Ractor
- Ruby’s Actor-like concurrent abstraction¶ ↑
Ractor
is designed to provide a parallel execution feature of Ruby without thread-safety concerns.
Summary¶ ↑
Multiple Ractors in an interpreter process¶ ↑
You can make multiple Ractors and they run in parallel.
-
Ractor.new{ expr }
creates a newRactor
andexpr
is run in parallel on a parallel computer. -
Interpreter invokes with the first
Ractor
(called main Ractor). -
If the main
Ractor
terminates, all other Ractors receive termination requests, similar to how threads behave. (if main thread (first invokedThread
), Ruby interpreter sends all running threads to terminate execution). -
Each
Ractor
contains one or more Threads. -
Threads within the same
Ractor
share a Ractor-wide global lock like GIL (GVL in MRI terminology), so they can’t run in parallel (without releasing GVL explicitly in C-level). Threads in different ractors run in parallel. -
The overhead of creating a
Ractor
is similar to overhead of oneThread
creation.
Limited sharing between multiple ractors¶ ↑
Ractors don’t share everything, unlike threads.
-
Most objects are Unshareable objects, so you don’t need to care about thread-safety problems which are caused by sharing.
-
Some objects are Shareable objects.
-
Immutable objects: frozen objects which don’t refer to unshareable-objects.
-
i = 123
:i
is an immutable object. -
s = "str".freeze
:s
is an immutable object. -
a = [1, [2], 3].freeze
:a
is not an immutable object becausea
refers unshareable-object[2]
(which is not frozen). -
h = {c: Object}.freeze
:h
is an immutable object becauseh
refersSymbol
:c
and shareableObject
class object which is not frozen.
-
-
Class/Module objects
-
Special shareable objects
-
Ractor
object itself. -
And more…
-
Communication between Ractors with Ractor::Port
¶ ↑
Ractors communicate with each other and synchronize the execution by message exchanging between Ractors. Ractor::Port
is provided for this communication.
port = Ractor::Port.new Ractor.new port do |port| # Other ractors can send to the port port << 42 end port.receive # get a message to the port. Only the creator Ractor can receive from the port #=> 42
Ractors have its own default port and Ractor#send
, Ractor.receive
will use it.
Copy & Move semantics to send messages¶ ↑
To send unshareable objects as messages, objects are copied or moved.
-
Copy: use deep-copy.
-
Move: move membership.
-
Sender can not access the moved object after moving the object.
-
Guarantee that at least only 1
Ractor
can access the object.
Thread-safety¶ ↑
Ractor
helps to write a thread-safe concurrent program, but we can make thread-unsafe programs with Ractors.
-
GOOD: Sharing limitation
-
Most objects are unshareable, so we can’t make data-racy and race-conditional programs.
-
Shareable objects are protected by an interpreter or locking mechanism.
-
BAD: Class/Module can violate this assumption
-
To make it compatible with old behavior, classes and modules can introduce data-race and so on.
-
Ruby programmers should take care if they modify class/module objects on multi
Ractor
programs. -
BAD:
Ractor
can’t solve all thread-safety problems -
There are several blocking operations (waiting send) so you can make a program which has dead-lock and live-lock issues.
-
Some kind of shareable objects can introduce transactions (STM, for example). However, misusing transactions will generate inconsistent state.
Without Ractor
, we need to trace all state-mutations to debug thread-safety issues. With Ractor
, you can concentrate on suspicious code which are shared with Ractors.
Creation and termination¶ ↑
Ractor.new
¶ ↑
-
Ractor.new{ expr }
generates anotherRactor
.
# Ractor.new with a block creates new Ractor r = Ractor.new do # This block will be run in parallel with other ractors end # You can name a Ractor with `name:` argument. r = Ractor.new name: 'test-name' do end # and Ractor#name returns its name. r.name #=> 'test-name'
Given block isolation¶ ↑
The Ractor
executes given expr
in a given block. Given block will be isolated from outer scope by the Proc#isolate
method (not exposed yet for Ruby users). To prevent sharing unshareable objects between ractors, block outer-variables, self
and other information are isolated.
