getting "reached maximum number of connections" error with large volume of http requests on google app engine

Question

I'm attempting to do some stress testing on my GAE application to see how it's performance holds up with a large number of simultaneous users. I tried having a 100 threads each send an https requests within 1 second, but half of them failed with a 503 status code the following message: "Error: Connection not allowed: reached maximum number of connections."

This is a paid app, so I tried upgrading the instance class and setting up some idle instances, but it doesn't seem to make any difference.

Is there a limit on the number of simultaneous connections? Or is this because all the requests are generated from the same host?

Thanks

EDIT: Response to Kyle: I'm using jmeter and sending 100 simultaneous requests to google.com doesn't ahve any issues. Response to Nick: I'm not expecting individual clients to send lots of simultaneous requests, I was trying to simulate 100 users sending 1 request each.

Answer 1

Unbeknownst to me, a colleague had added a custom throttling filter to our application :) I removed this from the web.xml and it solves the problem.