TITLE
Synopsis 17 - Concurrency [DRAFT]
AUTHOR
Elizabeth Mattijsen <liz@dijkmat.nl>
Audrey Tang <autrijus@autrijus.org>VERSION
Maintainer: Elizabeth Mattijsen <liz@dijkmat.nl>
Date: 13 Jun 2005
Last Modified: 13 Nov 2005
Number: 0
Version: 1SKETCH
This is a rough sketch of how concurrency works in Perl 6.
(actually these are just random notes, put here under the release-early release-often principle, slowly being integrated in a more textual format. Patches welcome!)
OVERVIEW
Concurrency can take many forms in Perl 6. With varying degrees of explicitness and control capabilities. This document attempts to describe what these capabilities are and in which form they can be accessed in Perl 6.
Processes, threads, fibers?
Concurrency comes in many shapes and forms. Most Perl users are used to the concept of a "process" or a "thread" (usually depending on the OS they work on). Some systems even are familiar with very lightweight threads called "fibers".
When discussing issues about concurrency with different people, it soon becomes apparent that everybody has his own set of "understandings" about what each word means, which doesn't make it any easier to describe Perl 6 concurrency.
It seemed the most natural to use the word "thread" to describe a process which has its own context, but also shares context with 0 or more concurrently running processes. Depending on your OS, or even specific version of your OS, this could still be a single "process" from the OS's point of view. Or it could contain an OS process for each thread. Or any mixture of these two implementations.
In this document we try to be agnostic about this: all we know in Perl 6 are "threads", which have their own context and share context with other concurrently running "threads". Whether they be process, threads or fibres at the OS level should not matter at the Perl 6 level.
And for sake of consistency, an unthreaded "normal" program is considered to be also running in a single thread.
Variables
In the past, there have been two models for concurrent processes in Perl. In general, these are referred to as "5.005 threads" (perldoc perlothrtut) and "ithreads" (perldoc perlthrtut).
The main difference between these two models from a programmer's point of view, is that variables in "5.005 threads" are shared by default. Whereas in the "ithreads" model, only variables that have been indicated to be "shared", are actually shared between threads. All other variable values are actually copies of the variable's value in the "parent" thread.
With regards to variables, the concurrency model of Perl 6 is closer to the "5.005 threads" model than it is to the "ithreads" model. In fact, all variables "visible" to a particular scope in Perl 6 will be accessible and modifiable from all of the concurrent processes that start from that scope. In that sense, one could consider the "ithreads" model as a historical diversion: the Perl 6 concurrency picks up where the "5.005 threads" path left off.
(EM: maybe point out that the "ithreads" behaviour can be simulated with some kind of copy-on-write magic to be automagically added to all variable access inside a thread, except for those with an explicit "is shared" attribute?)
No user accessible locks
Differently from any current concurrent process implementation in Perl, there are no user accessible locks. Instead, the concept of Software Transactionable Memory is used. This is in concept similar to the use of
BEGIN TRANSACTION
... do your uninterruptible actions
COMMITin the database world. More interestingly, this also includes the concept of rollback:
BEGIN TRANSACTION
... do your stuff, but impossible to complete: ROLLBACKThis causes the state of the process to be reverted to the state at the moment the BEGIN TRANSACTION was executed.
Perl 6 supports this concept through Code blocks which are marked "is atomic". These sections are guaranteed to either be completed totally (when the Code block is exited), or have their state reverted to the state at the start of the Code block (with the retry statement).
(EM: maybe point out if / how old style locks can be "simulated", for those needing a migration path?)
Atomic Code blocks
my ($x, $y);
sub c is atomic {
$x -= 3;
$y += 3;
if $x < 10 { retry }
};
$e = &c.retry_with( &d ); #
$e();
if $i { is atomic; ... } else { ...; }A Code block can be marked as "is atomic". This means that code executed inside that scope is guaranteed not to be interrupted in any way.
The start of a block marked "is atomic" also becomes a "checkpoint" to which execution can return (in exactly the same state) if a problem occurs (a.k.a. a retry is done) inside the scope of the Code block.
retry
The retry function basically restores the state of the thread at the last checkpoint and will wait there until an external event allows it to potentially run that atomic section of code again without having to retry again.
If there are no external events possible that could restart execution, an exception will be raised.
The last checkpoint is either the last atomic / non-atomic boundary, or the most immediate caller constructed with retry_with.
retry_with
The retry_with method on an atomic Code object causes a checkpoint to be made for retry, creating an alternate execution path to be followed when a retry is done.
limitations
Because Perl 6 must be able to revert its state to the state it had at the checkpoint, it is not allowed to perform any non-revertable actions. These would include reading / writing from file handles that do not support seek (such as sockets). Attempting to do so will cause a fatal error to occur.
If you're not interested in revertability, but are interested in uninteruptability, you could use the "is critical" trait.
Critical Code blocks
sub tricky is critical {
# code accessing external info, not to be interrupted
}
if ($update) {
is critical;
# code accessing external info, not to be interrupted
}A Code block marked "is critical" can not be interrupted in any way. But since it is able to access non-revertible data structures (such as non-seekable file handles), it cannot do a retry as it would be impossible to restore the state to the beginning of the Code block.
