=head1 NAME

mod_perl_tuning - mod_perl performance tuning

=head1 DESCRIPTION

Described here are examples and hints on how to configure a mod_perl
enabled Apache server, concentrating on tips for configuration for
high-speed performance.  The primary way to achieve maximal
performance is to reduce the resources consumed by the mod_perl
enabled HTTPD processes.

This document assumes familiarity with Apache configuration directives
some familiarity with the mod_perl configuration directives, and that
you have already built and installed a mod_perl enabled Apache server.
Please also read the mod_perl documentation that comes with mod_perl
for programming tips.  Some configurations below use features from
mod_perl version 1.03 which were not present in earlier versions.

These performance tuning hints are collected from my experiences in
setting up and running servers for handling large promotional sites,
such as The Weather Channel's "Blimp Site-ings" game, the MSIE 4.0
"Subscribe to Win" game, and the MSN Million Dollar Madness game.

=head1 BASIC CONFIGURATION

The basic configuration for mod_perl is as follows.  In the
F<httpd.conf> file, I add configuration parameters to make the
C<http://www.domain.com/programs> URL be the base location for all
mod_perl programs.  Thus, access to
C<http://www.domain.com/programs/printenv> will run the printenv
script, as we'll see below.  Also, any *.perl file will be interpreted
as a mod_perl program just as if it were in the programs directory,
and *.rperl will be mod_perl, but I<without> any HTTP headers
automatically sent; you must do this explicitly.  If you don't want
these last two, just leave it out of your configuration.

In the configuration files, I use F</var/www> as the C<ServerRoot>
directory, and F</var/www/docs> as the C<DocumentRoot>.  You will need
to change it to match your particular setup.  The network address below
in the access to perl-status should also be changed to match yours.

Additions to F<httpd.conf>:

 # put mod_perl programs here
 # startup.perl loads all functions that we want to use within mod_perl
 Perlrequire /var/www/perllib/startup.perl
 <Directory /var/www/docs/programs>
   AllowOverride None
   Options ExecCGI
   SetHandler perl-script
   PerlHandler Apache::Registry
   PerlSendHeader On
 </Directory>

 # like above, but no PerlSendHeaders
 <Directory /var/www/docs/rprograms>
   AllowOverride None
   Options ExecCGI
   SetHandler perl-script
   PerlHandler Apache::Registry
   PerlSendHeader Off
 </Directory>

 # allow arbitrary *.perl files to be scattered throughout the site.
 <Files *.perl>
   SetHandler perl-script
   PerlHandler Apache::Registry
   PerlSendHeader On
   Options +ExecCGI
 </Files>

 # like *.perl, but do not send HTTP headers
 <Files *.rperl>
   SetHandler perl-script
   PerlHandler Apache::Registry
   PerlSendHeader Off
   Options +ExecCGI
 </Files>

 <Location /perl-status>
   SetHandler perl-script
   PerlHandler Apache::Status
   order deny,allow
   deny from all
   allow from 204.117.82.
 </Location>

Now, you'll notice that I use a C<PerlRequire> directive to load in the
file F<startup.perl>.  In that file, I include all of the C<use>
statements that occur in any of my mod_perl programs (either from the
programs directory, or the *.perl files).  Here is an example:

 #! /usr/local/bin/perl
 use strict;

 # load up necessary perl function modules to be able to call from Perl-SSI
 # files.  These objects are reloaded upon server restart (SIGHUP or SIGUSR1)
 # if PerlFreshRestart is "On" in httpd.conf (as of mod_perl 1.03).

