add docs on task-specific buffering using threads

Co-Authored-By: Mason Protter <[email protected]>

Author: IanButterworth

base/threadingconstructs.jl

@@ -246,8 +246,8 @@ 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).
-- The value of [`threadid()`](@ref Threads.threadid) may change even within a single
-  iteration. See [`Task Migration`](@ref man-task-migration)
+- Unless the `:static` schedule is used, the value of [`threadid()`](@ref Threads.threadid)
+  may change even within a single iteration. See [`Task Migration`](@ref man-task-migration).
 
 ## Schedulers
 

doc/src/manual/multi-threading.md

@@ -239,6 +239,68 @@ julia> a
 
 Note that [`Threads.@threads`](@ref) does not have an optional reduction parameter like [`@distributed`](@ref).
 
+### Using `@threads` without data races
+
+Taking the example of a naive sum
+
+```julia-repl
+julia> function sum_single(a)
+           s = 0
+           for i in a
+               s += i
+           end
+           s
+       end
+sum_single (generic function with 1 method)
+
+julia> sum_single(1:1_000_000)
+500000500000
+```
+
+Simply adding `@threads` exposes a data race with multiple threads reading and writing `s` at the same time.
+```julia-repl
+julia> function sum_multi_bad(a)
+           s = 0
+           Threads.@threads for i in a
+               s += i
+           end
+           s
+       end
+sum_multi_bad (generic function with 1 method)
+
+julia> sum_multi_bad(1:1_000_000)
+70140554652
+```
+
+Note that the result is not `500000500000` as it should be, and will most likely change each evaluation.
+
+To fix this, buffers that are specific to the task may be used to segment the sum into chunks that are race-free.
+Here `sum_single` is reused, with its own internal buffer `s`, and vector `a` is split into `nthreads()`
+chunks for parallel work via `nthreads()` `@spawn`-ed tasks.
+
+```julia-repl
+julia> function sum_multi_good(a)
+           chunks = Iterators.partition(a, length(a) ÷ Threads.nthreads())
+           tasks = map(chunks) do chunk
+               Threads.@spawn sum_single(chunk)
+           end
+           chunk_sums = fetch.(tasks)
+           return sum_single(chunk_sums)
+       end
+sum_multi_good (generic function with 1 method)
+
+julia> sum_multi_good(1:1_000_000)
+500000500000
+```
+!!! Note
+    Buffers should not be managed based on `threadid()` i.e. `buffers = zeros(Threads.nthreads())` because concurrent tasks
+    can yield, meaning multiple concurrent tasks may use the same buffer on a given thread, introducing risk of data races.
+    Further, when more than one thread is available tasks may change thread at yield points, which is known as
+    [task migration](@ref man-task-migration).
+
+Another option is the use of atomic operations on variables shared across tasks/threads, which may be more performant
+depending on the characteristics of the operations.
+
 ## Atomic Operations
 
 Julia supports accessing and modifying values *atomically*, that is, in a thread-safe way to avoid
@@ -390,11 +452,13 @@ threads in Julia:
 
 ## [Task Migration](@id man-task-migration)
 
-After a task starts running on a certain thread (e.g. via [`@spawn`](@ref Threads.@spawn) or
-[`@threads`](@ref Threads.@threads)), it may move to a different thread if the task yields.
+After a task starts running on a certain thread it may move to a different thread if the task yields.
+
+Such tasks may have been started with [`@spawn`](@ref Threads.@spawn) or [`@threads`](@ref Threads.@threads),
+although the `:static` schedule option for `@threads` does freeze the threadid.
 
-This means that [`threadid()`](@ref Threads.threadid) should not be treated as constant within a task, and therefore
-should not be used to index into a vector of buffers or stateful objects.
+This means that in most cases [`threadid()`](@ref Threads.threadid) should not be treated as constant within a task,
+and therefore should not be used to index into a vector of buffers or stateful objects.
 
 !!! compat "Julia 1.7"
     Task migration was introduced in Julia 1.7. Before this tasks always remained on the same thread that they were