How can I prevent tons of apache processes spawning when I start apache and proceeding to kill my machine?

You have probably made the error of configuring Apache to use far more than all of your ram. This is an easy mistake to make.

I am assuming you are using a Prefork Apache, and an in-process application server (such as PHP or mod_perl). In this model, you will end up with a maximum of (MaxClients * max memory usage of your application per process) memory used. If you don't have nearly that much, it's time to decrease one, the other or both.

In the general case, this means decreasing MaxClients to the point where your server has enough ram to cope.

The default values typically used for MaxClients (150 is typical) are not suitable for running an in-process heavyweight application server on a modest machine if you are using the Prefork model (Most application servers either don't support, or discourage, the use of threaded models).

However, decreasing MaxClients will eventually cause the application to become unavailable, particularly if you have keepalives on and the keepalive timeout too long. Processes which are just keeping a connection alive (state K in server-status) still use a lot of RAM, and that may be a problem - try to minimise keepalive timeout, or turn it off altogether.

You need to keep an eye on server-status (as provided by mod_status).

Of course you should only make ANY of these changes if you understand the consequences. Think twice, change the config once. If you have ANY ability to test the changes with simulated load on a similar spec non-production machine, do so.

use ps -aux | grep apache to find out the number of processes that apache is running on. Look out for the "RSS" column which gives an estimate of the memory used by each process. Alternatively you can use "top", where you shift + f and then select the %MEM column to sort the processes by memory usage.

The number of processes is determined by "MaxClients" directive in your apache.conf file. The way you come to this figure is as described by this page;

1. SSH into your server as root.
2. Run top.
3. Press shift + m.
4. Note the highest RES memory used by httpd.
5. Hit Q to exit top.
6. Execute: service httpd stop (In debian, sudo service apache2 stop)
7. Once httpd is stopped, execute: free -m
8. Note the memory listed under "used".
9. Find the guaranteed memory for your VPS plan. Support can tell you how much you have guaranteed if you cannot find it.
10. Subtract the memory USED from the memory that your plan is GUARANTEED. This will give you your base FREE MEMORY POOL.
11. Multiply the value of your FREE MEMORY POOL by 0.8 to find your average AVAILABLE APACHE POOL (this will allow you a 20% memory reserve for burst periods).
12. Divide your AVAILABLE APACHE POOL by the highest RES memory used by httpd. This will give you the MaxClients value that should be set for your system. (Round it to the nearest integer less than this value if it has a fraction component.)

The right value for "MaxClients" will ensure the right memory allocation for your apache server. That's how I solved it.

In Debian, apache conf file is at /etc/apache2/apache2.conf

Have you changed your configuration file recently? If yes, I trust you keep the old version for diffing?

If not, search for the "StartServers", "MaxSpareServers" and "MinSpareServers" directives. Generally you want to leave these at defaults, but it's possible that they were intentionally set high (bad idea) or accidentally set that way due to a bad config edit.

If this doesn't help, it's time to look outside Apache, for some process that's opening connections at a fast rate (could be that there's a testing process that's run amok).

First step is the access log. Second step is to run netstat, to see where the connections might be coming from. And if it's running on the same system, you can look in /proc/*/fd to find the two ends of the connection.