TITLE

Timers and Timeouts

VERSION

  Maintainer: Uri Guttman <uri@sysarch.com>
  Date: 10 Aug 2000
  Mailing List: perl6-language-flow@perl.org
  Number: 87
  Version: 1
  Status: Developing

ABSTRACT

This RFC will discuss supporting multiple independent timers and timeouts for asynchronous I/O events and exiting eval blocks.

DESCRIPTION

Currently the only asynchronous timer mechanism in Perl is the alarm signal. It has many flaws including being delivered unsafely and that there is only one of them (which precludes nesting calls with their own timeouts).

Under the asynchronous I/O (AIO) system all calls which support a callback can also have an optional timeout. This event has a simple common set of attribute/value arguments that can be used with any AIO call. When no activity occurs on that AIO object before the timeout period ends, the timeout is triggered and a callback is made. When any activity occurs, the timeout is reset to its initial period (which can also be done with the timeout_reset method). Standalone timeouts with no AIO object can be created for use as independent timers.

Timers can be used to break out of any AIO operation and even out of eval blocks by having the callback execute die (or quit or whatever). So you can nest eval blocks and have control over how long each level takes to run. Also this means no more collisions with sharing alarm calls.

IMPLEMENTATION

There are no syntactical changes needed for timeouts. All AIO calls will be able to handle these additional optional arguments:

	timeout		=>	<number in seconds. float is OK>
	timeout_cb	=>	<timeout callback. see callback RFC>
	timeout_method	=>	<method to call other then the default>
	timeout_repeat	=>	<times to repeat>

The timeout arguments is in seconds but a float value can be used to get fractions. The callback argument is the same as all the others in the AIO system, it can be a coderef, an object or a class name. If you pass in a timeout_method, that will be used as the callback. If the repeat argument is passed the timeout will repeat that many times and forever with the value -1.

Here is a typical AIO call with timeouts:

package Foo ; $aio->read_event( cb => 'Foo', timeout => 5 ) ;

sub READABLE {

	my( $self ) = @_ ;

	my $cnt = $aio->read( $buffer, 1024 ) ;
}

sub READ_EVENT_TIMEOUT {

	print "nothing to read\n" ;
}

The default callback name for timeouts will be the AIO operation name in upper case and _TIMEOUT appended to it. So the default methods for read and connect are READ_TIMEOUT and CONNECT_TIMEOUT

Timers not associated with an AIO object can be created firectly with this call:

	$timer = AIO::Timer->new( timeout => 5, repeat => -1,
				  cb => sub{ print "annoying, isn't it!\n" }
	) ;

IMPACT

It has to be emphasized that all timing in this RFC is a minimum time. There is no guarantee for any speed of delivery of the event. Supporting event priorities is a possibility to improve the delivery speed some.

This requires one of the AIO delivery methods (event loops, mailboxes, in line) to be enabled. See the use aio RFC.

UNKNOWNS

REFERENCES

Event.pm - XS based event loop module.

RFC #47 - Universal Asynchronous I/O (the moby one)

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