Following somone from this forum's advice i installed eaccelerator, and was pleased to see the load decrease quite a bit.
However after several hours of everything running fine, i get the following error in apache:
[20678] EACCELERATOR: PHP crashed on opline 11 of wp_kses() at /home/unblog/www/wp-inst/wp-includes/kses.php:774
This happens for every single request and creates segmentation faults, resulting in "server error" pages being loaded on every single blog.
Any idea where this could be from ?
well, i found it:
http://eaccelerator.net/ticket/16
apparently its a EA problem, not WP. In case anyone else has those errors, solution seems to be to disable EA disk cache:
eaccelerator.shm_only="1"
in php.ini.
I found that the shm_only="1" solution didn't help my site as much as I'd like. I run other non-WP sites on my server, and this tweak affected site-wide performance. Instead, I added the following tweak to .htaccess in the root of wpmu.
***
php_flag eaccelerator.enable 0
php_flag eaccelerator.optimizer 0
***
I see no performace-loss anywhere on my server, and still haven't gotten the dread whitescreen-of-wordpress-death. I'll reply if the WSOWD comes back.
As a follow up, the above tweak I mentionned didn't really solve the problem, which reappeared later on.
I ended up removing EA and installing APC, which has worked wonders since.