apache 2.2 - How much free memory should I have on my webserver?

I have a webserver that's currently hosting two Wordpress sites and
some java-based collaboration software. The server has 2G of memory and is currently
using about 1.8G of the available memory. Right now what's on here is pretty much a
pilot project that's getting negligible traffic so I think it's pretty clear that I'll
be needing more memory.

I was wondering, if I
was to release it, how I might anticipate my memory needs based on the traffic it gets.
I've poked around on Google and what I've found has been a bit tenuous. Is there a good
heuristic that one should use when calculating memory demands as a function of the base
(no traffic) load on the server?

For
reference, the output of free -m can be seen
below:

 total used free shared
            buffers cached
Mem: 2048 1832 215 0 0 0
-/+ buffers/cache: 1832
            215
Swap: 0 0
            0

To me this looks
like actual memory used and isn't an illusion due to caching or anything
else.

I figure the demands of my
collaboration software will have to be experimentally tested so here's free
-m without that software
running:

 total used free shared
            buffers cached 
Mem: 2048 1109 938 0 0 0 
-/+ buffers/cache: 1109
            938 
Swap: 0 0 0
            

My plan B to figure
this out is to add a bunch of swap space to the server, give it some traffic and adjust
according the the amount that swap gets used. I was just wondering if anyone had a good
rule of thumb to estimate how much memory I should plan on in advance...or if what I'm
thinking is nuts.

Many thanks in
advance (I'm really quite new to this).

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class="normal">Answer

The
answer is basically: It depends. In your specific case, the quality & efficiency of
the sites you have installed can come into
question.

For example, this week I was working
on a server that is relatively moderate traffic, yet eating up 4GB of RAM. After doing a
code review—the whole site is using an off-the-shelf CMS similar to WordPress but with
customizations—we discovered a major bottleneck. Once we recoded the issue away, the
server now uses about 2GB of RAM on an average day. A 50% drop in RAM
usage!

I would not recommend doing something
like split the server to use Ngnix as well as Apache. Just use Apache. The headaches of
balancing the configs of two different servers is not worth the marginal benefit. Apache
is fine. But that said, you should consider your WordPress PHP sites versus Java app
versus MySQL in on the same box. In my experience, you should do the
following:

1. Properly
   configure Apache: Apache is a good piece of software, but right out of
   the box it’s a memory hog. For example, I believe the default is to allow 255
   connections per second? I can assure you most simple sites barely get 40 connections per
   second on a good day. So adjusting Apache to be realistic to your traffic will help.
   Also, there is a KeepAlive setting in Apache that works great!
   But out of the box, I believe it’s set to a
   MaxKeepAliveRequests of 100 which is fairly nuts. I usually set
   this to about 30 connections with a small KeepAliveTimeout of 2
   to 3 seconds. The key is to have the KeepAliveTimeout to match
   speed it takes for an average page to download with a little bit of room for
   overhead/slowness. So if a page loads in 1 second, do a
   KeepAliveTimeout of 2
   seconds.


2. Review the code for your WordPress
   sites for potential bottlenecks: Concentrate on the PHP core of it &
   clear up what you can. Look out for excessive MySQL calls & file system calls. This
   is where you will be able to make the app fly! Also, check the
   memory_limit in your php.ini and make
   sure it’s not higher than necessary. The default is 64M, but in many cases that can be
   lowered to 32M.


3. MySQL tuning or moving it
   onto it’s own server: After writing about MySQL above I realized you
   might be hosting your MySQL instance on the same box. Look into optimizing MySQL
   performance by running a script like href="https://launchpad.net/mysql-tuning-primer" rel="noreferrer">MySQL tuning
   primer. Without tuning, MySQL will eat up all resources & big the system
   down. With tuning, MySQL will run better/faster & resources can be freed for other
   purposes. Also, consider moving your MySQL DB to a standalone server. You might have to
   learn how to properly network & firewall the server to allow your servers access but
   protect against hackers, but the performance benefit will be
   great.


4. Consider moving the Java app to
   another server: My rule of thumb is one major Java application per
   server. In general, Java apps can be memory hogs when compared to PHP setups like
   WordPress. By giving the Java app it’s own server to use, the WordPress sites will be
   much happier.

Regarding
the MySQL tuning, that is something that can take a few weeks to nail down at the
beginning. The reason being tuning scripts are based on real traffic MySQL sees. So you
basically make your site live to the world, wait 2 days (at least), run the tuning
scripts & then wait a few more days to tune some more. After a week or so you should
be able to tune MySQL to work as well as it can with your setup.