I was about to write some thoughts down about a dot-matrix LCD display…. when my server failed to respond. It did reply to a PING, so the network connectivity was OK. When I was able to log in again a few minutes later, I noticed an insane load.
load average: 47.38, 80.48, 55.56
80?!?!?!?!?! what the fuck is my server doing!?!?!?! What has caused this behaviour? Is it a Denial-of-Service attack? If so, who has interest in bringing my server down?

Anyways, apart from the insane load, and the temporary being unavailable of the server, the real problem is this condition causes the server to run out of memory eventually, causing the kernel to start killing tasks, and when it does so, clamd is most likely the victim. When clamd is down, the mail server will refuse to handle any more mails. The default behaviour in case of out-of-memory is to kill the task using most memory. Clamav is a memory-hungry daemon. Why does it use that much memory? Anyhow, I am wondering if there is a way to exclude a specific process from the list of potential-to-be-killed tasks?

I believe the high load is caused due nearly running out-of-memory, causing heavy swap usage. Anyhow, everytime this event appears, it is caused by apache. So, that raises the question if there is a way to prevent apache (and anything that runs under it, such as php) from using too much memory?

Last this problem occurred, there were 128 instances of apache in memory. I guess, I should reduce this to a way lower value. I have been having problems like this before, and as solution I have started using mpm prefork. Its configuration allows me to set the maximum number of instances, and I have reduced this to a lower value. I hope this prevents this problem from appearing in the future.

So, in the end, it appears to be a simple misconfiguration. No attack whatsoever.