Focus: Linux, Apache 1.3+, [PHP], [MySQL]
Notes: Use at your own risk. If this has any errors, please let me know and I will correct them.

Summary
If you are reaching the limits of your server running Apache serving a lot of dynamic content, you can either spend thousands on new equipment or reduce bloat to increase your server capacity by anywhere from 2 to 10 times. This article concentrates on important and poorly-documented ways of increasing capacity without additional hardware.

Problems
There are a few common things that can cause server load problems, and a thousand uncommon. Let's focus on the common:
Drive Swapping - too many processes (or runaway processes) using too much RAM
CPU - poorly optimized DB queries, poorly optimized code, runaway processes
Network - hardware limits, moron attacks

Solutions: The Obvious
Briefly, and for completeness, here are the most obvious solutions:

Use "TOP" and "PS axu" to check for processes that are using too much CPU or RAM.
Use "netstat -anp | sort -u" to check for network problems.

Solutions: Apache's RAM Usage
First and most obvious, Apache processes use a ton a RAM. This minor issue becomes a major issue when you realize that after each process has done its job, the bloated process sits and spoon-feed data to the client, instead of moving on to bigger and better things. This is further compounded by a bit of essential info that should really be more common knowledge:

If you serve 100% static files with Apache, each httpd process will use around 2-3 megs of RAM.
If you serve 99% static files & 1% dynamic files with Apache, each httpd process will use from 3-20 megs of RAM (depending on your MOST complex dynamic page).

This occurs because a process grows to accommodate whatever it is serving, and NEVER decreases again unless that process happens to die. Quickly, unless you have very few dynamic pages and major traffic fluctuation, most of your httpd processes will take up an amount of RAM equal to the largest dynamic script on your system. A smart web server would deal with this automatically. As it is, you have a few options to manually improve RAM usage.

Reduce wasted processes by tweaking KeepAlive
This is a tradeoff. KeepAliveTimeout is the amount of time a process sits around doing nothing but taking up space. Those seconds add up in a HUGE way. But using KeepAlive can increase speed for both you and the client - disable KeepAlive and the serving of static files like images can be a lot slower. I think it's best to have KeepAlive on, and KeepAliveTimeout very low (like 1-2 seconds).

Limit total processes with MaxClients
If you use Apache to serve dynamic content, your simultaneous connections are severely limited. Exceed a certain number, and your system begins cannibalistic swapping, getting slower and slower until it dies. IMHO, a web server should automatically take steps to prevent this, but instead they seem to assume you have unlimited resources. Use trial & error to figure out how many Apache processes your server can handle, and set this value in MaxClients. Note: the Apache docs on this are misleading - if this limit is reached, clients are not "locked out", they are simply queued, and their access slows. Based on the value of MaxClients, you can estimate the values you need for StartServers, MinSpareServers, & MaxSpareServers.

Force processes to reset with MaxRequestsPerChild
Forcing your processes to die after a while makes them start over with low RAM usage, and this can reduce total memory usage in many situations. The less dynamic content you have, the more useful this will be. This is a game of catch-up, with your dynamic files constantly increasing total RAM usage, and restarting processes constantly reducing it. Experiment with MaxRequestsPerChild - even values as low as 20 may work well. But don't set it too low, because creating new processes does have overhead. You can figure out the best settings under load by examining "ps axu --sort:rss". A word of warning, using this is a bit like using heroin. The results can be impressive, but are NOT consistent - if the only way you can keep your server running is by tweaking this, you will eventually run into trouble. That being said, by tweaking MaxRequestsPerChild you may be able to increase MaxClients as much as 50%.

Apache Further Tweaking
For mixed purpose sites (say image galleries, download sites, etc.), you can often improve performance by running two different apache daemons on the same server. For example, we recently compiled apache to just serve up images (gifs,jpegs,png etc). This way for a site that has thousands of stock photos. We put both the main apache and the image apache on the same server and noticed a drop in load and ram usage. Consider a page had about 20-50 image calls -- the were all off-loaded to the stripped down apache, which could run 3x more servers with the same ram usage than the regular apache on the server.

