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| 579 | See the examples section earlier in this document for some actual |
579 | See the examples section earlier in this document for some actual |
| 580 | examples. |
580 | examples. |
| 581 | |
581 | |
| 582 | =back |
582 | =back |
| 583 | |
583 | |
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584 | =head1 ADVANCED TOPICS |
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585 | |
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586 | =head2 Choosing a backend |
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587 | |
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588 | So how do you decide which backend to use? Well, that's your problem to |
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589 | solve, but here are some thoughts on the matter: |
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590 | |
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591 | =over 4 |
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592 | |
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593 | =item Synchronous |
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594 | |
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595 | The synchronous backend does not rely on any external modules (well, |
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596 | except L<common::sense>, which works around a bug in how perl's warning |
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597 | system works). This keeps the process very small, for example, on my |
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598 | system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB |
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599 | after C<use warnings; use strict> (for people who grew up with C64s around |
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600 | them this is probably shocking every single time they see it). The worker |
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601 | process in the first example in this document uses 1792kB. |
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602 | |
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603 | Since the calls are done synchronously, slow jobs will keep newer jobs |
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604 | from executing. |
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605 | |
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606 | The synchronous backend also has no overhead due to running an event loop |
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607 | - reading requests is therefore very efficient, while writing responses is |
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608 | less so, as every response results in a write syscall. |
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609 | |
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610 | If the parent process is busy and a bit slow reading responses, the child |
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611 | waits instead of processing further requests. This also limits the amount |
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612 | of memory needed for buffering, as never more than one response has to be |
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613 | buffered. |
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614 | |
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615 | The API in the child is simple - you just have to define a function that |
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616 | does something and returns something. |
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617 | |
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618 | It's hard to use modules or code that relies on an event loop, as the |
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619 | child cannot execute anything while it waits for more input. |
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620 | |
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621 | =item Asynchronous |
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622 | |
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623 | The asynchronous backend relies on L<AnyEvent>, which tries to be small, |
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624 | but still comes at a price: On my system, the worker from example 1a uses |
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625 | 3420kB RSS (for L<AnyEvent>, which loads L<EV>, which needs L<XSLoader> |
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626 | which in turn loads a lot of other modules such as L<warnings>, L<strict>, |
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627 | L<vars>, L<Exporter>...). |
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628 | |
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629 | It batches requests and responses reasonably efficiently, doing only as |
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630 | few reads and writes as needed, but needs to poll for events via the event |
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631 | loop. |
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632 | |
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633 | Responses are queued when the parent process is busy. This means the child |
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634 | can continue to execute any queued requests. It also means that a child |
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635 | might queue a lot of responses in memory when it generates them and the |
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636 | parent process is slow accepting them. |
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637 | |
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638 | The API is not a straightforward RPC pattern - you have to call a |
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639 | "done" callback to pass return values and signal completion. Also, more |
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640 | importantly, the API starts jobs as fast as possible - when 1000 jobs |
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641 | are queued and the jobs are slow, they will all run concurrently. The |
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642 | child must implement some queueing/limiting mechanism if this causes |
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643 | problems. Alternatively, the parent could limit the amount of rpc calls |
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644 | that are outstanding. |
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645 | |
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646 | Using event-based modules such as L<IO::AIO>, L<Gtk2>, L<Tk> and so on is |
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647 | easy. |
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648 | |
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649 | =back |
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650 | |
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651 | =head2 Passing file descriptors |
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652 | |
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653 | Unlike L<AnyEvent::Fork>, this module has no in-built file handle or file |
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654 | descriptor passing abilities. |
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655 | |
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656 | The reason is that passing file descriptors is extraordinary tricky |
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657 | business, and conflicts with efficient batching of messages. |
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658 | |
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659 | There still is a method you can use: Create a |
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660 | C<AnyEvent::Util::portable_socketpair> and C<send_fh> one half of it to |
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661 | the process before you pass control to C<AnyEvent::Fork::RPC::run>. |
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662 | |
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663 | Whenever you want to pass a file descriptor, send an rpc request to the |
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664 | child process (so it expects the descriptor), then send it over the other |
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665 | half of the socketpair. The child should fetch the descriptor from the |
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666 | half it has passed earlier. |
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667 | |
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668 | Here is some (untested) pseudocode to that effect: |
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669 | |
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670 | use AnyEvent::Util; |
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671 | use AnyEvent::Fork::RPC; |
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672 | use IO::FDPass; |
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673 | |
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674 | my ($s1, $s2) = AnyEvent::Util::portable_socketpair; |
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675 | |
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676 | my $rpc = AnyEvent::Fork |
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677 | ->new |
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678 | ->send_fh ($s2) |
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679 | ->require ("MyWorker") |
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680 | ->AnyEvent::Fork::RPC::run ("MyWorker::run" |
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681 | init => "MyWorker::init", |
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682 | ); |
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683 | |
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684 | undef $s2; # no need to keep it around |
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685 | |
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686 | # pass an fd |
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687 | $rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv); |
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688 | |
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689 | IO::FDPass fileno $s1, fileno $handle_to_pass; |
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690 | |
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691 | $cv->recv; |
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692 | |
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693 | The MyWorker module could look like this: |
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694 | |
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695 | package MyWorker; |
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696 | |
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697 | use IO::FDPass; |
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698 | |
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699 | my $s2; |
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700 | |
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701 | sub init { |
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702 | $s2 = $_[0]; |
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703 | } |
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704 | |
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705 | sub run { |
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706 | if ($_[0] eq "i'll send some fd now, please expect it!") { |
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707 | my $fd = IO::FDPass::recv fileno $s2; |
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708 | ... |
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709 | } |
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710 | } |
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711 | |
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712 | Of course, this might be blocking if you pass a lot of file descriptors, |
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713 | so you might want to look into L<AnyEvent::FDpasser> which can handle the |
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714 | gory details. |
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715 | |
| 584 | =head1 SEE ALSO |
716 | =head1 SEE ALSO |
| 585 | |
717 | |
| 586 | L<AnyEvent::Fork> (to create the processes in the first place), |
718 | L<AnyEvent::Fork> (to create the processes in the first place), |
| 587 | L<AnyEvent::Fork::Pool> (to manage whole pools of processes). |
719 | L<AnyEvent::Fork::Pool> (to manage whole pools of processes). |
| 588 | |
720 | |
