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reduce.jl
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reduce.jl
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"""
reduce(step, xf, reducible; [init, simd, basesize, stoppable]) :: T
Thread-based parallelization of [`foldl`](@ref). The "bottom"
reduction function `step(::T, ::T) :: T` must be associative and
`init` must be its identity element.
Transducers composing `xf` must be stateless (e.g., [`Map`](@ref),
[`Filter`](@ref), [`Cat`](@ref), etc.) except for [`ScanEmit`](@ref).
Note that [`Scan`](@ref) is not supported (although possible in
theory). Early termination requires Julia ≥ 1.3.
Use [`tcollect`](@ref) or [`tcopy`](@ref) to collect results into a
container.
See also: [Parallel processing tutorial](@ref tutorial-parallel),
[`foldl`](@ref), [`dreduce`](@ref).
# Keyword Arguments
- `basesize::Integer = amount(reducible) ÷ nthreads()`: A size of
chunk in `reducible` that is processed by each worker. A smaller
size may be required when:
* computation time for processing each item fluctuates a lot
* computation can be terminated by [`reduced`](@ref) or
transducers using it, such as [`ReduceIf`](@ref)
- `stoppable::Bool`: [This option usually does not have to be set
manually.] Transducers.jl's `reduce` executed in the "stoppable"
mode used for optimizing reduction with [`reduced`](@ref) has a
slight overhead if `reduced` is not used. This mode can be disabled
by passing `stoppable = false`. It is usually automatically
detected and set appropriately. Note that this option is purely for
optimization and does not affect the result value.
- For other keyword arguments, see [`foldl`](@ref).
!!! compat "Transducers.jl 0.4.23"
Keyword option `stoppable` requires at least Transducers.jl 0.4.23.
# Examples
```jldoctest
julia> using Transducers
julia> reduce(+, Map(exp) |> Map(log), 1:3)
6.0
julia> using BangBang: append!!
julia> reduce(append!!, Map(x -> 1:x), 1:2; basesize=1, init=Union{}[])
3-element Array{Int64,1}:
1
1
2
```
"""
Base.reduce
"""
mapreduce(xf, step, reducible; init, simd)
!!! warning
`mapreduce` exists primary for backward compatibility. It is
recommended to use `reduce`.
Like [`reduce`](@ref) but `step` is _not_ automatically wrapped by
[`Completing`](@ref).
"""
Base.mapreduce
struct SizedReducible{T} <: Reducible
reducible::T
basesize::Int
end
foldable(reducible::SizedReducible) = reducible.reducible
"""
Transducers.issmall(reducible, basesize) :: Bool
Check if `reducible` collection is considered small compared to
`basesize` (an integer). Fold functions such as [`reduce`](@ref)
switches to sequential `__foldl__` when `issmall` returns `true`.
Default implementation is `amount(reducible) <= basesize`.
"""
issmall
issmall(reducible, basesize) = amount(reducible) <= basesize
issmall(reducible::SizedReducible) =
issmall(reducible.reducible, max(reducible.basesize, 1))
function _halve(reducible::SizedReducible)
left, right = halve(reducible.reducible)
return (
SizedReducible(left, reducible.basesize),
SizedReducible(right, reducible.basesize),
)
end
struct TaskContext
listening::Vector{Threads.Atomic{Bool}}
cancellables::Vector{Threads.Atomic{Bool}}
end
TaskContext() = TaskContext([], [])
function splitcontext(ctx::TaskContext)
c = Threads.Atomic{Bool}(false)
return (
fg = TaskContext(ctx.listening, vcat(ctx.cancellables, c)),
bg = TaskContext(vcat(ctx.listening, c), ctx.cancellables),
)
end
function should_abort(ctx::TaskContext)
for c in ctx.listening
c[] && return true
end
return false
end
function cancel!(ctx::TaskContext)
for c in ctx.cancellables
c[] = true
end
end
struct DummyTask end
Base.schedule(::DummyTask) = nothing
function transduce_assoc(
xform::Transducer,
step,
init,
coll;
simd::SIMDFlag = Val(false),
basesize::Integer = amount(coll) ÷ Threads.nthreads(),
stoppable::Union{Bool,Nothing} = nothing,
)
rf = _reducingfunction(xform, step; init = init, simd = simd)
if stoppable === nothing
stoppable = _might_return_reduced(rf, init, coll)
end
acc = @return_if_reduced _transduce_assoc_nocomplete(
rf,
init,
coll,
basesize,
stoppable,
)
result = complete(rf, acc)
if unreduced(result) isa DefaultInit
throw(EmptyResultError(rf))
# See how `transduce(rf, init, coll)` is implemented in ./processes.jl
end
return result
end
if VERSION >= v"1.3-alpha"
maybe_collect(coll) = coll
else
maybe_collect(coll::AbstractArray) = coll
maybe_collect(coll) = collect(coll)
end
function _transduce_assoc_nocomplete(rf, init, coll, basesize, stoppable = true)
reducible = SizedReducible(maybe_collect(coll), basesize)
@static if VERSION >= v"1.3-alpha"
return _reduce(TaskContext(), stoppable, DummyTask(), rf, init, reducible)
else
return _reduce_threads_for(rf, init, reducible)
end
end
@noinline _reduce_basecase(rf::F, init::I, reducible) where {F,I} =
restack(foldl_nocomplete(rf, _start_init(rf, init), foldable(reducible)))
