The many arrays of GHC

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What is this ARR_WORDS?

Sometimes when profiling Haskell code you will find yourself wondering how some large array will manifest itself in the cost-center profiler (namely in the “closure description” heap profile variety, +RTS -hd). Here is a brief summary.

Type What appears in -hd profile? Size
Primitive arrays
GHC.Prim.ArrayArray# ARR_PTRS \(n\) words
GHC.Prim.MutableArrayArray# MUT_ARR_PTRS_* \(n\) words
GHC.Prim.SmallArray# SMALL_ARR_PTRS_* \(n\) words
GHC.Prim.SmallMutableArray# SMALL_MUT_ARR_PTRS_* \(n\) words
GHC.Prim.ByteArray# ARR_WORDS \(n\) bytes
GHC.Prim.MutableByteArray# MUT_ARR_WORDS_* \(n\) bytes
GHC.Prim.Array# ARR_PTRS \(n\) words
GHC.Prim.MutableArray# MUT_ARR_PTRS_* \(n\) words
array package
Data.Array ARR_WORDS
*bytestring package
Data.ByteString.ByteString ARR_WORDS \(n\) bytes
Data.ByteString.Short.ShortByteString ARR_WORDS \(n\) bytes
vector package
Data.Vector.Vector ARR_PTRS \(s * n\) bytes + \(n\) words
Data.Vector.Vector.Mutable.MVector ARR_PTRS \(s * n\) bytes + \(n\) words
Data.Vector.Storable.Vector ARR_WORDS \(s * n\) bytes
Data.Vector.Storable.Vector.Mutable.MVector ARR_WORDS \(s * n\) bytes
Data.Vector.Unboxed.Vector ARR_WORDS \(s * n\) bytes
Data.Vector.Unboxed.Vector.Mutable.MVector ARR_WORDS \(s * n\) bytes
text package
Data.Text.Text ARR_WORDS \(\approx 2*n\) bytes

In the above \(n\) refers to the length of the array, \(s\) refers to the element size in bytes (where appropriate).

Of course, it would be great if the profiler could give us more helpful output. This issue has been tracked as GHC #7275 for several years now. Maybe you could be the one to fix it!