Tasks
Core.Task
— TypeTask(func)
Create a Task
(i.e. coroutine) to execute the given function func
(which must be callable with no arguments). The task exits when this function returns.
Examples
julia> a() = sum(i for i in 1:1000);
julia> b = Task(a);
In this example, b
is a runnable Task
that hasn't started yet.
Base.@task
— Macro@task
Wrap an expression in a Task
without executing it, and return the Task
. This only creates a task, and does not run it.
Examples
julia> a1() = sum(i for i in 1:1000);
julia> b = @task a1();
julia> istaskstarted(b)
false
julia> schedule(b);
julia> yield();
julia> istaskdone(b)
true
Base.@async
— Macro@async
Wrap an expression in a Task
and add it to the local machine's scheduler queue.
Values can be interpolated into @async
via $
, which copies the value directly into the constructed underlying closure. This allows you to insert the value of a variable, isolating the aysnchronous code from changes to the variable's value in the current task.
Interpolating values via $
is available as of Julia 1.4.
Base.asyncmap
— Functionasyncmap(f, c...; ntasks=0, batch_size=nothing)
Uses multiple concurrent tasks to map f
over a collection (or multiple equal length collections). For multiple collection arguments, f
is applied elementwise.
ntasks
specifies the number of tasks to run concurrently. Depending on the length of the collections, if ntasks
is unspecified, up to 100 tasks will be used for concurrent mapping.
ntasks
can also be specified as a zero-arg function. In this case, the number of tasks to run in parallel is checked before processing every element and a new task started if the value of ntasks_func
is less than the current number of tasks.
If batch_size
is specified, the collection is processed in batch mode. f
must then be a function that must accept a Vector
of argument tuples and must return a vector of results. The input vector will have a length of batch_size
or less.
The following examples highlight execution in different tasks by returning the objectid
of the tasks in which the mapping function is executed.
First, with ntasks
undefined, each element is processed in a different task.
julia> tskoid() = objectid(current_task());
julia> asyncmap(x->tskoid(), 1:5)
5-element Array{UInt64,1}:
0x6e15e66c75c75853
0x440f8819a1baa682
0x9fb3eeadd0c83985
0xebd3e35fe90d4050
0x29efc93edce2b961
julia> length(unique(asyncmap(x->tskoid(), 1:5)))
5
With ntasks=2
all elements are processed in 2 tasks.
julia> asyncmap(x->tskoid(), 1:5; ntasks=2)
5-element Array{UInt64,1}:
0x027ab1680df7ae94
0xa23d2f80cd7cf157
0x027ab1680df7ae94
0xa23d2f80cd7cf157
0x027ab1680df7ae94
julia> length(unique(asyncmap(x->tskoid(), 1:5; ntasks=2)))
2
With batch_size
defined, the mapping function needs to be changed to accept an array of argument tuples and return an array of results. map
is used in the modified mapping function to achieve this.
julia> batch_func(input) = map(x->string("args_tuple: ", x, ", element_val: ", x[1], ", task: ", tskoid()), input)
batch_func (generic function with 1 method)
julia> asyncmap(batch_func, 1:5; ntasks=2, batch_size=2)
5-element Array{String,1}:
"args_tuple: (1,), element_val: 1, task: 9118321258196414413"
"args_tuple: (2,), element_val: 2, task: 4904288162898683522"
"args_tuple: (3,), element_val: 3, task: 9118321258196414413"
"args_tuple: (4,), element_val: 4, task: 4904288162898683522"
"args_tuple: (5,), element_val: 5, task: 9118321258196414413"
Currently, all tasks in Julia are executed in a single OS thread co-operatively. Consequently, asyncmap
is beneficial only when the mapping function involves any I/O - disk, network, remote worker invocation, etc.
Base.asyncmap!
— Functionasyncmap!(f, results, c...; ntasks=0, batch_size=nothing)
Like asyncmap
, but stores output in results
rather than returning a collection.
Base.current_task
— Functioncurrent_task()
Get the currently running Task
.
Base.istaskdone
— Functionistaskdone(t::Task) -> Bool
Determine whether a task has exited.
Examples
julia> a2() = sum(i for i in 1:1000);
julia> b = Task(a2);
julia> istaskdone(b)
false
julia> schedule(b);
julia> yield();
julia> istaskdone(b)
true
Base.istaskstarted
— Functionistaskstarted(t::Task) -> Bool
Determine whether a task has started executing.
Examples
julia> a3() = sum(i for i in 1:1000);
julia> b = Task(a3);
julia> istaskstarted(b)
false
Base.istaskfailed
— Functionistaskfailed(t::Task) -> Bool
Determine whether a task has exited because an exception was thrown.
Examples
julia> a4() = error("task failed");
julia> b = Task(a4);
julia> istaskfailed(b)
false
julia> schedule(b);
julia> yield();
julia> istaskfailed(b)
true
Base.task_local_storage
— Methodtask_local_storage(key)
Look up the value of a key in the current task's task-local storage.
Base.task_local_storage
— Methodtask_local_storage(key, value)
Assign a value to a key in the current task's task-local storage.