Proc#isolate
is called at Ractor
creation time (when Ractor.new
is called). If given Proc
object is not able to isolate because of outer variables and so on, an error will be raised.
begin a = true r = Ractor.new do a #=> ArgumentError because this block accesses `a`. end r.join # see later rescue ArgumentError end
-
The
self
of the given block is theRactor
object itself.
r = Ractor.new do p self.class #=> Ractor self.object_id end r.value == self.object_id #=> false
Passed arguments to Ractor.new()
becomes block parameters for the given block. However, an interpreter does not pass the parameter object references, but send them as messages (see below for details).
r = Ractor.new 'ok' do |msg| msg #=> 'ok' end r.value #=> 'ok'
# almost similar to the last example r = Ractor.new do msg = Ractor.receive msg end r.send 'ok' r.value #=> 'ok'
An execution result of given block¶ ↑
Return value of the given block becomes an outgoing message (see below for details).
r = Ractor.new do 'ok' end r.value #=> `ok`
Error in the given block will be propagated to the receiver of an outgoing message.
r = Ractor.new do raise 'ok' # exception will be transferred to the receiver end begin r.value rescue Ractor::RemoteError => e e.cause.class #=> RuntimeError e.cause.message #=> 'ok' e.ractor #=> r end
Communication between Ractors¶ ↑
Communication between Ractors is achieved by sending and receiving messages. There are two ways to communicate with each other.
-
(1) Message sending/receiving via
Ractor::Port
-
(2) Using shareable container objects
-
Ractor::TVar gem (ko1/ractor-tvar)
-
more?
Users can control program execution timing with (1), but should not control with (2) (only manage as critical section).
For message sending and receiving, there are two types of APIs: push type and pull type.
-
(1) send/receive via
Ractor::Port
. -
Ractor::Port#send(obj)
(Ractor::Port#<<(obj)
is an alias) send a message to the port. Ports are connected to the infinite size incoming queue soRactor::Port#send
will never block. -
Ractor::Port#receive
dequeue a message from its own incoming queue. If the incoming queue is empty,Ractor::Port#receive
calling will block the execution of a thread. -
Ractor.select()
can wait for the success ofRactor::Port#receive
. -
You can close
Ractor::Port
byRactor::Port#close
only by the creatorRactor
of the port. -
If the port is closed, you can’t
send
to the port. IfRactor::Port#receive
is blocked for the closed port, then it will raise an exception. -
When a
Ractor
is terminated, the Ractor’s ports are closed. -
There are 3 ways to send an object as a message
-
(1) Send a reference: Sending a shareable object, send only a reference to the object (fast)
-
(2) Copy an object: Sending an unshareable object by copying an object deeply (slow). Note that you can not send an object which does not support deep copy. Some
T_DATA
objects (objects whose class is defined in a C extension, such asStringIO
) are not supported. -
(3) Move an object: Sending an unshareable object reference with a membership. Sender
Ractor
can not access moved objects anymore (raise an exception) after moving it. Current implementation makes new object as a moved object for receiverRactor
and copies references of sending object to moved object.T_DATA
objects are not supported. -
You can choose “Copy” and “Move” by the
move:
keyword,Ractor#send(obj, move: true/false)
andRactor.yield(obj, move: true/false)
(default isfalse
(COPY)).
Wait for multiple Ractors with Ractor.select
¶ ↑
You can wait multiple Ractor
port’s receiving. The return value of Ractor.select()
is [port, msg]
where port
is a ready port and msg
is received message.
To make convenient, Ractor.select
can also accept Ractors to wait the termination of Ractors. The return value of Ractor.select()
is [r, msg]
where r
is a terminated Ractor
and msg
is the value of Ractor’s block.
Wait for a single ractor (same as Ractor#value
):
r1 = Ractor.new{'r1'} r, obj = Ractor.select(r1) r == r1 and obj == 'r1' #=> true
Waiting for two ractors:
r1 = Ractor.new{'r1'} r2 = Ractor.new{'r2'} rs = [r1, r2] as = [] # Wait for r1 or r2's Ractor.yield r, obj = Ractor.select(*rs) rs.delete(r) as << obj # Second try (rs only contain not-closed ractors) r, obj = Ractor.select(*rs) rs.delete(r) as << obj as.sort == ['r1', 'r2'] #=> true
TODO: Current Ractor.select()
has the same issue of select(2)
, so this interface should be refined.
