perl - 需要帮助理解 Perl 代码 - 多进程/fork

Question

我正在寻找一个示例来限制同时运行的 fork 进程的数量，但我遇到了这段旧代码

#!/usr/bin/perl
#total forks, max childs, what to run
#function takes 2 scalars and a reference to code to run
sub mfork ($$&) {
        my ($count, $max, $code) = @_;
        # total number of processes to spawn
        foreach my $c (1 .. $count) {
                #what is happening here? why wait vs waitpid?
                wait unless $c <= $max;
                die "Fork failed: $!\n" unless defined (my $pid = fork);
                # i don't undestand the arrow notation here and how it calls a function, 
                #also unless $pid is saying run function unless you're the parent
                exit $code -> ($c) unless $pid;
        }
        #no idea what's happening here, why are we waiting twice? for the last process?
        #why 1 until (-1 == wait)? what's 1 doing here
        1 until -1 == wait;
}


#code to run
mfork 10, 3, sub {
        print "$$: " . localtime() . ": Starting\n";
        select undef, undef, undef, rand 2;
        print "$$: " . localtime() . ": Exiting\n";
};

Answer 1

让我们看一下代码。代码是你的，你的大部分评论都被删除了。所有其他评论都是我的。

#!/usr/bin/perl
# total forks, max childs, what to run
# function takes 2 scalars and a reference to code to run

sub mfork ($$&) {
        my ($count, $max, $code) = @_;

      # total number of processes to spawn
        foreach my $c (1 .. $count) {

                # wait waits for any child to return,
                # waitpid for a specific one
                wait unless $c <= $max;

                die "Fork failed: $!\n" unless defined (my $pid = fork);

                # the arrow is used to call the coderef in $code
                # and the argument is $c. It's confusing because it has
                # the space. It's a deref arrow, but looks like OOp.
                # You're right about the 'unless $pid' part.
                # If there is $pid it's in the parent, so it does
                # nothing. If it is the child, it will run the
                # code and exit.

                exit $code -> ($c) unless $pid;
        }

        # This is reached after the parent is done with the foreach.
        # It will wait in the first line of the foreach while there are
        # still $count tasks remaining. Once it has spawned all of those
        # (some finish and exit and make room for new ones inside the
        # loop) it gets here, where it waits for the remaining ones.
        # wait will return -1 when there are no more children.
        # The '1 until' is just short for having an until loop that
        # doesn't have the block. The 1; is not a costly operation.
        # When wait == -1 it passes the line, returning from the sub.
        1 until -1 == wait;
}


# because of the prototype above there are no () needed here
mfork 10, 3, sub {
        print "$$: " . localtime() . ": Starting\n";
        select undef, undef, undef, rand 2;
        print "$$: " . localtime() . ": Exiting\n";
};

让我们详细了解一下。

• 有wait和 waitpid . wait 将等待，直到任何 child 返回。这很有用，因为程序不关心释放了哪个 slot。一旦一个完成，就可以产生一个新的。 waitpid 采用特定 $pid 的参数。这在这里没有帮助。
• $code->($c) 语法运行代码引用。就像 %{ $foo }{bar} 将取消引用 hashref，&{ $baz }() 将取消引用(并运行，这就是 ()) 代码引用。一种更容易阅读的方式是 $foo->{bar}。 $baz->() 也是如此。 arraow 取消引用它。参见 perlref和 perlreftut .

---

虽然这很好用，但使用 Parallel::Forkmanager 可能更有意义，它以更少的代码行提供了此功能，您无需担心它的工作原理。

use strict;
use warnings;
use Parallel::ForkManager;

my $pm = Parallel::ForkManager->new(3); # max 3 at the same time

DATA_LOOP:
foreach my $data (1 .. 10) {
  # Forks and returns the pid for the child:
  my $pid = $pm->start and next DATA_LOOP;

  ... do some work with $data in the child process ...
  print "$$: " . localtime() . ": Starting\n";
  select undef, undef, undef, rand 2;
  print "$$: " . localtime() . ": Exiting\n";

  $pm->finish; # Terminates the child process
}

就是这样。阅读起来更清晰。 :)