Thanks for your explaination. It helps me understannd better.
That is why in the script, it reads 1024 bytes each time in a loop for each filehandel. For non-blocking filehandel, it has to be done by this kind of chunk-reading.
And using "while (<FH>)", is more for blocking filehandle: I mean, if "Open" not failed, for sure, you can read all lines from the filehandle.

My third question is about " EAGAIN() and retry'. I didn't undersntand this part of the code: 

    $hl->{$_}->{retry} = 0;
    $hl->{$_}->{retries} = 0;

    my $start = time;
    my $blocksize = 1024;
    while (scalar keys %hltodo) {
        machine: for (keys %hltodo) {
            my $out = $hl->{$_}->{chld_out};
            # begin to read
            my $bytes_read = -1;
            while ($bytes_read) {
                my $buf;
                my $bytes_read = sysread($out, $buf, $blocksize);
                if (defined($bytes_read)) {
                    if ($bytes_read == 0) {
                        # eof
                        close($out);
                        last;
                    } else {
                $hl->{$_}->{data}.= $buf;
                    }
                } else {
                    if ($! == EAGAIN()) {
                        # retry
                        $hl->{$_}->{retry}++;
                        $hl->{$_}->{retries}++;
                        if ($hl->{$_}->{retry}) {
                            $hl->{$_}->{retry} = 0;
                            next machine;
                        }
                        usleep 10;
                    } else {
                        last;
                    }
                }
            }
            delete $hl->{$_}->{"chld_out"};
            delete $hltodo{$_};
        }
        # kill remaining pids if timeout reached
        if ($opt{timeout} && time > $start + $opt{timeout}) {
            print STDERR "Timeout for: ", join (" ", keys %hltodo), " 
+killing ", join (" ", values %hltodo) ;
            kill 1, values %hltodo;
            %hltodo = ();
        }
    }

[download]

When the non-blocking filehandel is blocked dur to what ever the reason,it send$! to EAGAIN, then $hl->{$_}->{retry}++ will be 1, so it goes to " $hl->{$_}->{retry} = 0" and "next machine", ti will never do usleep 10 microsecond? I must miss something for this part?

In reply to Re^2: parallel process on remote machines,read results and hanle timeout of those process by x12345
in thread parallel process on remote machines,read results and hanle timeout of those process by x12345

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