Apache + PHP memory usage tuning #56

Though I don't know yet how we'll implement this, we'd like to — as part of the provisioning process — calculate a php memory_limit and several apache directives, based on the server's total RAM capacity.

Our target machine for a genesis install would have 2GB or more, while the local vagrant machine currently is allocated 1GB.

Note: I'm assuming #53 would mean Vagrant's memory usage would be negligible and roll into the overall system reserved memory.

First, we should calculate memory set aside for specific non-apache uses. I'm thinking the following:

• System reserved memory: 512M for a machine with 2G or more, otherwise 1/4 of total system memory
• MySQL reserved memory: 512M for a machine with 2G or more, otherwise 320M
• PHP memory_limit: 256M for a machine with 2G or more, otherwise 128M

And apache-specific settings:

• StartServers: 5 for a machine with 2G or more, otherwise 2
• MinSpareServers: 5 for a machine with 2G or more, otherwise 2
• MaxSpareServers: 10 for a machine with 2G or more, otherwise 3
• MaxRequestsPerChild: Pulling a figure out of thin air, but I'm thinking 5000 should work?
• MaxClients: the greater of these two:
  • ($TotalMemory - $ReservedSystemMemory - $ReservedMysqlMemory) / $PhpMemoryLimit
  • The number of vcpus the machine has

Thoughts?