内存使用率异常（内存泄漏？）

Question

我具有以下类型和两个相关功能，作为大型清单的一部分，我打算对其进行测量：

类型和访问功能：

data Aggregate a = Aggregate (Maybe a) (a -> Aggregate a)

get :: Aggregate a -> Maybe a
get (Aggregate get' _) = get'

put :: Aggregate a -> a -> Aggregate a
put (Aggregate _ put') = put'

第一个功能：

updateFirst :: Maybe a -> a -> Aggregate a
updateFirst cur val = Aggregate new (updateFirst new)
  where new = mplus cur (Just val)

first :: Aggregate a
first = Aggregate Nothing (updateFirst Nothing)

第二功能：

updateMinimum :: Ord a => Maybe a -> a -> Aggregate a
updateMinimum cur val = Aggregate new (updateMinimum new)
  where new = min <$> (mplus cur (Just val)) <*> Just val

minimum :: Ord a => Aggregate a
minimum = Aggregate Nothing (updateMinimum Nothing)

函数的编写方式应使内存保持恒定。因此，我选择在GHC中使用Strict语言扩展名，该扩展名要求评估这些代码。 Weigh库用于执行分配测量：

test :: A.Aggregate Double -> Int -> Maybe Double
test agg len = A.get $ F.foldl' A.put agg (take len $ iterate (+0.3) 2.0)

testGroup :: String -> A.Aggregate Double -> Weigh ()
testGroup name agg = sequence_ $ map (\cnt -> func (str cnt) (test agg) cnt) counts
  where
    counts  = map (10^) [0 .. 6]
    str cnt = name ++ (show cnt)

main :: IO ()
main =
  mainWith
    (do setColumns [Case, Allocated, Max, GCs]
        testGroup "fst" A.first
        testGroup "min" A.minimum
    )

Weigh输出如下：

Case          Allocated          Max  GCs
fst1                304           96    0
fst10             2,248           96    0
fst100           21,688           96    0
fst1000         216,088           96    0
fst10000      2,160,088           96    2
fst100000    21,600,088           96   20
fst1000000  216,000,088           96  207
min1                552           96    0
min10             4,728           96    0
min100           46,488           96    0
min1000         464,088           96    0
min10000      4,967,768           96    4
min100000    49,709,656    6,537,712   44
min1000000  497,226,840  103,345,512  445

为什么GHC突然在大小为10 ^ 5和10 ^ 6的输入中分配更多的内存？我的GHC版本是8.4.4。

Answer 1

Haskell中的严格注释可以说是“关系”的。他们说，每当对WHNF进行评估时，都必须对WHNF（weak head normal form）进行评估。