After a day of trying to figure out why one of my web servers was locking up, I found that it was using a bit too much memory. But I had no idea how much, and Linux memory reporting is a bit arcane at best. Especially with something like apache + PHP using shared memory pools. So after some analysis, I came up with the following script:

apachemem.pl

It had the following output:

Total memory available: 3.21G
Total used by apache2 (451 instances): 3.80G
Total used by other processes: 0.12G

Average memory used per apache2 process: 8.63M
Recommended number of processes based on Average: 381
Needed memory for 500 processes based on Average: 4.21G

Max memory used for apache2 process: 17.61M
Recommended number of processes based on Max: 186
Needed memory for 500 processes based on Max: 8.60G

Mean plus two Standard Deviations (bulk of usage under max): 11.28M
Recommended number of processes based on Mean + 2*Stdev: 291
Needed memory for 500 processes based on Mean + 2*Stdev: 5.51G

At the time I ran it, I had 450 instances out of a configured max of 700 active. Come to find out I only had the ram to support about 380 of those at best. :(

Hopefully some other people find this useful, as it's pretty hard to get a straight answer on "How many processes can I support?" Now if only php and the standard modules would officially work with the worker MPM.