DiskArrays.jl

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This package is an attempt to collect utilities for working with n-dimensional array-like data structures that do not have considerable overhead for single read operations. Most important examples are arrays that represent data on hard disk that are accessed through a C library or that are compressed in chunks. It can be inadvisable to make these arrays a subtype of AbstractArray many functions working with AbstractArrays assume fast random access into single values (including basic things like getindex, show, reduce, etc...). Currently supported features are:

  • getindex/setindex with the same rules as base (trailing or singleton dimensions etc)
  • views into DiskArrays
  • a fallback Base.show method that does not call getindex repeatedly
  • implementations for mapreduce and mapreducedim, that respect the chunking of the underlying dataset. This greatly increases performance of higher-level reductions like sum(a,dims=d)
  • an iterator over the values of a DiskArray that caches a chunk of data and returns the values within. This allows efficient usage of e.g. using DataStructures; counter(a)
  • customization of broadcast when there is a DiskArray on the LHS. This at least makes things like a.=5 possible and relatively fast

There are basically two ways to use this package. Either one makes the abstraction directly a subtype of AbstractDiskArray which requires to implement a single readblock! method that reads a Cartesian range of data points. The remaining getindex methods will come for free then. The second way is to use the interpret_indices_disk function to get a translation of the user-supplied indices into a set of ranges and then use these to read the data from disk.

Example

Here we define a new array type that wraps a normal AbstractArray. The only access method that we define is a readblock! function where indices are strictly given as unit ranges along every dimension of the array. This is a very common API used in libraries like HDF5, NetCDF and Zarr. We also define a chunking, which will control the way iteration and reductions are computed. In order to understand how exactly data is accessed, we added the additional print statements in the readblock! and writeblock! functions.

using DiskArrays

struct PseudoDiskArray{T,N,A<:AbstractArray{T,N}} <: AbstractDiskArray{T,N}
  parent::A
  chunksize::NTuple{N,Int}
end
PseudoDiskArray(a;chunksize=size(a)) = PseudoDiskArray(a,chunksize)
haschunks(a::PseudoDiskArray) = Chunked()
eachchunk(a::PseudoDiskArray) = GridChunks(a,a.chunksize)
Base.size(a::PseudoDiskArray) = size(a.parent)
function DiskArrays.readblock!(a::PseudoDiskArray,aout,i::AbstractUnitRange...)
  ndims(a) == length(i) || error("Number of indices is not correct")
  all(r->isa(r,AbstractUnitRange),i) || error("Not all indices are unit ranges")
  println("Reading at index ", join(string.(i)," "))
  aout .= a.parent[i...]
end
function DiskArrays.writeblock!(a::PseudoDiskArray,v,i::AbstractUnitRange...)
  ndims(a) == length(i) || error("Number of indices is not correct")
  all(r->isa(r,AbstractUnitRange),i) || error("Not all indices are unit ranges")
  println("Writing to indices ", join(string.(i)," "))
  view(a.parent,i...) .= v
end
a = PseudoDiskArray(rand(4,5,1))

Disk Array with size 10 x 9 x 1

Now all the Base indexing behaviors work for our array, while minimizing the number of reads that have to be done:

a[:,3]

Reading at index Base.OneTo(10) 3:3 1:1

10-element Array{Float64,1}:
 0.8821177068878834
 0.6220977650963209
 0.22676949571723437
 0.3177934541451004
 0.08014908894614026
 0.9989838001681182
 0.5865160181790519
 0.27931778627456216
 0.449108677620097  
 0.22886146620923808

As can be seen from the read message, only a single call to readblock is performed, which will map to a single call into the underlying C library.

mask = falses(4,5,1)
mask[3,2:4,1] .= true
a[mask]

3-element Array{Int64,1}:
 6
 7
 8

One can check in a similar way, that reductions respect the chunks defined by the data type:

sum(a,dims=(1,3))

Reading at index 1:5 1:3 1:1
Reading at index 6:10 1:3 1:1
Reading at index 1:5 4:6 1:1
Reading at index 6:10 4:6 1:1
Reading at index 1:5 7:9 1:1
Reading at index 6:10 7:9 1:1

1×9×1 Array{Float64,3}:
[:, :, 1] =
 6.33221  4.91877  3.98709  4.18658  …  6.01844  5.03799  3.91565  6.06882

When a DiskArray is on the LHS of a broadcasting expression, the results with be written chunk by chunk:

va = view(a,5:10,5:8,1)
va .= 2.0
a[:,:,1]

Writing to indices 5:5 5:6 1:1
Writing to indices 6:10 5:6 1:1
Writing to indices 5:5 7:8 1:1
Writing to indices 6:10 7:8 1:1
Reading at index Base.OneTo(10) Base.OneTo(9) 1:1

10×9 Array{Float64,2}:
 0.929979   0.664717  0.617594  0.720272   …  0.564644  0.430036  0.791838
 0.392748   0.508902  0.941583  0.854843      0.682924  0.323496  0.389914
 0.761131   0.937071  0.805167  0.951293      0.630261  0.290144  0.534721
 0.332388   0.914568  0.497409  0.471007      0.470808  0.726594  0.97107
 0.251657   0.24236   0.866905  0.669599      2.0       2.0       0.427387
 0.388476   0.121011  0.738621  0.304039   …  2.0       2.0       0.687802
 0.991391   0.621701  0.210167  0.129159      2.0       2.0       0.733581
 0.371857   0.549601  0.289447  0.509249      2.0       2.0       0.920333
 0.76309    0.648815  0.632453  0.623295      2.0       2.0       0.387723
 0.0882056  0.842403  0.147516  0.0562536     2.0       2.0       0.107673

Accessing strided Arrays

There are situations where one wants to read every other value along a certain axis or provide arbitrary strides. Some DiskArray backends may want to provide optimized methods to read these strided arrays. In this case a backend can define readblock!(a,aout,r::OrdinalRange...) and the respective writeblock method which will overwrite the fallback behavior that would read the whol block of data and only return the desired range.

Arrays that do not implement eachchunk

There are arrays that live on disk but which are not split into rectangular chunks, so that the haschunks trait returns Unchunked(). In order to still enable broadcasting and reductions for these arrays, a chunk size will be estimated in a way that a certain memory limit per chunk is not exceeded. This memory limit defaults to 100MB and can be modified by changing DiskArrays.default_chunk_size[]. Then a chunk size is computed based on the element size of the array. However, there are cases where the size of the element type is undefined, e.g. for Strings or variable-length vectors. In these cases one can overload the DiskArrays.element_size function for certain container types which returns an approximate element size (in bytes). Otherwise the size of an element will simply be assumed to equal the value stored in DiskArrays.fallback_element_size which defaults to 100 bytes.