I know this is an old posting. Could you solve your problem in the mean time?
> the OS is standard Ubuntu 10.04 package (Apache 2.2.14, MySQL 5.1.41 and PHP 5.3.2), 4Gb RAM .
Good choice. You can always add RAM, first make sure the available RAM is utilized.
> with installed APC Version 3.1.3p1. and Moodle 1.9.10
How did you install?
> When I set the parameter on admin->server->performance :
> 1. cachetype : eaccelerator and
> 2. rcache : Yes (as recommended for database records)
> There was a message (debugging on) :
> $CFG->eaccelerator is set to true but the required functions are not available. You need to have either eaccelerator or turckmmcache extensions installed, compiled with the shmem keys option enabled.
Aren't these three things: APC, eaccelerator and turckmmcache?
I haven't tried eaccelerator, but at least on Debian 5 you had to install it from the source: http://www.sysadminslife.com/linux/eaccelerator-php-beschleuniger-installation-unter-debian-5-lenny-und-ubuntu/ (Geman but you can follow the commands).
> * line 29 of lib/eaccelerator.class.php: call to debugging()
Possibly due to missing eacceleator.
It would be nice, if you could summerize you experience in the dicussion "Update on PHP-accelerators" http://moodle.org/mod/forum/discuss.php?d=168965
> I am running an elearning site with 2000 students active and approximately 350 courses.
More important is, how intensively the site is being used. See http://docs.moodle.org/en/Performance and http://docs.moodle.org/en/Performance_FAQ