multithreaded (threadsafe) not working for requests to the same url

Hi all,

In my application, a lot of time is spent on /inbox.
I'm running rails multithreaded (config.threadsafe) and verified this
is working OK.
However, it seems that requests to the same url, will block.

Simple test app:
./script/generate controller slowdown action1 action2
def action1
  sleep 15
end
def action2
  sleep 15
end
and turn on config.threadsafe in config/environments/production.rb
now run server in production mode

open 3 browser tabs, enter these urls:
- http://localhost:3000/slowdown/action1
- http://localhost:3000/slowdown/action1
- http://localhost:3000/slowdown/action2

You'll see that if you start the requests roughly at the same time,
the first action1 and action2 will complete at the same time.
The second action1 comes 15sec behind.

Passing non-used params (localhost:3000/slowdown/action1?blah=3) will
trick the app to
handle the request in parallel. Also, aliasing localhost does the
trick.

So it looks as if rails/rack does some locking per url.
I couldn't find anything though.

Is anyone aware of this behavior? What is the cause?
Thanks,
Mathijs