GHC/Memory Footprint

This page is concerned with the memory footprint of Haskell data structures stored in the heap.

The heap is the garbage collected area of memory in which the running program introduces heap nodes.

An in-depth explanation of the GHC internals can be found in the GHC Commentary: The Layout of Heap Objects.

A good introduction on how to compute the size of Haskell datastructures can be found in Johan Tibell's Computing the size of a HashMap.

Memory Footprints of common data types

See also Memory footprints of some common data types which is the origin of the table below.

The following tables assumes fully evaluated data structures (i.e. no thunks)

A "word" is 4 bytes on 32bit archs, and 8 bytes on 64bit archs. Sizes are usually rounded up to word-boundaries.

Constructors with no fields are instantiated only once on the heap. This is expressed in the sizeof()-formulas below with italic numbers which can be ignored for practical considerations.

Basic Types

Data type	sizeof(T)	Notes
()	0 words	Single shared ()
Bool	0 words	Single shared True/False
Char	2 words	Char-sharing pool
Int	2 words
Int8	2 words	Due to alignment
Int16	2 words	Due to alignment
Int32	2 words	Due to alignment
Int64 (on 64-bit arch)	2 words
Int64 (on 32-bit arch)	3 words
Word	2 words
Word8	2 words	Due to alignment
Word16	2 words	Due to alignment
Word32	2 words	Due to alignment
Word64 (on 64-bit arch)	2 words
Word64 (on 32-bit arch)	3 words
Double (on 64-bit arch)	2 words
Double (on 32-bit arch)	3 words
Integer (small)	2 words
Integer (bignum rep.)	3 words + sizeof(bignum-repr)	FIXME

Container Types

Data type	sizeof(T)	Notes
(va,vb)	3 words + sizeof(va) + sizeof(vb)
[v]	(1 + 3N) words + N * sizeof(v)	single shared []
Data.ByteString	9 words + N bytes
Data.Text	6 words + 2N bytes
Data.Map k v	6N words + N * (sizeof(k) + sizeof(v))
Data.Set v	5N words + N * sizeof(v)
Data.IntMap v	(3N + 5(N-1) words) + sizeof(v)
Data.IntSet	(2N + 5(N-1)) words
Data.HashMap k v	4.5N words + N * (sizeof(k) + sizeof(v))
Data.HashSet	4.5N words + N * sizeof(v)
Data.Vector v	(4 + (2+N)) words + N * sizeof(v)	O(1) slicing shares Array#