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The previous machine was a Duron 1.3gHz with 512M and 7200 RPM IDE, and the new one is a Pentium 4 2.0gHz with 1 GB o' RAM and a 7200 RPM drive, both running Apache 1.3.x. On the old machine I had my MaxClients set at 250 (w/o keepalive) and it seemed okay with acceptable performance -- on this new I've gone so far as to raise the the MaxClients to 400 in desperation (with 'ps' showing a count of 402 all the time) and it's still not keeping up (again, w/o keepalive), with requests timing out left and right. I don't think it's the connection itself as it's a 10mbps line and I have mod_throttle set at 500kb/sec and not yet getting any warnings; I don't think I'm saturating the line. I've copied the config file I used on the old machine to the new one so the configs are more or less the same. Other than the Apache config changes I've made the machine is otherwise a virgin RedHat 7.2 install (with Ensim, disabled).
The only two explainations I can figure out is a) it's trying to do reverse DNS lookups, or b) the disk is just too slow. I but I really think it should be able to handle this.
I have considered moving to an "alternative" web server that are said to handle high loads better such as Boa or Tux, but if I can salvage Apache it would be preferable.
It's times like this I think I'd like to move to Montana and keep some sheep and deal with no unit of time shorter than a season.
What have we learned? : High collision rates are not normal for a switched ethernet port and they will hurt performance if too high. All Apache problems are not caused by Apache directly. You can monitor your network performance in a very cursory manner using ifconfig under *nix or similar tools (ipconfig?) under Windows.
A lot of this stuff appears in no faq or manual, unfortunatly; it's one of those things you stumble on by accident and then kick yourself for not having checked it before.