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We're hosting 10 websites with (most of them using Drupal 6 or Drupal 7) using nginx + php-fpm. Our VPS is low end (only 1GB of RAM with 64 bit OS and limited CPU). The amount of visitors is moderate to low, but simultaneous traffic in many of them is enough to get timeouts.
1) Is it correct to use php-fpm in this scenario, or it would be better to go back to Apache and mod_php?
2) Is it better to keep a fpm pool for each site, or in this extreme condition using one pool for many sites would make any difference? (Security issues aside)
3) Is there a formula for calculating the best values for 'pm.max_children', 'pm.start_servers', 'pm.min_spare_servers', 'pm.max_spare_servers' and APC settings (enabled using default settings under Ubuntu 12.04 LTS)?
In general, any recommendations about running php-fpm with low resources would be greatly appreciated.