# `restack` here is crucial when using heap-allocated accumulator.
# See `ThreadsX.unique` and the MWE extracted from it:
# https://github.com/tkf/Restacker.jl/blob/master/benchmark/bench_unique.jl
function _reduce(
ctx,
stoppable,
next_task,
rf::R,
init::I,
reducible::Reducible,
) where {R,I}
if should_abort(ctx)
# As other tasks may be calling `fetch` on `next_task`, it
# _must_ be scheduled at some point to avoid dead lock:
stoppable && schedule(next_task)
# Maybe use `error=false`? Or pass something and get it via `yieldto`?
return init
end
if issmall(reducible)
stoppable && schedule(next_task)
acc = _reduce_basecase(rf, init, reducible)
if acc isa Reduced
cancel!(ctx)
end
return acc
else
left, right = _halve(reducible)
fg, bg = splitcontext(ctx)
task = nonsticky!(@task _reduce(bg, stoppable, next_task, rf, init, right))
stoppable || schedule(task)
a0 = _reduce(fg, stoppable, task, rf, init, left)
b0 = fetch(task)
a = @return_if_reduced a0
should_abort(ctx) && return a # slight optimization
b0 isa Reduced && return combine_right_reduced(rf, a, b0)
return combine(rf, a, b0)
end
end
combine_right_reduced(rf, a, b0::Reduced) =
reduced(combine(_realbottomrf(rf), a, unreduced(b0)))
function _reduce_threads_for(rf, init, reducible::SizedReducible{<:AbstractArray})
arr = reducible.reducible
basesize = reducible.basesize
nthreads = max(
1,
basesize <= 1 ? length(arr) : length(arr) ÷ basesize
)
if nthreads == 1
return foldl_nocomplete(rf, _start_init(rf, init), arr)
else
w = length(arr) ÷ nthreads
results = Vector{Any}(undef, nthreads)
Threads.@threads for i in 1:nthreads
if i == nthreads
chunk = @view arr[(i - 1) * w + 1:end]
else
chunk = @view arr[(i - 1) * w + 1:i * w]
end
results[i] = foldl_nocomplete(rf, _start_init(rf, init), chunk)
end
# It can be done in `log2(n)` for loops but it's not clear if
# `combine` is compute-intensive enough so that launching
# threads is worth enough. Let's merge the `results`
# sequentially for now.
return combine_all(rf, results)
end
end
function combine_all(rf, results)
step = combine_step(rf)
return transduce(ensurerf(Completing(step)), Init(step), results)
end
combine_step(rf) =
asmonoid() do a0, b0
a = @return_if_reduced a0
b0 isa Reduced && return combine_right_reduced(rf, a, b0)
return combine(rf, a, b0)
end
# The output of `reduce` is correct regardless of the value of
# `stoppable`. Thus, we can use `return_type` here purely for
# optimization.
_might_return_reduced(rf, init, coll) =
Base.typeintersect(
Core.Compiler.return_type(
_reduce_dummy, # simulate the output type of `_reduce`
typeof((rf, init, coll)),
),
Reduced,
) !== Union{}
_reduce_dummy(rf, init, coll) =
__reduce_dummy(rf, init, SizedReducible(maybe_collect(coll), 1))
function __reduce_dummy(rf, init, reducible)
if issmall(reducible)
return _reduce_basecase(rf, init, reducible)
else
left, right = halve(reducible)
a = _reduce_dummy(rf, init, left)
b = _reduce_dummy(rf, init, right)
a isa Reduced && return a
b isa Reduced && return combine_right_reduced(rf, a, b)
return combine(rf, a, b)
end
end
# AbstractArray for disambiguation
Base.mapreduce(xform::Transducer, step, itr::AbstractArray;
init = MissingInit(), kwargs...) =
unreduced(transduce_assoc(xform, step, init, itr; kwargs...))
Base.mapreduce(xform::Transducer, step, itr;
init = MissingInit(), kwargs...) =
unreduced(transduce_assoc(xform, step, init, itr; kwargs...))
Base.reduce(step, xform::Transducer, itr; kwargs...) =
mapreduce(xform, Completing(step), itr; kwargs...)
Base.reduce(step, foldable::Foldable; kwargs...) =
reduce(step, extract_transducer(foldable)...; kwargs...)
"""
tcopy(xf::Transducer, T, reducible; basesize) :: Union{T, Empty{T}}
tcopy(xf::Transducer, reducible::T; basesize) :: Union{T, Empty{T}}
tcopy([T,] itr; basesize) :: Union{T, Empty{T}}
Thread-based parallel version of [`copy`](@ref).
Keyword arguments are passed to [`reduce`](@ref).
See also: [Parallel processing tutorial](@ref tutorial-parallel)
(especially [Example: parallel `collect`](@ref tutorial-parallel-collect)).