TODO: select
syntax of go-language uses round-robin technique to make fair scheduling. Now Ractor.select()
doesn’t use it.
Closing Ractor’s ports¶ ↑
-
Ractor::Port#close
close the ports (similar toQueue#close
). -
port.send(obj)
whereport
is closed, will raise an exception. -
When the queue connected to the port is empty and port is closed,
Ractor::Port#receive
raises an exception. If the queue is not empty, it dequeues an object without exceptions. -
When a
Ractor
terminates, the ports are closed automatically.
Example (try to get a result from closed Ractor
):
r = Ractor.new do 'finish' end r.join # success (wait for the termination) r.value # success (will return 'finish') # the first Ractor which success the `Ractor#value` can get the result Ractor.new r do |r| r.value #=> Ractor::Error end
Example (try to send to closed (terminated) Ractor
):
r = Ractor.new do end r.join # wait terminate begin r.send(1) rescue Ractor::ClosedError 'ok' else 'ng' end
Send a message by copying¶ ↑
Ractor::Port#send(obj)
copy obj
deeply if obj
is an unshareable object.
obj = 'str'.dup r = Ractor.new obj do |msg| # return received msg's object_id msg.object_id end obj.object_id == r.value #=> false
Some objects are not supported to copy the value, and raise an exception.
obj = Thread.new{} begin Ractor.new obj do |msg| msg end rescue TypeError => e e.message #=> #<TypeError: allocator undefined for Thread> else 'ng' # unreachable here end
Send a message by moving¶ ↑
Ractor::Port#send(obj, move: true)
moves obj
to the destination Ractor
. If the source Ractor
touches the moved object (for example, call the method like obj.foo()
), it will be an error.
# move with Ractor#send r = Ractor.new do obj = Ractor.receive obj << ' world' end str = 'hello' r.send str, move: true modified = r.value #=> 'hello world' # str is moved, and accessing str from this Ractor is prohibited begin # Error because it touches moved str. str << ' exception' # raise Ractor::MovedError rescue Ractor::MovedError modified #=> 'hello world' else raise 'unreachable' end
Some objects are not supported to move, and an exception will be raised.
r = Ractor.new do Ractor.receive end r.send(Thread.new{}, move: true) #=> allocator undefined for Thread (TypeError)
To achieve the access prohibition for moved objects, class replacement technique is used to implement it.
Shareable objects¶ ↑
The following objects are shareable.
-
Immutable objects
-
Small integers, some symbols,
true
,false
,nil
(a.k.a.SPECIAL_CONST_P()
objects in internal) -
Frozen native objects
-
Frozen
String
andRegexp
objects (their instance variables should refer only shareable objects) -
Class, Module objects (
T_CLASS
,T_MODULE
andT_ICLASS
in internal) -
Ractor
and other special objects which care about synchronization.
Implementation: Now shareable objects (RVALUE
) have FL_SHAREABLE
flag. This flag can be added lazily.
To make shareable objects, Ractor.make_shareable(obj)
method is provided. In this case, try to make shareable by freezing obj
and recursively traversable objects. This method accepts copy:
keyword (default value is false).Ractor.make_shareable(obj, copy: true)
tries to make a deep copy of obj
and make the copied object shareable.
Language changes to isolate unshareable objects between Ractors¶ ↑
To isolate unshareable objects between Ractors, we introduced additional language semantics on multi-Ractor Ruby programs.
Note that without using Ractors, these additional semantics is not needed (100% compatible with Ruby 2).
Global variables¶ ↑
Only the main Ractor
(a Ractor
created at starting of interpreter) can access global variables.
$gv = 1 r = Ractor.new do $gv end begin r.join rescue Ractor::RemoteError => e e.cause.message #=> 'can not access global variables from non-main Ractors' end
Note that some special global variables, such as $stdin
, $stdout
and $stderr
are Ractor-local. See [Bug #17268] for more details.
