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change default threading scheduler to :static #50019

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change default threading scheduler to :static #50019

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dfca21e
change default threading scheduler to :static
Moelf May 31, 2023
5ff8038
fix whitespace
Moelf May 31, 2023
e98626b
fix test
Moelf Jun 1, 2023
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21 changes: 14 additions & 7 deletions base/threadingconstructs.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -185,11 +185,11 @@ function _threadsfor(iter, lbody, schedule)
end
end
end
if $(schedule === :dynamic || schedule === :default)
if $(schedule === :dynamic)
threading_run(threadsfor_fun, false)
elseif ccall(:jl_in_threaded_region, Cint, ()) != 0 # :static
error("`@threads :static` cannot be used concurrently or nested")
else # :static
elseif $(schedule === :static || schedule === :default) # :static
threading_run(threadsfor_fun, true)
end
nothing
Expand Down Expand Up @@ -234,12 +234,12 @@ For example, the above conditions imply that:
## Schedulers

Without the scheduler argument, the exact scheduling is unspecified and varies across Julia
releases. Currently, `:dynamic` is used when the scheduler is not specified.
releases. Currently, `:static` is used when the scheduler is not specified.

!!! compat "Julia 1.5"
The `schedule` argument is available as of Julia 1.5.

### `:dynamic` (default)
### `:dynamic`

`:dynamic` scheduler executes iterations dynamically to available worker threads. Current
implementation assumes that the workload for each iteration is uniform. However, this
Expand All @@ -256,9 +256,15 @@ smaller than the cost of spawning and synchronizing a task (typically less than
microseconds).

!!! compat "Julia 1.8"
The `:dynamic` option for the `schedule` argument is available and the default as of Julia 1.8.
The `:dynamic` option for the `schedule` argument is available as of Julia 1.8.

### `:static`
!!! warning
If your code rely on indexing buffer with [`Threads.threadid()`](@ref), you should
use `:static` shcedule. For example, the now-discouraged pattern from
[1.3 release blog post](https://julialang.org/blog/2019/07/multithreading/#thread-local_state)
Specifically, the use of `temps[Threads.threadid()]` is incorrect when using dynamic scheduler.

### `:static` (default)

`:static` scheduler creates one task per thread and divides the iterations equally among
them, assigning each task specifically to each thread. In particular, the value of
Expand All @@ -267,10 +273,11 @@ Specifying `:static` is an error if used from inside another `@threads` loop or
thread other than 1.

!!! note
`:static` scheduling exists for supporting transition of code written before Julia 1.3.
`:static` scheduling exists for supporting transition of code written earlier Julia.
In newly written library functions, `:static` scheduling is discouraged because the
functions using this option cannot be called from arbitrary worker threads.


## Example

To illustrate of the different scheduling strategies, consider the following function
Expand Down
4 changes: 2 additions & 2 deletions test/threads_exec.jl
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Expand Up @@ -566,8 +566,8 @@ test_load_and_lookup_18020(10000)
# This may not be efficient/fully supported but should work without crashing.....
function test_nested_loops()
a = zeros(Int, 100, 100)
@threads for i in 1:100
@threads for j in 1:100
@threads :dynamic for i in 1:100
@threads :dynamic for j in 1:100
a[j, i] = i + j
end
end
Expand Down