Benchmarking on OSX: HTTP timeouts!

I’ve been doing some HTTP benchmarking on OSX lately, using ab (ApacheBench). After a large volume of requests, I always ended up with connection timeouts. I used to blame my application and mentally filed it as “must investigate”.

I was wrong.

The problem here was OSX, which seems to only have roughly 16000 ports available for connections. A port that was used by a closed connection is only released after 15 seconds. Quick calculation shows that you can only do a sustained rate of 1000 connections per second. Try to do more and you’ll end up with timeouts.

That’s not acceptable for testing pretty much anything that scales.


Here’s the workaround: you can control the 15 seconds release delay with sysctl:

sudo sysctl -w net.inet.tcp.msl=100

There’s probably a good reason why it’s in there, so you might want to revert this value once you are done testing:

sudo sysctl -w net.inet.tcp.msl=15000


Alternatively, you could just use Linux if you want to get some real work done.

April 5, 2014 18:57 #performance