Finally, think outside the box: replace or supplement Apache

Use a 2nd server
You can use a tiny, lightning fast server to handle static documents & images, and pass any more complicated requests on to Apache on the same machine. This way Apache won't tie up its multi-megabyte processes serving simple streams of bytes. You can have Apache only get used, for example, when a php script needs to be executed. Good options for this are:

TUX / "Red Hat Content Accelerator" - http://www.redhat.com/docs/manuals/tux/
kHTTPd - http://www.fenrus.demon.nl/
thttpd - http://www.acme.com/software/thttpd/
Try lingerd
Lingerd takes over the job of feeding bytes to the client after Apache has fetched the document, but requires kernel modification. Sounds pretty good, haven't tried it. lingerd - http://www.iagora.com/about/software/lingerd/

Use a proxy cache
A proxy cache can keep a duplicate copy of everything it gets from Apache, and serve the copy instead of bothering Apache with it. This has the benefit of also being able to cache dynamically generated pages, but it does add a bit of bloat.

Replace Apache completely
If you don't need all the features of Apache, simply replace it with something more scalable. Currently, the best options appear to be servers that use a non-blocking I/O technology and connect to all clients with the same process. That's right - only ONE process. The best include:

thttpd - http://www.acme.com/software/thttpd/
Caudium - http://caudium.net/index.html
Roxen - http://www.roxen.com/products/webserver/
Zeus ($$) - http://www.zeus.co.uk
Solutions: PHP's CPU & RAM Usage
Compiling PHP scripts is usually more expensive than running them. So why not use a simple tool that keeps them precompiled? I highly recommend Turck MMCache. Alternatives include PHP Accelerator, APC, & Zend Accelerator. You will see a speed increase of 2x-10x, simple as that. I have no stats on the RAM improvement at this time.

Solutions: Optimize Database Queries
This is covered in detail everywhere, so just keep in mind a few important notes: One bad query statement running often can bring your site to its knees. Two or three bad query statements don't perform much different than one. In other words, if you optimize one query you may not see any server-wide speed improvement. If you find & optimize ALL your bad queries you may suddenly see a 5x server speed improvement. The log-slow-queries feature of MySQL can be very helpful.

How to log slow queries:

Code: 
# vi /etc/rc.d/init.d/mysqld
Find this line:
Code: 
SAFE_MYSQLD_OPTIONS="--defaults-file=/etc/my.cnf"
change it to:
Code: 
SAFE_MYSQLD_OPTIONS="--defaults-file=/etc/my.cnf --log-slow-queries=/var/log/slow-queries.log"
As you can see, we added the option of logging all slow queries to /var/log/slow-queries.log
Code: 
Close and save mysqld. Shift + Z + Z

touch /var/log/slow-queries.log
chmod 644 /var/log/slow-queries.log

restart mysql
service myslqd restart
mysqld will log all slow queries to this file.

References
These sites contain additional, more well known methods for optimization.

Tuning Apache and PHP for Speed on Unix - http://php.weblogs.com/tuning_apache_unix
Getting maximum performance from MySQL - Objekt nicht gefunden!
System Tuning Info for Linux Servers - Server Oriented System Tuning Info
mod_perl Performance Tuning (applies outside perl) - mod_perl: Performance Tuning
Once again, if this has any errors or important omissions, please let me know and I will correct them.
If you experience a capacity increase on your server after trying the optimizations, let me know!

Article written by spagmoid additions by: albo,huck on the ev1 forums.
Areon Reviewed by Areon on . Optimize and Tweak High-Traffic Servers Focus: Linux, Apache 1.3+, , Notes: Use at your own risk. If this has any errors, please let me know and I will correct them. Summary If you are reaching the limits of your server running Apache serving a lot of dynamic content, you can either spend thousands on new equipment or reduce bloat to increase your server capacity by anywhere from 2 to 10 times. This article concentrates on important and poorly-documented ways of increasing capacity without additional hardware. Problems Rating: 5