 # only library-type routines should go in this directory.

 use lib "/var/www/perllib";

 # make sure we are in a sane environment.
 $ENV{GATEWAY_INTERFACE} =~ /^CGI-Perl/ or die "GATEWAY_INTERFACE not Perl!";

 use Apache::Registry ();	# for things in the "/programs" URL

 # pull in things we will use in most requests so it is read and compiled
 # exactly once
 use CGI (); CGI->compile(':all');
 use CGI::Carp ();
 use DBI ();
 use DBD::mysql ();

 1;

What this does is pull in all of the code used by the programs (but
does not C<import> any of the module methods) into the main HTTPD
process, which then creates the child processes with the code already
in place.  You can also put any new modules you like into the
F</var/www/perllib> directory and simply C<use> them in your
programs.  There is no need to put C<use lib "/var/www/perllib";> in
all of your programs.  You do, however, still need to C<use> the
modules in your programs.  Perl is smart enough to know it doesn't
need to recompile the code, but it does need to C<import> the module
methods into your program's name space.

If you only have a few modules to load, you can use the PerlModule
directive to pre-load them with the same effect.

The biggest benefit here is that the child process never needs to
recompile the code, so it is faster to start, and the child process
actually shares the same physical copy of the code in memory due to
the way the virtual memory system in modern operating systems works.

You will want to replace the C<use> lines above with modules you
actually need.

=head2 Simple Test Program

Here's a sample script called F<printenv> that you can stick in the
F<programs> directory to test the functionality of the configuration.

 #! /usr/local/bin/perl
 use strict;
 # print the environment in a mod_perl program under Apache::Registry

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

 print "<HEAD><TITLE>Apache::Registry Environment</TITLE></HEAD>\n";

 print "<BODY><PRE>\n";
 print map { "$_ = $ENV{$_}\n" } sort keys %ENV;
 print "</PRE></BODY>\n";

When you run this, check the value of the GATEWAY_INTERFACE variable
to see that you are indeed running mod_perl.

=head1 REDUCING MEMORY USE

As a side effect of using mod_perl, your HTTPD processes will be
larger than without it.  There is just no way around it, as you have
this extra code to support your added functionality.

On a very busy site, the number of HTTPD processes can grow to be
quite large.  For example, on one large site, the typical HTTPD was
about 5Mb large.  With 30 of these, all of RAM was exhausted, and we
started to go to swap.  With 60 of these, swapping turned into
thrashing, and the whole machine slowed to a crawl.

To reduce thrashing, limiting the maximum number of HTTPD processes to
a number that is just larger than what will fit into RAM (in this
case, 45) is necessary.  The drawback is that when the server is
serving 45 requests, new requests will queue up and wait; however, if
you let the maximum number of processes grow, the new requests will
start to get served right away, I<but> they will take much longer to
complete.

One way to reduce the amount of real memory taken up by each process
is to pre-load commonly used modules into the primary HTTPD process so
that the code is shared by all processes.  This is accomplished by
inserting the C<use Foo ();> lines into the F<startup.perl> file for
any C<use Foo;> statement in any commonly used Registry program.  The
idea is that the operating system's VM subsystem will share the data
across the processes.

You can also pre-load Apache::Registry programs using the
C<Apache::RegistryLoader> module so that the code for these programs
is shared by all HTTPD processes as well.

B<NOTE>: When you pre-load modules in the startup script, you may
need to kill and restart HTTPD for changes to take effect.  A simple
C<kill -HUP> or C<kill -USR1> will not reload that code unless you
have set the C<PerlFreshRestart> configuration parameter in
F<httpd.conf> to be "On".

=head1 REDUCING THE NUMBER OF LARGE PROCESSES

Unfortunately, simply reducing the size of each HTTPD process is not
enough on a very busy site.  You also need to reduce the quantity of
these processes.  This reduces memory consumption even more, and
results in fewer processes fighting for the attention of the CPU.  If
you can reduce the quantity of processes to fit into RAM, your
response time is increased even more.

The idea of the techniques outlined below is to offload the normal
document delivery (such as static HTML and GIF files) from the
mod_perl HTTPD, and let it only handle the mod_perl requests.  This
way, your large mod_perl HTTPD processes are not tied up delivering
simple content when a smaller process could perform the same job more
efficiently.