!!! compat "Transducers.jl 0.4.5"
New in version 0.4.5.
!!! compat "Transducers.jl 0.4.8"
`tcopy` now accepts iterator comprehensions and eductions.
# Examples
```jldoctest
julia> using Transducers
julia> tcopy(Map(x -> x => x^2), Dict, 2:2)
Dict{Int64,Int64} with 1 entry:
2 => 4
julia> using TypedTables
julia> @assert tcopy(Map(x -> (a=x,)), Table, 1:1) == Table(a=[1])
julia> using StructArrays
julia> @assert tcopy(Map(x -> (a=x,)), StructVector, 1:1) == StructVector(a=[1])
```
`tcopy` works with iterator comprehensions and eductions (unlike
[`copy`](@ref), there is no need for manual conversion with
[`eduction`](@ref)):
```jldoctest; setup = :(using Transducers, StructArrays, DataFrames)
julia> table = StructVector(a = [1, 2, 3], b = [5, 6, 7]);
julia> @assert tcopy(
(A = row.a + 1, B = row.b - 1) for row in table if isodd(row.a)
) == StructVector(A = [2, 4], B = [4, 6])
julia> @assert tcopy(
DataFrame,
(A = row.a + 1, B = row.b - 1) for row in table if isodd(row.a)
) == DataFrame(A = [2, 4], B = [4, 6])
julia> @assert tcopy(eduction(
Filter(row -> isodd(row.a)) |> Map(row -> (A = row.a + 1, B = row.b - 1)),
table,
)) == StructVector(A = [2, 4], B = [4, 6])
```
If you have [`Cat`](@ref) or [`MapCat`](@ref) at the end of the
transducer, consider using [`reduce`](@ref) directly:
```jldoctest
julia> using Transducers
using DataFrames
julia> @assert tcopy(
Map(x -> DataFrame(a = [x])) |> MapCat(eachrow),
DataFrame,
1:2;
basesize = 1,
) == DataFrame(a = [1, 2])
julia> using BangBang: Empty, append!!
julia> @assert reduce(
append!!,
Map(x -> DataFrame(a = [x])),
1:2;
basesize = 1,
# init = Empty(DataFrame),
) == DataFrame(a = [1, 2])
```
Note that above snippet assumes that it is OK to mutate the dataframe
returned by the transducer. Use `init = Empty(DataFrame)` if this is
not the case.
This approach of using `reduce` works with other containers; e.g.,
with `TypedTables.Table`:
```jldoctest; setup = :(using Transducers)
julia> using TypedTables
julia> @assert reduce(
append!!,
Map(x -> Table(a = [x])),
1:2;
basesize = 1,
# init = Empty(Table),
) == Table(a = [1, 2])
```
"""
tcopy(xf, T, reducible; kwargs...) =
reduce(append!!, xf |> Map(SingletonVector), reducible; init = Empty(T), kwargs...)
tcopy(xf, reducible; kwargs...) = tcopy(xf, _materializer(reducible), reducible; kwargs...)
function tcopy(::Type{T}, itr; kwargs...) where {T}
xf, foldable = extract_transducer(itr)
return tcopy(xf, T, foldable; kwargs...)
end
function tcopy(itr; kwargs...)
xf, foldable = extract_transducer(itr)
return tcopy(xf, foldable; kwargs...)
end
tcopy(xf, T::Type{<:AbstractSet}, reducible; kwargs...) =
reduce(union!!, xf |> Map(SingletonVector), reducible; init = Empty(T), kwargs...)
function tcopy(
::typeof(Map(identity)),
T::Type{<:AbstractSet},
array::PartitionableArray;
basesize::Integer = max(1, length(array) ÷ Threads.nthreads()),
kwargs...,
)
@argcheck basesize >= 1
return reduce(
union!!,
Map(identity),
Iterators.partition(array, basesize);
init = Empty(T),
basesize = 1,
kwargs...,
)
end
"""
tcollect(xf::Transducer, reducible; basesize) :: Union{Vector, Empty{Vector}}
tcollect(itr; basesize) :: Union{Vector, Empty{Vector}}
Thread-based parallel version of [`collect`](@ref).
This is just a short-hand notation of `tcopy(xf, Vector, reducible)`.
Use [`tcopy`](@ref) to get a container other than a `Vector`.
See also: [Parallel processing tutorial](@ref tutorial-parallel)
(especially [Example: parallel `collect`](@ref tutorial-parallel-collect)).
!!! compat "Transducers.jl 0.4.5"
New in version 0.4.5.
!!! compat "Transducers.jl 0.4.8"
`tcollect` now accepts iterator comprehensions and eductions.
# Examples
```jldoctest
julia> using Transducers
julia> tcollect(Map(x -> x^2), 1:2)
2-element Array{Int64,1}:
1
4
julia> tcollect(x^2 for x in 1:2)
2-element Array{Int64,1}:
1
4
```
"""
tcollect(xf, reducible; kwargs...) = tcopy(xf, Vector, reducible; kwargs...)
tcollect(itr; kwargs...) = tcollect(extract_transducer(itr)...; kwargs...)