Mixing Atomic and Critical
Both "atomic" as well as "critical" propagate down the call chain. This means that any subroutine that in itself is not "atomic" or "critical" becomes uninterruptible if called inside a code block that is marked as "atomic" or "critical".
Atomic Code blocks called inside the call chain of a "critical" code block do not pose a problem, as they are more restrictive.
Any code that attempts to perform any non-revertible action (e.g. reading from a socket) will cause a fatal error when called inside the call chain of an Atomic Code block.
Co-Routines
The execution of co-routine (or "coro" for short) could be considered as a short "side-step" from the normal path of execution, much like the normal calling of a subroutine.
The main difference with a normal subroutine, is that the co-routine supports a special type of return, called "yield".
(EM: not sure whether the "threads->yield" causes so much mental interference that we should use something else for "yield" in the coro context. And whether we should have a seperate "coro" keyword at all: after all, the "yield" could be in a normal subroutine called from a coro, so it's not like the compiler would be allowed to flag "yield" in a sub as an error).
####################################################################### Below here still the more or less unorganized stuff
CORE::GLOBAL::exit; # kills all the threads
# We intententionally do not list cross-machine parallelism Conc:: classes here. # Consult your local 6PAN mirror with a time machine. use Conc::Processes; # fork() or createProcess based implementation use Conc::Threads; # maybe it just exports &async to override the default one, yay use Conc::Multiplex; # this is default
my $thr = async { ...do something... END { } };
Conc::Thread.this Conc::Proc.this
Conc object # name is still up for grabs! - numify to TIDs (as in pugs) - stringify to something sensible (eg. "<Conc:tid=5>"); - enumerable with Conc.list - Conc.yield (if this is to live but deprecated, maybe call it sleep(0)?) - sleep() always respects other threads, thank you very much - standard methods: - .join # wait for invocant to finish (always item cxt) - .die # throw exception in the invocant thread - .alarm # set up alarms - .alarms # query existing alarms - .suspend # pause a thread; fail if already paused - .resume # revive a thread; fail if already running - .detach # survives parent thread demise (promoted to process) # process-local changes no longer affects parent # tentatively, the control methods still applies to it # including wait (which will always return undef) # also needs to discard any atomicity context - attributes: - .started # time - .finished # time - .waiting # suspened (not diff from block on wakeup signal) # waiting on a handle, a condition, a lock, et cetera # otherwise returns false for running threads # if it's finished then it's undef(?) - .current_continuation # the CC currently running in that thread
- "is throttled" trait
method throttled::trait_auxillary:<is> ($limit=1, :$key=gensym()) {
# "is throttled" limits max connection to this Code object
# the throttling is shared among closures with the same key
# the limit may differ on closures with the same key.
# if the counter with the "key" equals or exceeds a closure's limit,
# the closure can't be entered until it's released
# (this can be trivially implmented using atomic+retry)
}
class Foo {
method a is throttled(:limit(3) :key<blah>) { ... }
method b is throttled(:limit(2) :key<blah>) { ... }
}
my Foo $f .= new;
async { $f.a }
async { $f.b }- Thread::Status - IO objects and containers gets concurrency love! - $obj.wake_on_readable - $obj.wake_on_writable - $obj.wake_on_either_readable_or_writable_or_passed_time(3); # fixme fixme - $obj.wake_on:{.readable} # busy wait, probably
my @a is Array::Chan = 1..Inf;
async { @a.push(1) };
async { @a.blocking_shift({ ... }) };
async { @a.unshift({ ... }) };Communication abstractions - shared, transactional variables by default
# program will wait for _all_ threads # unjoined threads will be joined at the beginning of the END block batch # of the parent thread that spawned them
### INTERFACE BARRIER ### module Blah; {
is atomic; # retry/orelse/whatever other rollback stuff
# limitation: no external IO (without lethal warnings anyway)
# can't do anything irreversible
is critical; # free to do anything irreversible
# means "don't interrupt me"
# in system with critical section, no interrupts from
# other threads will happen during execution
# you can't suspend me
my $boo is export;
$boo = 1;
# We decree that this part forms the static interface
# it's run once during initial compilation under the
# Separate Compilation doctrine and the syms sealed off
# to form part fo bytecode syms headers
%CALLER::<&blah> = { 1 }; # work - adds to export set
die "Eureka!" if %CALLER::<$sym>; # never dies
# BEGIN { $boo = time };
sub IMPORT {
# VERY DYNAMIC!
our $i = time;
%CALLER::<&blah> = { 1 }; # work - adds to export set
die "Eureka!" if %CALLER::<$sym>; # probes interactively
}
}
### INTERFACE BARRIER ###my $sym; threads.new({ use Blah; BEGIN { require(Blah).import }
my $boo; BEGIN { eval slurp<Blah.pm>; $boo := $Blah::boo };
...
});Signals
Asynchronous exceptions are just like user-initiated exceptions with die, so you can also catch it with regular CATCH blocks as specified in S04.
