Add special region support to @tasks (#93)

* critical section

* minor docstring update

* section single; overhaul of forbody escaping

* update docstring

* drop comments

* one_by_one and one_only

* one_only -> only_one

Author: carstenbauer

CHANGELOG.md

@@ -1,6 +1,11 @@
 OhMyThreads.jl Changelog
 =========================
 
+Version 0.5.1
+-------------
+- ![Feature][badge-feature] Within a parallel `@tasks` block one can now mark a region with `@one_by_one`. This region will be run by one task at a time ("critical region").
+- ![Feature][badge-feature] Within a `@tasks` block one can now mark a region as with `@only_one`. This region will be run by a single parallel task only (other tasks will skip over it).
+
 Version 0.5.0
 -------------
 

docs/src/refs/api.md

@@ -11,6 +11,8 @@ CollapsedDocStrings = true
 @tasks
 @set
 @local
+@only_one
+@one_by_one
 ```
 
 ### Functions

src/OhMyThreads.jl

@@ -20,7 +20,7 @@ using .Schedulers: Scheduler, DynamicScheduler, StaticScheduler, GreedyScheduler
                    SerialScheduler
 include("implementation.jl")
 
-export @tasks, @set, @local
+export @tasks, @set, @local, @one_by_one, @only_one
 export treduce, tmapreduce, treducemap, tmap, tmap!, tforeach, tcollect
 export Scheduler, DynamicScheduler, StaticScheduler, GreedyScheduler, SerialScheduler
 

src/macro_impl.jl

@@ -1,3 +1,5 @@
+using OhMyThreads.Tools: OnlyOneRegion, try_enter!
+
 function tasks_macro(forex)
     if forex.head != :for
         throw(ErrorException("Expected a for loop after `@tasks`."))
@@ -17,11 +19,26 @@ function tasks_macro(forex)
 
     settings = Settings()
 
+    # Escape everything in the loop body that is not used in conjuction with one of our
+    # "macros", e.g. @set or @local. Code inside of these macro blocks will be escaped by
+    # the respective "macro" handling functions below.
+    for i in findall(forbody.args) do arg
+        !(arg isa Expr && arg.head == :macrocall && arg.args[1] == Symbol("@set")) &&
+            !(arg isa Expr && arg.head == :macrocall && arg.args[1] == Symbol("@local")) &&
+            !(arg isa Expr && arg.head == :macrocall &&
+              arg.args[1] == Symbol("@only_one")) &&
+            !(arg isa Expr && arg.head == :macrocall &&
+              arg.args[1] == Symbol("@one_by_one"))
+    end
+        forbody.args[i] = esc(forbody.args[i])
+    end
+
     locals_before, locals_names = _maybe_handle_atlocal_block!(forbody.args)
     tls_names = isnothing(locals_before) ? [] : map(x -> x.args[1], locals_before)
     _maybe_handle_atset_block!(settings, forbody.args)
+    setup_onlyone_blocks = _maybe_handle_atonlyone_blocks!(forbody.args)
+    setup_onebyone_blocks = _maybe_handle_atonebyone_blocks!(forbody.args)
 
-    forbody = esc(forbody)
     itrng = esc(itrng)
     itvar = esc(itvar)
 
@@ -39,19 +56,25 @@ function tasks_macro(forex)
     end
     q = if isgiven(settings.reducer)
         quote
+            $setup_onlyone_blocks
+            $setup_onebyone_blocks
             $make_mapping_function
             tmapreduce(mapping_function, $(settings.reducer),
                 $(itrng))
         end
     elseif isgiven(settings.collect)
         maybe_warn_useless_init(settings)
         quote
+            $setup_onlyone_blocks
+            $setup_onebyone_blocks
             $make_mapping_function
             tmap(mapping_function, $(itrng))
         end
     else
         maybe_warn_useless_init(settings)
         quote
+            $setup_onlyone_blocks
+            $setup_onebyone_blocks
             $make_mapping_function
             tforeach(mapping_function, $(itrng))
         end
@@ -68,7 +91,7 @@ function tasks_macro(forex)
     for (k, v) in settings.kwargs
         push!(kwexpr.args, Expr(:kw, k, v))
     end
-    insert!(q.args[4].args, 2, kwexpr)
+    insert!(q.args[8].args, 2, kwexpr)
 
     # wrap everything in a let ... end block
     # and, potentially, define the `TaskLocalValue`s.
@@ -151,16 +174,15 @@ function _atlocal_assign_to_exprs(ex)
         tls_type = esc(left_ex.args[2])
         local_before = :($(tl_storage) = TaskLocalValue{$tls_type}(() -> $(tls_def)))
     else
-        tls_sym  = esc(left_ex)
+        tls_sym = esc(left_ex)
         local_before = :($(tl_storage) = let f = () -> $(tls_def)
-                             TaskLocalValue{Core.Compiler.return_type(f, Tuple{})}(f)
-                         end)
+            TaskLocalValue{Core.Compiler.return_type(f, Tuple{})}(f)
