mod_perl: sharing data across httpd childs ?

Posted by howa on October 15, 2008, 4:36 am

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I am refering to the tutorial (http://modperlbook.org/html/4-2-3-
PerlModule-and-PerlRequire.html),
about setting up Perl module memory sharing across http childs:

==================================

1. in httpd.conf

Added PerlRequire /home/www/cgi-bin/startup.pl

the its contents is:

use strict;

use lib "/home/www/cgi-bin/";

use TestPM ();

1;

==================================

2. The TestPM's content

use strict;

package TestPM;

my $data;

sub new {

        my ($class) = @_;

        my $self = {};

        bless $self, $class;

        return $self;
}

sub init {
        $data = "1234567890" x 1000000;        # 10M of data
}

==================================

3. test.cgi

#!/usr/bin/perl

print "Content-type:text/html\n\n";

use strict;

use TestPM;

TestPM::init();

==================================

By stress testing the test.cgi, I found memory is not shared at all,
using the top command, e.g.

show/hide quoted text

11290 web 25 0 100m 23m 1620 R 47 0.3 0:00.29 /usr/
local/apache_1.3.41/bin/httpd
11247 web 20 0 101m 23m 1628 R 41 0.3 0:05.15 /usr/
local/apache_1.3.41/bin/httpd

As you can see, each httpd is using 23m, and 1620bytes shared, which I
belive the much data is shared....

Any idea?

Thanks.

Posted by Joost Diepenmaat on October 15, 2008, 6:48 am

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The only data that might be shared is data that is already initialized at
the startup phase. Data that's copied / assigned during at later stages
will not be shared. The manual and the book both mention this.

--
Joost Diepenmaat | blog: http://joost.zeekat.nl/ | work: http://zeekat.nl/

Posted by howa on October 15, 2008, 7:23 am

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Hello,

show/hide quoted text

Thanks for reply.

Even I put the line:

show/hide quoted text

inside the startup.pl, still the same

Posted by xhoster on October 15, 2008, 2:07 pm

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In addition to what the other post said, it should be pointed out that
the SHR column of the "top" command does not reflect all modes of memory
sharing. For example, Copy-On-Write sharing does not seem to accounted
for as shared. I think it reflects only shared libraries (*.so files) and
not any kind of data sharing. It would be nice if the "man top" did a
better job of explaining this. You need a better way of diagnosing how
much memory is actually being used.

Xho

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