Make @threads :dynamic default (#44136)

Co-authored-by: Takafumi Arakaki <[email protected]>

NEWS.md

@@ -73,9 +73,9 @@ Command-line option changes
 Multi-threading changes
 -----------------------
 
-* A new `:dynamic` schedule option for `Threads.@threads` which is similar to the default behavior except iterations
- will be scheduled dynamically to available worker threads rather than pinned to each thread. This option is more
- composable with (possibly nested) `@spawn` and `@threads` loops ([#43919])
+* `Threads.@threads` now defaults to a new `:dynamic` schedule option which is similar to the previous behavior except
+ that iterations will be scheduled dynamically to available worker threads rather than pinned to each thread. This
+ behavior is more composable with (possibly nested) `@spawn` and `@threads` loops ([#43919], [#44136])
 
 Build system changes
 --------------------

base/threadingconstructs.jl

@@ -85,16 +85,11 @@ function _threadsfor(iter, lbody, schedule)
  end
  end
  end
- if $(schedule === :dynamic)
+ if $(schedule === :dynamic || schedule === :default)
  threading_run(threadsfor_fun, false)
- elseif ccall(:jl_in_threaded_region, Cint, ()) != 0
- $(if schedule === :static
- :(error("`@threads :static` cannot be used concurrently or nested"))
- else
- # only use threads when called from outside @threads
- :(threadsfor_fun(onethread = true))
- end)
- else
+ elseif ccall(:jl_in_threaded_region, Cint, ()) != 0 # :static
+ error("`@threads :static` cannot be used concurrently or nested")
+ else # :static
  threading_run(threadsfor_fun, true)
  end
  nothing
@@ -126,21 +121,21 @@ For example, the above conditions imply that:
 - Communicating between iterations using blocking primitives like `Channel`s is incorrect.
 - Write only to locations not shared across iterations (unless a lock or atomic operation is used).
 
-
 Schedule options are:
 - `:static` creates one task per thread and divides the iterations equally among
  them, assigning each task specifically to each thread.
  Specifying `:static` is an error if used from inside another `@threads` loop
  or from a thread other than 1.
-- `:dynamic` will schedule iterations dynamically to available worker threads,
+- `:dynamic` (default) will schedule iterations dynamically to available worker threads,
  assuming that the workload for each iteration is uniform.
 
-Without the scheduler argument, the exact scheduling is unspecified; i.e. it may be
-different across Julia releases. Currently, the behavior is dependent on the calling thread.
-The default is `:static` when called from thread 1. The loop will be executed without threading
-when called from other threads.
+Without the scheduler argument, the exact scheduling is unspecified and varies across Julia releases.
 
-The default schedule (used when no `schedule` argument is present) is subject to change.
+!!! compat "Julia 1.5"
+ The `schedule` argument is available as of Julia 1.5.
+
+!!! compat "Julia 1.8"
+ The `:dynamic` option for the `schedule` argument is available and the default as of Julia 1.8.
 
 For example, an illustration of the different scheduling strategies where `busywait`
 is a non-yielding timed loop that runs for a number of seconds.
@@ -172,12 +167,6 @@ julia> @time begin
 The `:dynamic` example takes 2 seconds since one of the non-occupied threads is able
 to run two of the 1-second iterations to complete the for loop.
 
-!!! compat "Julia 1.5"
- The `schedule` argument is available as of Julia 1.5.
-
-!!! compat "Julia 1.8"
- The `:dynamic` option for the `schedule` argument is available as of Julia 1.8.
-
 See also: [`@spawn`](@ref Threads.@spawn), [`nthreads()`](@ref Threads.nthreads),
 [`threadid()`](@ref Threads.threadid), `pmap` in [`Distributed`](@ref man-distributed),
 `BLAS.set_num_threads` in [`LinearAlgebra`](@ref man-linalg).

test/threads_exec.jl

@@ -503,7 +503,7 @@ function test_thread_cfunction()
  @test cfs[2] == cf(fs[2])
  @test length(unique(cfs)) == 1000
  ok = zeros(Int, nthreads())
- @threads for i in 1:10000
+ @threads :static for i in 1:10000
  i = mod1(i, 1000)
  fi = fs[i]
  cfi = cf(fi)
@@ -705,9 +705,9 @@ let ch = Channel{Char}(0), t
  @test String(collect(ch)) == "hello"
 end
 
-# errors inside @threads
+# errors inside @threads :static
 function _atthreads_with_error(a, err)
- Threads.@threads for i in eachindex(a)
+ Threads.@threads :static for i in eachindex(a)
  if err
  error("failed")
  end