Apache (with php and MySQL) was using too much memory, so I was told lighttpd would help. I installed lighttpd and disabled Apache. Lighttpd took over just fine.

But it's using even more mem. Apache (php+MySQL) was using around 256MB. Lighttpd (php+MySQL) is now using 315+ MB.

Did I do something wrong?

Doesn't sound like an apache, PHP, MySQL or Lighttpd problem but rather a code problem or you're system can't keep up with the amount of usage from users.

Describe using too much memory though? Was it constantly being used, causing the system to go down in performance or was most or some of it cached at all?

I run phpbb+mediawiki and I don't even get 80 unique hits a day.

Too much memory means above 256MB.

How much memory does the server have? Is the site getting slow or unresponsive? Why do you think it's too much memory? Do you have any idea how memory works in Linux?

Because past 256MB is Burst RAM, and my host says once I go above 256 I may be accessing RAM that is not available.

I personally run servers that run Apache, PHP and MySQL, hosting probably a dozen different sites with around 12 databases for each, ranging from Blogs to Forums, etc.

Right now, one system is using 1.5GB of memory out of 2GB of memory, probably a good 90% of the memory being consumed currently is split between mysql and apache. 1/3 of the 1.5GB is cached. Let me also mention this is a DNS and Postfix server as well.

So telling me 256MB is too much, I fail to see without any further details and I think you're wasting your time switching to lighttpd for something that probably isn't an issue to begin with.

Okay, now were getting somewhere.

So you have a host provider? Well, if they've only given you 256MB of memory, I fail to see how they allow you to go above that. Linux will fill all available memory it is allocated if it can. Most of it ends up getting cached. Have you run any other commands to find out if this is the case?

The hard limit is 512MB. Up to 256MB is guaranteed RAM, after that it's Burst RAM.