Instance variables of shareable objects¶ ↑
Instance variables of classes/modules can be get from non-main Ractors if the referring values are shareable objects.
class C @iv = 1 end p Ractor.new do class C @iv end end.value #=> 1
Otherwise, only the main Ractor
can access instance variables of shareable objects.
class C @iv = [] # unshareable object end Ractor.new do class C begin p @iv rescue Ractor::IsolationError p $!.message #=> "can not get unshareable values from instance variables of classes/modules from non-main Ractors" end begin @iv = 42 rescue Ractor::IsolationError p $!.message #=> "can not set instance variables of classes/modules by non-main Ractors" end end end.join
shared = Ractor.new{} shared.instance_variable_set(:@iv, 'str') r = Ractor.new shared do |shared| p shared.instance_variable_get(:@iv) end begin r.join rescue Ractor::RemoteError => e e.cause.message #=> can not access instance variables of shareable objects from non-main Ractors (Ractor::IsolationError) end
Note that instance variables for class/module objects are also prohibited on Ractors.
Class variables¶ ↑
Only the main Ractor
can access class variables.
class C @@cv = 'str' end r = Ractor.new do class C p @@cv end end begin r.join rescue => e e.class #=> Ractor::IsolationError end
Constants¶ ↑
Only the main Ractor
can read constants which refer to the unshareable object.
class C CONST = 'str' end r = Ractor.new do C::CONST end begin r.join rescue => e e.class #=> Ractor::IsolationError end
Only the main Ractor
can define constants which refer to the unshareable object.
class C end r = Ractor.new do C::CONST = 'str' end begin r.join rescue => e e.class #=> Ractor::IsolationError end
To make multi-ractor supported library, the constants should only refer shareable objects.
TABLE = {a: 'ko1', b: 'ko2', c: 'ko3'}
In this case, TABLE
references an unshareable Hash
object. So that other ractors can not refer TABLE
constant. To make it shareable, we can use Ractor.make_shareable()
like that.
TABLE = Ractor.make_shareable( {a: 'ko1', b: 'ko2', c: 'ko3'} )
To make it easy, Ruby 3.0 introduced new shareable_constant_value
Directive.
# shareable_constant_value: literal TABLE = {a: 'ko1', b: 'ko2', c: 'ko3'} #=> Same as: TABLE = Ractor.make_shareable( {a: 'ko1', b: 'ko2', c: 'ko3'} )
shareable_constant_value
directive accepts the following modes (descriptions use the example: CONST = expr
):
-
none: Do nothing. Same as:
CONST = expr
-
literal:
-
if
expr
consists of literals, replaced toCONST = Ractor.make_shareable(expr)
. -
otherwise: replaced to
CONST = expr.tap{|o| raise unless Ractor.shareable?(o)}
. -
experimental_everything: replaced to
CONST = Ractor.make_shareable(expr)
. -
experimental_copy: replaced to
CONST = Ractor.make_shareable(expr, copy: true)
.
Except the none
mode (default), it is guaranteed that the assigned constants refer to only shareable objects.
See doc/syntax/comments.rdoc for more details.
Implementation note¶ ↑
-
Each
Ractor
has its own thread, it means eachRactor
has at least 1 native thread. -
Each
Ractor
has its own ID (rb_ractor_t::pub::id
). -
On debug mode, all unshareable objects are labeled with current Ractor’s id, and it is checked to detect unshareable object leak (access an object from different
Ractor
) in VM.
