Month: May 2012

Storing PHP sessions in memcached instead of in files

in php.ini:

session.save_handler = memcache
session.save_path = "tcp://serv01:11211,tcp://serv02:11211,tcp://serv03:11211"

Obviously, you need php5-memcache installed, replace “serv1” “serv2” and “serv3” with valid server address(es), and you’ll need to restart your Apache server after making the change.

Why would you need to do this? Well, this week I had to deal with a web application server pool that kept slowly increasing its number of children all the way up to MaxCli, no matter what. It was unpredictable, other than being a slow creep. Eventually, stracing the pids showed that they were getting stuck in FLOCK on files in /var/lib/php5/php_sess*. This turns out to be an endemic problem with PHP that the devs don’t seem inclined to fix: php’s garbage collector will delete session files dirtily if a php process (which in the case of mod_php, means an Apache process) violates any of the php limits, such as max_execution_time (among many, many others). So you end up with your php script trying to lock a session file (file descriptor 3) that php’s garbage collector already deleted, and therefore an infinitely hung process that will never go away on its own.

Changing over to using memcache to store php sessions eliminated this file lock issue and resulted in a much more stable situation – a server pool that had been creeping up to 800 children per server over the course of a couple hours has been running stable and sweet on less than 150 children per server for days now.

Enabling core dumps on Apache2.2 on Debian

It was quite an adventure today, figuring out how to get a segfaulting Apache to give me core dumps to analyze on Debian Squeeze. What SHOULD have been easy… wasn’t. Here’s what all you must do:

First of all, you’ll need to set ulimit -c unlimited in your /etc/init.d/apache2 script’s start section.

case $1 in
start)
log_daemon_msg "Starting web server" "apache2"
# set ulimit for debugging
# ulimit -c unlimited

Now make a directory for core dumps – mkdir /tmp/apache2-dumps ; chmod 777 /tmp/apache2-dumps – then you’ll need to apt-get install apache2-dbg libapr1-dbg libaprutil1-dbg …

And, the current (Debian Squeeze, in May 2012) version of Debian does not have PIE support in the default gdb, so you’ll need to install gdb from backports. So, add deb http://backports.debian.org/debian-backports squeeze-backports main to /etc/apt/sources.list, then apt-get update && apt-get install -t squeeze-backports gdb …

Now add “CoreDumpDirectory /tmp/apache2-dumps” to /etc/apache2/apache2.conf (or its own file in conf.d, whatever), then /etc/init.d/apache2 stop ; /etc/init.d/apache2 start …

And once you start getting segfaults, you’ll get a core in /tmp/apache2-dumps/core.

Finally, now that you have your core, you can gdb apache2 /tmp/apache2-dumps/core, bt full, and debug to your heart’s content. WHEW.