Want to wreck your afternoon? Just have a poorly configured WordPress install linked from Hacker News. Here's the postmortem.

In my case, my slice was freezing. I didn't know what the problem was until I ran top and saw this. Yikes.

The problem was the Apache is configured by default to allow up to 150 Apache processes. Each process took 5-10MB of real memory so my slice's 512MB was quickly overwhelmed. But why was it creating 150 processes in the first place? Shouldn't WP-SuperCache respond very quickly, such that the process can serve many requests per second? Yes, but...


Keep-Alives try to help client performance. This is a performance tweak that will kill you. By default, Apache is configured to hold the process locked for a given socket for 15 seconds (!!?) in case that socket makes another request. That's a terrible, terrible default: you should never lock resources waiting for human input. So in 15 seconds, Hacker News delivered me 50-100 requests. These requests all generated their own process, quickly overwhelming my RAM and swap and effectively freezing my slice.

I lowered the maximum number of processes (MaxClients) to 20 and the keep-alive timeout from 15 to 2 seconds. Before I was seeing load averages in the 100s and since reconfiguration, my slice's load average has been under 1 all afternoon. Here's the config I changed:

# KeepAliveTimeout: Number of seconds to wait for the next request from the
# same client on the same connection.

KeepAliveTimeout 2

## Server-Pool Size Regulation (MPM specific)

# prefork MPM
# StartServers: number of server processes to start
# MinSpareServers: minimum number of server processes which are kept spare
# MaxSpareServers: maximum number of server processes which are kept spare
# MaxClients: maximum number of server processes allowed to start

StartServers 5
MinSpareServers 5
MaxSpareServers 10
MaxClients 20

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