php Memory usage

20MB seems pretty much impossible for PHP5, even on a 32 bit architecture system (64 bit binaries are 50-100% larger than 32 bit binaries, and dynamic languages like PHP are hit dramatically harder by the 64 bit penalty than lower level static languages, so if 20MB PHP is possible on 32 bit machines, then 30-40MB would be the low end for PHP on 64 bit machines...and since it's dynamic, it's probably closer to 40 than 30). But, it is worth mentioning a few things...

PHP memory is heavily shared because a very large portion of PHP is just wrapping up existing system libraries. So, every PHP process uses a lot of the same shared libraries for functionality. Shared libraries are practically free beyond the first instance of php-cgi. So, while you're seeing 208MB, the unique memory usage for that process is very likely more like 70MB (that's about what we're seeing on our system, though total php-cgi process size is 219MB on our 64bit system; though the line between shared and not shared is very hard to determine, and tools like top never give the whole picture).

PHP memory usage can be modified by modifying what features you load into your PHP. The fewer features you load, the smaller it will be. The default php.ini on most modern Linux distros has everything and the kitchen sink turned on, by default. I have no idea how to guide you on which features you can safely disable. On RHEL/CentOS, you'll find that a number of modules are loaded automatically from /etc/php.d. Possibly there are many you don't need. I dunno. I've never experimented much.

Yes, we were misled a couple of years ago about what that option meant, but it's long been unset in Virtualmin by default. ;-)

Howdy -- a few comments --

The MySQL defaults tend to be pretty decent, for servers that aren't getting clobbered. Which are most servers. I suspect that's not your issue, but we can come back to that :-)

You asked if it's possible to limit the php-cgi programs per-server. There can only be one php-cgi running per Apache process, so you wouldn't ever have more than what MaxClients is set to in Apache.

What Joe is suggesting may be a good start -- you might want to tell PHP to stop loading every possible module.

That might help -- but if your system load is high, the thing to figure out is how many requests are you serving, and what are people requesting?

If your app is system intensive, you might need to streamline it a bit. Some apps support caching, for example.

Also, how much RAM do you have? What does the "free -m" command show?

Look at 'top' to see which processes are most demanding. Then we can figure out what steps to take to reduce load. If you have a lot of memory (4GB+), you'd want to switch to the my-huge.cnf configuration for MySQL. It increases the size of various caches in MySQL, and can reduce disk usage at the expense of having a bigger MySQL process. On the other hand, if you have little memory, we'd want to look for more ways to reduce memory usage--swapping in/out is a very demanding task, and so pretty much anything to reduce memory usage would be a win.

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