Base.task_local_storage
— Methodtask_local_storage(body, key, value)
Call the function body
with a modified task-local storage, in which value
is assigned to key
; the previous value of key
, or lack thereof, is restored afterwards. Useful for emulating dynamic scoping.
Scheduling
Base.yield
— Functionyield()
Switch to the scheduler to allow another scheduled task to run. A task that calls this function is still runnable, and will be restarted immediately if there are no other runnable tasks.
yield(t::Task, arg = nothing)
A fast, unfair-scheduling version of schedule(t, arg); yield()
which immediately yields to t
before calling the scheduler.
Base.yieldto
— Functionyieldto(t::Task, arg = nothing)
Switch to the given task. The first time a task is switched to, the task's function is called with no arguments. On subsequent switches, arg
is returned from the task's last call to yieldto
. This is a low-level call that only switches tasks, not considering states or scheduling in any way. Its use is discouraged.
Base.sleep
— Functionsleep(seconds)
Block the current task for a specified number of seconds. The minimum sleep time is 1 millisecond or input of 0.001
.
Base.schedule
— Functionschedule(t::Task, [val]; error=false)
Add a Task
to the scheduler's queue. This causes the task to run constantly when the system is otherwise idle, unless the task performs a blocking operation such as wait
.
If a second argument val
is provided, it will be passed to the task (via the return value of yieldto
) when it runs again. If error
is true
, the value is raised as an exception in the woken task.
Examples
julia> a5() = sum(i for i in 1:1000);
julia> b = Task(a5);
julia> istaskstarted(b)
false
julia> schedule(b);
julia> yield();
julia> istaskstarted(b)
true
julia> istaskdone(b)
true
Synchronization
Base.@sync
— Macro@sync
Wait until all lexically-enclosed uses of @async
, @spawn
, @spawnat
and @distributed
are complete. All exceptions thrown by enclosed async operations are collected and thrown as a CompositeException
.
Base.wait
— FunctionSpecial note for Threads.Condition
:
The caller must be holding the lock
that owns c
before calling this method. The calling task will be blocked until some other task wakes it, usually by calling notify
on the same Condition object. The lock will be atomically released when blocking (even if it was locked recursively), and will be reacquired before returning.
wait(r::Future)
Wait for a value to become available for the specified Future
.
wait(r::RemoteChannel, args...)
Wait for a value to become available on the specified RemoteChannel
.
wait([x])
Block the current task until some event occurs, depending on the type of the argument:
Channel
: Wait for a value to be appended to the channel.Condition
: Wait fornotify
on a condition.Process
: Wait for a process or process chain to exit. Theexitcode
field of a process can be used to determine success or failure.Task
: Wait for aTask
to finish. If the task fails with an exception, aTaskFailedException
(which wraps the failed task) is thrown.RawFD
: Wait for changes on a file descriptor (see theFileWatching
package).
If no argument is passed, the task blocks for an undefined period. A task can only be restarted by an explicit call to schedule
or yieldto
.
Often wait
is called within a while
loop to ensure a waited-for condition is met before proceeding.
Base.fetch
— Methodfetch(t::Task)
Wait for a Task to finish, then return its result value. If the task fails with an exception, a TaskFailedException
(which wraps the failed task) is thrown.
Base.timedwait
— Functiontimedwait(testcb::Function, timeout::Real; pollint::Real=0.1)
Waits until testcb
returns true
or for timeout
seconds, whichever is earlier. testcb
is polled every pollint
seconds. The minimum duration for timeout
and pollint
is 1 millisecond or 0.001
.
Returns :ok or :timed_out
Base.Condition
— TypeCondition()
Create an edge-triggered event source that tasks can wait for. Tasks that call wait
on a Condition
are suspended and queued. Tasks are woken up when notify
is later called on the Condition
. Edge triggering means that only tasks waiting at the time notify
is called can be woken up. For level-triggered notifications, you must keep extra state to keep track of whether a notification has happened. The Channel
and Threads.Event
types do this, and can be used for level-triggered events.
This object is NOT thread-safe. See Threads.Condition
for a thread-safe version.
Base.notify
— Functionnotify(condition, val=nothing; all=true, error=false)
Wake up tasks waiting for a condition, passing them val
. If all
is true
(the default), all waiting tasks are woken, otherwise only one is. If error
is true
, the passed value is raised as an exception in the woken tasks.
Return the count of tasks woken up. Return 0 if no tasks are waiting on condition
.
Base.Semaphore
— TypeSemaphore(sem_size)
Create a counting semaphore that allows at most sem_size
acquires to be in use at any time. Each acquire must be matched with a release.
Base.acquire
— Functionacquire(s::Semaphore)
Wait for one of the sem_size
permits to be available, blocking until one can be acquired.
Base.release
— Functionrelease(s::Semaphore)
Return one permit to the pool, possibly allowing another task to acquire it and resume execution.
Base.AbstractLock
— TypeAbstractLock
Abstract supertype describing types that implement the synchronization primitives: lock
, trylock
, unlock
, and islocked
.
