARKS: 2 (0 converted, 0 overflowed, 0 dud, 1 GC'd, 1 fizzled)

  INIT    time    0.000s  (  0.001s elapsed)
  MUT     time    1.809s  (  1.870s elapsed)
  GC      time    0.180s  (  0.074s elapsed)
  EXIT    time    0.000s  (  0.000s elapsed)
  Total   time    1.991s  (  1.945s elapsed)

  Alloc rate    1,728,514,772 bytes per MUT second

  Productivity  91.0% of total user, 93.1% of total elapsed

gc_alloc_block_sync: 238
whitehole_spin: 0
gen[0].sync: 0
gen[1].sync: 0

-N1 获得类似的结果(省略).

正如#haskell-beginners中的其他人所指出的那样,GC集合的#似乎是可疑的,所以我-A16M在运行时尝试添加.结果看起来更符合预期:

  % ./strat +RTS -N2 -l -s -A16M
(14930352,24157817)
   3,127,179,920 bytes allocated in the heap
         260,960 bytes copied during GC
          69,984 bytes maximum residency (2 sample(s))
          28,320 bytes maximum slop
              33 MB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       115 colls,   115 par    0.105s   0.002s     0.0000s    0.0003s
  Gen  1         2 colls,     1 par    0.000s   0.000s     0.0002s    0.0002s

  Parallel GC work balance: 71.25% (serial 0%, perfect 100%)

  TASKS: 6 (1 bound, 5 peak workers (5 total), using -N2)

  SPARKS: 2 (1 converted, 0 overflowed, 0 dud, 0 GC'd, 1 fizzled)

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    1.579s  (  1.087s elapsed)
  GC      time    0.106s  (  0.002s elapsed)
  EXIT    time    0.000s  (  0.000s elapsed)
  Total   time    1.686s  (  1.091s elapsed)

  Alloc rate    1,980,993,138 bytes per MUT second

  Productivity  93.7% of total user, 144.8% of total elapsed

gc_alloc_block_sync: 27
whitehole_spin: 0
gen[0].sync: 0
gen[1].sync: 0

问题是:为什么这是行为？即使频繁使用GC,我仍然可以直观地预计在其他90%的运行时间内,2个火花并行运行.

Answer 1

是的，这实际上是 GHC 8.0.1 及更早版本中的一个错误（我正在努力修复 8.0.2）。问题在于fib 35和fib 36表达式是恒定的，因此 GHC 将它们提升到 CAF 的顶层，而 RTS 错误地认为 CAF 无法访问，因此垃圾收集火花。

您可以通过在命令行上传递参数来使表达式变得非常量来解决这个问题：

main = do
     [a,b] <- map read <$> getArgs
     let pair = (fib a, fib b) `using` parPair
     print pair

然后运行该程序./strat 35 36。