我正在寻找一个限制同时运行的分支进程数量的示例,我偶然发现了这个旧代码
#!/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() . ": Startingn";
select undef, undef, undef, rand 2;
print "$$: " . localtime() . ": Exitingn";
};
让我们看一下代码。代码是你的,你的大部分评论被删除了。所有其他评论都是我的。
#!/usr/bin/perl
# total forks, max childs, what to run
# function takes 2 scalars and a reference to code to run
sub mfork ($$&) {
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 "$$: " . localtime() . ": Startingn";
select undef, undef, undef, rand 2;
print "$$: " . localtime() . ": Exitingn";
};
让我们仔细看看。
- 有
wait
和waitpid
。wait
将等待直到任何子进程返回。这很有用,因为程序并不关心哪个槽被释放。一旦一个完成,一个新的可以产生。waitpid
接受特定$pid
的参数。这在这里没有帮助。 -
$code->($c)
语法运行代码。就像%{ $foo }{bar}
将解引用一个hashref一样,&{ $baz }()
将解引用(并运行,这是()
)一个coderef。更容易阅读的方法是$foo->{bar}
。$baz->()
也是如此。箭头是应得的。参见perlref和perlreftut
虽然这很好也很有用,但也许使用Parallel::Forkmanager会更有意义,因为它用更少的代码行提供了强大的功能,并且您不需要担心它是如何工作的。
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 "$$: " . localtime() . ": Startingn";
select undef, undef, undef, rand 2;
print "$$: " . localtime() . ": Exitingn";
$pm->finish; # Terminates the child process
}
就是这样。读起来更清楚了。:)