Haskell String types memory overheads

memory_overhead.md

Memory overheads

These calculations are the total words required modulo the payload.

List

data [] a = [] | a : (List a)

• List constructor closure: 3 words
  • header: 1 word
  • Char closure ptr: 1 word
  • List closure ptr: 1 word
• Empty list constructor header: 1 word
• Char closure:
  • header: 1 word
  • paylod

=> 4 words per char + 1 word for empty list

Text

data ByteArray = ByteArray ByteArray#

data Text = Text
    {-# UNPACK #-} !ByteArray
    {-# UNPACK #-} !Int       -- offset
    {-# UNPACK #-} !Int       -- length

• Text constructor closure: 7 words
  • header: 1 word
  • offset Int field: 1 word
  • length Int field: 1 word
  • ByteArray constructor closure: 4 words
    • header: 1 word
    • ByteArray# closure ptr: 1 word
    • ByteArray# primitive: 2 words
      • header: 1 word
      • size: 1 words
      • payload

=> 7 words

Lazy Text

data ByteArray = ByteArray ByteArray#

data Text = Text
    {-# UNPACK #-} !ByteArray
    {-# UNPACK #-} !Int       -- offset
    {-# UNPACK #-} !Int       -- length

data Text = Empty
          | Chunk {-# UNPACK #-} !T.Text Text

• Chunk constructor closure: 8 words
  • header: 1 word
  • strict Text: 7 words
  • Lazy Text closure ptr: 1 word
• Empty Text constructor header: 1 word

=> 9 words per chunk + 1 word for Empty

ShortText

newtype ShortText = ShortText ShortByteString

• ShortText newtype: 4 words
  • ShortByteString newtype: 4 words
    • ByteArray closure: 4 words
      • header: 1 word
      • ByteArray constructor closure: 4 words
        • header: 1 word
        • ByteArray# closure ptr: 1 word
        • ByteArray# primitive: 2 words
          • header: 1 word
          • size: 1 words
          • payload

=> 4 words

ByteString

data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents

data ForeignPtrContents
  = PlainForeignPtr !(IORef Finalizers)
  | FinalPtr
  | MallocPtr (MutableByteArray# RealWorld) !(IORef Finalizers)
  | PlainPtr (MutableByteArray# RealWorld)

-- | ByteString API uses PlainPtr
data ByteString = BS {-# UNPACK #-} !(ForeignPtr Word8) -- payload
                     {-# UNPACK #-} !Int                -- length

• BS constructor closure: 5 words
  • header: 1 word
  • ForeignPtr constructor closure: 4 word
    • header: 1 word
    • Addr# closure ptr: 1 word
      • Addr# primitive: 1 word
    • ForeignPtrContents closure ptr: 1 word
• ForeignPtrContents closure: 4 words
  • header: 1 word
  • PlainPtr field ptr : 1 word
    • MutableByteArray# primitive: 2 words
      • header: 1 word
      • size: 1 words
      • payload
• length Int field: 1 word

=> 10 words

Lazy ByteString

data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents

data ForeignPtrContents
  = PlainForeignPtr !(IORef Finalizers)
  | FinalPtr
  | MallocPtr (MutableByteArray# RealWorld) !(IORef Finalizers)
  | PlainPtr (MutableByteArray# RealWorld)

-- | ByteString API uses PlainPtr
data ByteString = BS {-# UNPACK #-} !(ForeignPtr Word8) -- payload
                     {-# UNPACK #-} !Int                -- length

data ByteString = Empty
                | Chunk  {-# UNPACK #-} !S.StrictByteString ByteString

• Chunk constructor closure: 11 words
  • header: 1 word
  • strict ByteString: 10 words
  • Lazy ByteString closure ptr: 1 word
• Empty ByteString constructor header: 1 word

=> 12 words per chunk + 1 word for Empty

ShortByteString

data ByteArray = ByteArray ByteArray#

newtype ShortByteString = ShortByteString { unShortByteString :: ByteArray }

• ShortByteString newtype: 4 words
  • ByteArray closure: 4 words
    • header: 1 word
    • ByteArray# closure ptr: 1 word
    • ByteArray# primitive: 2 words
      • header: 1 word
      • size: 1 words
      • payload

=> 4 words

Bytes

data ByteArray = ByteArray ByteArray#

data Bytes = Bytes
  { array :: {-# UNPACK #-} !ByteArray
  , offset :: {-# UNPACK #-} !Int
  , length :: {-# UNPACK #-} !Int
  }

• Bytes constructor closure: 6 words
  • header: 1 word
  • offset Int field: 1 word
  • length Int field: 1 word
  • ByteArray constructor closure: 4 words
    • header: 1 word
    • ByteArray# closure ptr: 1 word
    • ByteArray# primitive: 2 words
      • header: 1 word
      • size: 1 words
      • payload

=> 7 words

Chunks

data ByteArray = ByteArray ByteArray#

data Bytes = Bytes
  { array :: {-# UNPACK #-} !ByteArray
  , offset :: {-# UNPACK #-} !Int
  , length :: {-# UNPACK #-} !Int
  }

data Chunks
  = ChunksCons {-# UNPACK #-} !Bytes !Chunks
  | ChunksNil

• Chunks constructor closure: 8 words
  • header: 1 word
  • Bytes: 7 words
  • Chunks closure ptr: 1 word
• ChunksNil constructor header: 1 word

=> 9 words per chunk + 1 word for ChunksNil