Weird processor usage, when redirecing IO to /dev/null

[briandfoy]

This ticket was imported from rt.cpan.org 43780

• Requestor: ki11er_bob@yahoo.dk
• Date: Mon Mar 02 14:31:01 2009
• Original subject: Weird processor usage, when redirecing IO to /dev/null
• Original link: rt.cpan.org 43780

---

Hi'

I am targeting a simple perl loop, in which I with ssh->cmd will execute a command on a remote host. This code should run in the back ground on the mother host.

The following simple code shows my observation.

use Net::SSH::Perl;

$host = Net::SSH::Perl->new("remotehost");
$host->login();

while (1) {
    print `date`;
    ($out, $err, $exit) = $host->cmd("sleep 10");
}

Now, call the code test.pl and run "./test.pl". Command "top" running in another shell on the mother host does not show anything suspicious. Now I would like to have the code running in the background and thus I do "./test.pl </dev/null 1>/dev/null 2>/dev/null &". Now the "top" command shows my little program eating up one full processor. This puzzles me very much indeed and I am speculating, if this is a bug or a feature.

Thanks for any help in advance.

Best regards.

Thomas.

[briandfoy]

from schwigon@cpan.org

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On Mon Mar 02 14:31:05 2009, ki11er_bob@yahoo.dk wrote:

"./test.pl".
"./test.pl </dev/null 1>/dev/null 2>/dev/null &".
Now the "top" command shows my little program eating up one full
processor.

Not the "to /dev/null" is an issue, but the "from /dev/null".

It tries to read from STDIN, maybe during its filehandle hijacking.
You can see the shell stopping it when you don't feed its stdin.

./test.pl 1>/dev/null 2>/dev/null &

The cpu usage in such a situation is normal, similar to
a process searching for its lost tty.

The way I understand Net::SSH::Perl so far I don't think
this gets resolved.

Kind regards,
Steffen