In the techniques below where there are two HTTPD configurations, the
same httpd executable can be used for both configurations; there is no
need to build HTTPD both with and without mod_perl compiled into it.
With Apache 1.3 this can be done with the DSO configuration -- just
configure one httpd invocation to dynamically load mod_perl and the
other not to do so.

These approaches work best when most of the requests are for static
content rather than mod_perl programs.  Log file analysis become a bit
of a challenge when you have multiple servers running on the same
host, since you must log to different files.

=head2 TWO MACHINES

The simplest way is to put all static content on one machine, and all
mod_perl programs on another.  The only trick is to make sure all
links are properly coded to refer to the proper host.  The static
content will be served up by lots of small HTTPD processes (configured
I<not> to use mod_perl), and the relatively few mod_perl requests
can be handled by the smaller number of large HTTPD processes on the
other machine.

The drawback is that you must maintain two machines, and this can get
expensive.  For extremely large projects, this is the best way to go.

=head2 TWO IP ADDRESSES

Similar to above, but one HTTPD runs bound to one IP address, while
the other runs bound to another IP address.  The only difference is
that one machine runs both servers.  Total memory usage is reduced
because the majority of files are served by the smaller HTTPD
processes, so there are fewer large mod_perl HTTPD processes sitting
around.

This is accomplished using the F<httpd.conf> directive C<BindAddress>
to make each HTTPD respond only to one IP address on this host.  One
will have mod_perl enabled, and the other will not.

=head2 TWO PORT NUMBERS

If you cannot get two IP addresses, you can also split the HTTPD
processes as above by putting one on the standard port 80, and the
other on some other port, such as 8042.  The only configuration
changes will be the C<Port> and log file directives in the httpd.conf
file (and also one of them does not have any mod_perl directives).

The major flaw with this scheme is that some firewalls will not allow
access to the server running on the alternate port, so some people
will not be able to access all of your pages.

If you use this approach or the one above with dual IP addresses, you
probably do not want to have the *.perl and *.rperl sections from the
sample configuration above, as this would require that your primary
HTTPD server be mod_perl enabled as well.

Thanks to Gerd Knops for this idea.

=head2 USING ProxyPass WITH TWO SERVERS

To overcome the limitation of the alternate port above, you can use
dual Apache HTTPD servers with just slight difference in
configuration.  Essentially, you set up two servers just as you would
with the two port on same IP address method above.  However, in your
primary HTTPD configuration you add a line like this:

 ProxyPass /programs http://localhost:8042/programs

Where your mod_perl enabled HTTPD is running on port 8042, and has
only the directory F<programs> within its DocumentRoot.  This assumes
that you have included the mod_proxy module in your server when it was
built.

Now, when you access http://www.domain.com/programs/printenv it will
internally be passed through to your HTTPD running on port 8042 as the
URL http://localhost:8042/programs/printenv and the result relayed
back transparently.  To the client, it all seems as if it is just one
server running.  This can also be used on the dual-host version to
hide the second server from view if desired.

=begin html
<P>
A complete configuration example of this technique is provided by
two HTTPD configuration files.
<A HREF="httpd.conf.txt">httpd.conf</A> is for the main server for all
regular pages, and <A HREF="httpd%2bperl.conf.txt">httpd+perl.conf</A> is
for the mod_perl programs accessed in the <CODE>/programs</CODE> URL.
</P>

The directory structure assumes that F</var/www/documents> is the
C<DocumentRoot> directory, and the the mod_perl programs are in
F</var/www/programs> and F</var/www/rprograms>.  I start them as
follows:

 daemon httpd
 daemon httpd -f conf/httpd+perl.conf

=end html

Thanks to Bowen Dwelle for this idea.

=head2 SQUID ACCELERATOR

Another approach to reducing the number of large HTTPD processes on
one machine is to use an accelerator such as Squid (which can be found
at http://squid.nlanr.net/Squid/ on the web) between the clients and
your large mod_perl HTTPD processes.  The idea here is that squid will
handle the static objects from its cache while the HTTPD processes
will handle mostly just the mod_perl requests once the cache is
primed.  This reduces the number of HTTPD processes and thus reduces
the amount of memory used.