Base.lock
— Functionlock(lock)
Acquire the lock
when it becomes available. If the lock is already locked by a different task/thread, wait for it to become available.
Each lock
must be matched by an unlock
.
lock(f::Function, lock)
Acquire the lock
, execute f
with the lock
held, and release the lock
when f
returns. If the lock is already locked by a different task/thread, wait for it to become available.
When this function returns, the lock
has been released, so the caller should not attempt to unlock
it.
Base.unlock
— Functionunlock(lock)
Releases ownership of the lock
.
If this is a recursive lock which has been acquired before, decrement an internal counter and return immediately.
Base.trylock
— Functiontrylock(lock) -> Success (Boolean)
Acquire the lock if it is available, and return true
if successful. If the lock is already locked by a different task/thread, return false
.
Each successful trylock
must be matched by an unlock
.
Base.islocked
— Functionislocked(lock) -> Status (Boolean)
Check whether the lock
is held by any task/thread. This should not be used for synchronization (see instead trylock
).
Base.ReentrantLock
— TypeReentrantLock()
Creates a re-entrant lock for synchronizing Task
s. The same task can acquire the lock as many times as required. Each lock
must be matched with an unlock
.
Channels
Base.Channel
— TypeChannel{T=Any}(size::Int=0)
Constructs a Channel
with an internal buffer that can hold a maximum of size
objects of type T
. put!
calls on a full channel block until an object is removed with take!
.
Channel(0)
constructs an unbuffered channel. put!
blocks until a matching take!
is called. And vice-versa.
Other constructors:
Channel()
: default constructor, equivalent toChannel{Any}(0)
Channel(Inf)
: equivalent toChannel{Any}(typemax(Int))
Channel(sz)
: equivalent toChannel{Any}(sz)
The default constructor Channel()
and default size=0
were added in Julia 1.3.
Base.Channel
— MethodChannel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false)
Create a new task from func
, bind it to a new channel of type T
and size size
, and schedule the task, all in a single call.
func
must accept the bound channel as its only argument.
If you need a reference to the created task, pass a Ref{Task}
object via the keyword argument taskref
.
If spawn = true
, the Task created for func
may be scheduled on another thread in parallel, equivalent to creating a task via Threads.@spawn
.
Return a Channel
.
Examples
julia> chnl = Channel() do ch
foreach(i -> put!(ch, i), 1:4)
end;
julia> typeof(chnl)
Channel{Any}
julia> for i in chnl
@show i
end;
i = 1
i = 2
i = 3
i = 4
Referencing the created task:
julia> taskref = Ref{Task}();
julia> chnl = Channel(taskref=taskref) do ch
println(take!(ch))
end;
julia> istaskdone(taskref[])
false
julia> put!(chnl, "Hello");
Hello
julia> istaskdone(taskref[])
true
The spawn=
parameter was added in Julia 1.3. This constructor was added in Julia 1.3. In earlier versions of Julia, Channel used keyword arguments to set size
and T
, but those constructors are deprecated.
julia> chnl = Channel{Char}(1, spawn=true) do ch
for c in "hello world"
put!(ch, c)
end
end
Channel{Char}(sz_max:1,sz_curr:1)
julia> String(collect(chnl))
"hello world"
Base.put!
— Methodput!(c::Channel, v)
Append an item v
to the channel c
. Blocks if the channel is full.
For unbuffered channels, blocks until a take!
is performed by a different task.
v
now gets converted to the channel's type with convert
as put!
is called.
Base.take!
— Methodtake!(c::Channel)
Remove and return a value from a Channel
. Blocks until data is available.
For unbuffered channels, blocks until a put!
is performed by a different task.
Base.isready
— Methodisready(c::Channel)
Determine whether a Channel
has a value stored to it. Returns immediately, does not block.
For unbuffered channels returns true
if there are tasks waiting on a put!
.
Base.fetch
— Methodfetch(c::Channel)
Wait for and get the first available item from the channel. Does not remove the item. fetch
is unsupported on an unbuffered (0-size) channel.
Base.close
— Methodclose(c::Channel[, excp::Exception])
Close a channel. An exception (optionally given by excp
), is thrown by:
Base.bind
— Methodbind(chnl::Channel, task::Task)
Associate the lifetime of chnl
with a task. Channel
chnl
is automatically closed when the task terminates. Any uncaught exception in the task is propagated to all waiters on chnl
.
The chnl
object can be explicitly closed independent of task termination. Terminating tasks have no effect on already closed Channel
objects.
When a channel is bound to multiple tasks, the first task to terminate will close the channel. When multiple channels are bound to the same task, termination of the task will close all of the bound channels.
Examples
julia> c = Channel(0);
julia> task = @async foreach(i->put!(c, i), 1:4);
julia> bind(c,task);
julia> for i in c
@show i
end;
i = 1
i = 2
i = 3
i = 4
julia> isopen(c)
false
julia> c = Channel(0);
julia> task = @async (put!(c, 1); error("foo"));
julia> bind(c, task);
julia> take!(c)
1
julia> put!(c, 1);
ERROR: TaskFailedException:
foo
Stacktrace:
[...]