+        end)
     end
     local_name = :($(tls_sym))
     return local_before, local_name
 end
 
-
 function _maybe_handle_atset_block!(settings, args)
     idcs = findall(args) do arg
         arg isa Expr && arg.head == :macrocall && arg.args[1] == Symbol("@set")
@@ -201,3 +223,43 @@ function _handle_atset_single_assign!(settings, ex)
         push!(settings.kwargs, sym => esc(def))
     end
 end
+
+function _maybe_handle_atonlyone_blocks!(args)
+    idcs = findall(args) do arg
+        arg isa Expr && arg.head == :macrocall && arg.args[1] == Symbol("@only_one")
+    end
+    isnothing(idcs) && return # no @only_one blocks
+    setup_onlyone_blocks = quote end
+    for i in idcs
+        body = args[i].args[3]
+        @gensym onlyone
+        init_onlyone_ex = :($(onlyone) = $(OnlyOneRegion()))
+        push!(setup_onlyone_blocks.args, init_onlyone_ex)
+        args[i] = quote
+            Tools.try_enter!($(onlyone)) do
+                $(esc(body))
+            end
+        end
+    end
+    return setup_onlyone_blocks
+end
+
+function _maybe_handle_atonebyone_blocks!(args)
+    idcs = findall(args) do arg
+        arg isa Expr && arg.head == :macrocall && arg.args[1] == Symbol("@one_by_one")
+    end
+    isnothing(idcs) && return # no @one_by_one blocks
+    setup_onebyone_blocks = quote end
+    for i in idcs
+        body = args[i].args[3]
+        @gensym onebyone
+        init_lock_ex = :($(onebyone) = $(Base.ReentrantLock()))
+        push!(setup_onebyone_blocks.args, init_lock_ex)
+        args[i] = quote
+            $(esc(:lock))($(onebyone)) do
+                $(esc(body))
+            end
+        end
+    end
+    return setup_onebyone_blocks
+end

src/macros.jl

@@ -151,3 +151,58 @@ end
         error("The @local macro may only be used inside of a @tasks block.")
     end
 end
+
+"""
+    @only_one begin ... end
+
+This can be used inside a `@tasks for ... end` block to mark a region of code to be
+executed by only one of the parallel tasks (all other tasks skip over this region).
+
+## Example
+
+```julia
+using OhMyThreads: @tasks
+
+@tasks for i in 1:10
+    @set ntasks = 10
+
+    println(i, ": before")
+    @only_one begin
+        println(i, ": only printed by a single task")
+        sleep(1)
+    end
+    println(i, ": after")
+end
+```
+"""
+macro only_one(args...)
+    error("The @only_one macro may only be used inside of a @tasks block.")
+end
+
+"""
+    @one_by_one begin ... end
+
+This can be used inside a `@tasks for ... end` block to mark a region of code to be
+executed by one parallel task at a time (i.e. exclusive access). The order may be arbitrary
+and non-deterministic.
+
+## Example
+
+```julia
+using OhMyThreads: @tasks
+
+@tasks for i in 1:10
+    @set ntasks = 10
+
+    println(i, ": before")
+    @one_by_one begin
+        println(i, ": one task at a time")
+        sleep(0.5)
+    end
+    println(i, ": after")
+end
+```
+"""
+macro one_by_one(args...)
+    error("The @one_by_one macro may only be used inside of a @tasks block.")
+end

src/tools.jl

@@ -24,4 +24,62 @@ Return a `UInt` identifier for the current running [Task](https://docs.julialang
 """
 taskid() = objectid(current_task())
 
+"""
+May be used to mark a region in parallel code to be executed by a single task only
+(all other tasks shall skip over it).
+
+See [`try_enter!`](@ref) and [`reset!`](@ref).
+"""
+mutable struct OnlyOneRegion
+    @atomic latch::Bool
+    OnlyOneRegion() = new(false)
+end
+
+"""
+    try_enter!(f, s::OnlyOneRegion)
+
+When called from multiple parallel tasks (on a shared `s::OnlyOneRegion`) only a single
+task will execute `f`.
+
+## Example
+
+```julia
+using OhMyThreads: @tasks
+using OhMyThreads.Tools: OnlyOneRegion, try_enter!
+
+only_one = OnlyOneRegion()
+
+@tasks for i in 1:10
+    @set ntasks = 10
+
+    println(i, ": before")
+    try_enter!(only_one) do
+        println(i, ": only printed by a single task")
+        sleep(1)
+    end
+    println(i, ": after")
+end
+```
+"""
+function try_enter!(f, s::OnlyOneRegion)
+    latch = @atomic :monotonic s.latch
+    if latch
+        return
+    end
+    (_, success) = @atomicreplace s.latch false=>true
+    if !success
+        return
+    end
+    f()
+    return
+end
+
+"""
+Reset the `OnlyOneRegion` (so that it can be used again).
+"""
+function reset!(s::OnlyOneRegion)
+    @atomicreplace s.latch true=>false
+    nothing
+end
+
 end # Tools