To set this up, just install the current version of Squid (at this
writing, this is version 1.1.22) and use the RunAccel script to start
it.  You will need to reconfigure your HTTPD to use an alternate port,
such as 8042, rather than its default port 80.  To do this, you can
either change the F<httpd.conf> line C<Port> or add a C<Listen>
directive to match the port specified in the F<squid.conf> file.
Your URLs do not need to change.  The benefit of using the C<Listen>
directive is that redirected URLs will still use the default port 80
rather than your alternate port, which might reveal your real server
location to the outside world and bypass the accelerator.

In the F<squid.conf> file, you will probably want to add C<programs>
and C<perl> to the C<cache_stoplist> parameter so that these are
always passed through to the HTTPD server under the assumption that
they always produce different results.

This is very similar to the two port, ProxyPass version above, but the
Squid cache may be more flexible to fine tune for dynamic documents
that do not change on every view.  The Squid proxy server also seems
to be more stable and robust than the Apache 1.2.4 proxy module.

One drawback to using this accelerator is that the logfiles will
always report access from IP address 127.0.0.1, which is the local
host loopback address.  Also, any access permissions or other user
tracking that requires the remote IP address will always see the local
address.  The following code uses a feature of recent mod_perl
versions (tested with mod_perl 1.16 and Apache 1.3.3) to trick Apache
into logging the real client address and giving that information to
mod_perl programs for their purposes.

First, in your F<startup.perl> file add the following code:

 use Apache::Constants qw(OK);

 sub My::SquidRemoteAddr ($) {
   my $r = shift;

   if (my ($ip) = $r->header_in('X-Forwarded-For') =~ /([^,\s]+)$/) {
     $r->connection->remote_ip($ip);
   }

   return OK;
 }

Next, add this to your F<httpd.conf> file:

 PerlPostReadRequestHandler My::SquidRemoteAddr

This will cause every request to have its C<remote_ip> address
overridden by the value set in the C<X-Forwarded-For> header added by
Squid.  Note that if you have multiple proxies between the client and
the server, you want the IP address of the last machine before your
accelerator.  This will be the right-most address in the
X-Forwarded-For header (assuming the other proxies append their
addresses to this same header, like Squid does.)

If you use apache with mod_proxy at your frontend, you can use Ask
Bjørn Hansen's mod_proxy_add_forward module from
ftp://ftp.netcetera.dk/pub/apache/ to make it insert the
C<X-Forwarded-For> header.

=head1 SUMMARY

To gain maximal performance of mod_perl on a busy site, one must
reduce the amount of resources used by the HTTPD to fit within what
the machine has available.  The best way to do this is to reduce
memory usage.  If your mod_perl requests are fewer than your static
page requests, then splitting the servers into mod_perl and
non-mod_perl versions further allows you to tune the amount of
resources used by each type of request.  Using the C<ProxyPass>
directive allows these multiple servers to appear as one to the
users.  Using the Squid accelerator also achieves this effect, but
Squid takes care of deciding when to acccess the large server
automatically.

If all of your requests require processing by mod_perl, then the only
thing you can really do is throw a I<lot> of memory on your machine
and try to tweak the perl code to be as small and lean as possible,
and to share the virtual memory pages by pre-loading the code.

=head1 AUTHOR

This document is written by Vivek Khera.  If you need to contact me,
just send email to the mod_perl mailing list.

This document is copyright (c) 1997-1998 by Vivek Khera.

If you have contributions for this document, please post them to the
mailing list.  Perl POD format is best, but plain text will do, too.

If you need assistance, contact the mod_perl mailing list at
modperl@perl.apache.org first (send 'subscribe' to modperl-request@apache.org
to subscribe). There are lots of people there that can help. Also, 
check the web pages http://perl.apache.org/ and http://www.apache.org/
for explanations of the configuration options.

$Revision: 177689 $
$Date: 2002-03-24 21:57:59 -0500 (Sun, 24 Mar 2002) $