Examples¶ ↑
Traditional Ring example in Actor-model¶ ↑
RN = 1_000 CR = Ractor.current r = Ractor.new do p Ractor.receive CR << :fin end RN.times{ r = Ractor.new r do |next_r| next_r << Ractor.receive end } p :setup_ok r << 1 p Ractor.receive
Fork-join¶ ↑
def fib n if n < 2 1 else fib(n-2) + fib(n-1) end end RN = 10 rs = (1..RN).map do |i| Ractor.new i do |i| [i, fib(i)] end end until rs.empty? r, v = Ractor.select(*rs) rs.delete r p answer: v end
Worker pool¶ ↑
(1) One ractor has a pool
require 'prime' N = 1000 RN = 10 # make RN workers workers = (1..RN).map do Ractor.new do |; result_port| loop do n, result_port = Ractor.receive result_port << [n, n.prime?, Ractor.current] end end end result_port = Ractor::Port.new results = [] (1..N).each do |i| if workers.empty? # receive a result n, result, w = result_port.receive results << [n, result] else w = workers.pop end # send a task to the idle worker ractor w << [i, result_port] end # receive a result while results.size != N n, result, _w = result_port.receive results << [n, result] end pp results.sort_by{|n, result| n}
Pipeline¶ ↑
# pipeline with send/receive r3 = Ractor.new Ractor.current do |cr| cr.send Ractor.receive + 'r3' end r2 = Ractor.new r3 do |r3| r3.send Ractor.receive + 'r2' end r1 = Ractor.new r2 do |r2| r2.send Ractor.receive + 'r1' end r1 << 'r0' p Ractor.receive #=> "r0r1r2r3"
Supervise¶ ↑
# ring example again r = Ractor.current (1..10).map{|i| r = Ractor.new r, i do |r, i| r.send Ractor.receive + "r#{i}" end } r.send "r0" p Ractor.receive #=> "r0r10r9r8r7r6r5r4r3r2r1"
# ring example with an error r = Ractor.current rs = (1..10).map{|i| r = Ractor.new r, i do |r, i| loop do msg = Ractor.receive raise if /e/ =~ msg r.send msg + "r#{i}" end end } r.send "r0" p Ractor.receive #=> "r0r10r9r8r7r6r5r4r3r2r1" r.send "r0" p Ractor.select(*rs, Ractor.current) #=> [:receive, "r0r10r9r8r7r6r5r4r3r2r1"] r.send "e0" p Ractor.select(*rs, Ractor.current) #=> # <Thread:0x000056262de28bd8 run> terminated with exception (report_on_exception is true): # Traceback (most recent call last): # 2: from /home/ko1/src/ruby/trunk/test.rb:7:in `block (2 levels) in <main>' # 1: from /home/ko1/src/ruby/trunk/test.rb:7:in `loop' # /home/ko1/src/ruby/trunk/test.rb:9:in `block (3 levels) in <main>': unhandled exception # Traceback (most recent call last): # 2: from /home/ko1/src/ruby/trunk/test.rb:7:in `block (2 levels) in <main>' # 1: from /home/ko1/src/ruby/trunk/test.rb:7:in `loop' # /home/ko1/src/ruby/trunk/test.rb:9:in `block (3 levels) in <main>': unhandled exception # 1: from /home/ko1/src/ruby/trunk/test.rb:21:in `<main>' # <internal:ractor>:69:in `select': thrown by remote Ractor. (Ractor::RemoteError)
# resend non-error message r = Ractor.current rs = (1..10).map{|i| r = Ractor.new r, i do |r, i| loop do msg = Ractor.receive raise if /e/ =~ msg r.send msg + "r#{i}" end end } r.send "r0" p Ractor.receive #=> "r0r10r9r8r7r6r5r4r3r2r1" r.send "r0" p Ractor.select(*rs, Ractor.current) [:receive, "r0r10r9r8r7r6r5r4r3r2r1"] msg = 'e0' begin r.send msg p Ractor.select(*rs, Ractor.current) rescue Ractor::RemoteError msg = 'r0' retry end #=> <internal:ractor>:100:in `send': The incoming-port is already closed (Ractor::ClosedError) # because r == r[-1] is terminated.
# ring example with supervisor and re-start def make_ractor r, i Ractor.new r, i do |r, i| loop do msg = Ractor.receive raise if /e/ =~ msg r.send msg + "r#{i}" end end end r = Ractor.current rs = (1..10).map{|i| r = make_ractor(r, i) } msg = 'e0' # error causing message begin r.send msg p Ractor.select(*rs, Ractor.current) rescue Ractor::RemoteError r = rs[-1] = make_ractor(rs[-2], rs.size-1) msg = 'x0' retry end #=> [:receive, "x0r9r9r8r7r6r5r4r3r2r1"]