To declare your main program catches INT signals, put a CATCH block anywhere in the toplevel to handle exceptions like this:
CATCH {
when Error::Signal::INT { ... }
}Alarm
An alarm is just a pre-arranged exception to be delivered to your program.
By the time alarm has arrived, the current block may have already finished executing, so you would need to set up CATCH blocks in places where an alarm can rise to handle it properly.
You can request an alarm using the number of seconds, or with a target date. It returns a proxy alarm object that you can do interesting things with.
multi Alarm *alarm (Num $seconds = $CALLER::_, &do = {die Sig::ALARM}, :$repeat = 1)
multi Alarm *alarm (Date $date, &do = {die Sig::ALARM}, :$repeat = 1)Perl 6's alarm has three additional features over traditional alarms:
Multiple and Lexical Alarms
One can set up multiple alarms using repeated alarm calls:
{
my $a1 = alarm(2);
my $a2 = alarm(2);
sleep 10;
CATCH {
is critical; # if you don't want $a2 to be raised inside this
when Sig::ALARM { ... }
}
}To stop an alarm, call $alarm.stop. The alarms method for Conc objects (including process and threads) returns a list of alarms currently scheduled for that concurrent context.
When an alarm object is garbage collected, the alarm is stopped automatically. Under void context, the implicit alarm object can only be stopped by querying .alarms on the current process.
We are not sure what alarm(0) would mean. Probably a deprecation warning?
Repeated Alarms
If you request a repeated alarm using the repeated named argument, it will attempt to fire off the alarm that many times. However, the alarm will be supressed when inside a CATCH block that's already handling the exception raised by same alarm.
To repeat 0 times is to not fire off any alarms at all. To repeat +Inf times is to repeat over and over again.
Callbacks in Alarms
You can arrange a callback (like JavaScript's setTimeOut) in alarm, which will then be invoked with the then-current code as caller.
If you set up such a callback to another Conc object, what happens is just like when you called .die on behalf of that object -- namely, the callback closure, along with anything it referenced, is shared to the target Conc context.
Unlike in Perl 5's ithreads where you cannot share anything after the fact, this allows passing shared objects in an ad-hoc fashion across concurrent parts of the program. Under the default (multiplexing) concurrency model, this is basically a no-op.
Continuations
Coroutines
## braindump of coro meeting by Liz and Autri, more to follow
- Coros are _like_ processes
coro dbl { yield $_ * 2; yield $_; return }; my @x = 1..10; my %y = map &dbl, @x; # 2 => 2, 6 => 4, 10 => 6, ...
coro perm (@x) { @x.splice(rand(@x),1).yield while @x; }
my &p1 := &perm.start(1..10); my &p2 := &perm.start(1..20);
p1(); p1(); p2(); p2();
coro foo { yield 42 };
(1..10).pick;
coro foo ($x) { yield $x; yield $x+2; cleanup(); while (2) { while (1) { &?SUB.kill; # seppuku } } } # implicit falloff return + return() means startover without yielding # return() means yielding and restart + no implicit falloff (I LIKE THIS)
&foo.finished; # true on return() and false on midway yield()
foo(4); # and that's all she wrote
coro foo ($x) { yield $x; # this point with $x bound to 10 yield $x+1; return 5; ... # this is never reached, I think we all agree }
# If you don't want your variables to get rebound, use "is copy": coro foo ($x is copy) {...} # which is sugar for coro foo ($x) { { my $x := $OUTER::x; ...; # Further calls of &foo rebound $OUTER::x, not $x. } }
sub foo { return undef if rand; ... }
use overload { '&{}' => sub { ... } }
class Coro is Conc::Multiplex does Code { method postcircumfix:<( )> { # start the thread, block stuff (we are in the caller's context) } }
class Hash is extended { method postcircumfix:<( )> (&self: *@_) { &self = ./start(@_); } method start { # remember self # upon return() or normal falloff, restore self } }
%ENV(123);
&foo_continued := &foo.start(10); &foo.start(20);
foo(10); # returns 10
foo(); # be "insufficient param" error or just return 11? foo(20); # returns 21
# continuation coros multi foo () { ...no rebinding... } multi foo ($x) { ...rebinding... }
&foo.kill;
my $first_ret = zoro( type => <even> ); &zoro.variant(:type<even>).kill; &zoro.variant(type => 'even').kill;
zoro( type => <odd> );
zoro( even => 1 ); zoro( odd => 1 );
multi coro zoro ($type where 'even') {} multi coro zoro ($type where 'odd') {}
multi coro zoro ($even is named) {} multi coro zoro ($odd is named) {}
# iblech's thoughts: # Coroutine parameters should never be rebound. Instead, yield(...)s return # value is an Arglist object containing the new arguments: coro bar ($a, $b) { ...; my $new_set_of_args = yield(...); my $sum_of_old_a_and_new_a = $a + $new_set_of_args<$a>; ...; } bar(42, 23); # $a is 42, $b is 23 bar(17, 19); # $a still 42, $b still 19, # $new_set_of_args is \(a => 17, b => 19)
Junctive Autothreading and Hyper Operations
Live in userland for the time being.