Support for websocket is not yet fully baked into Mongrel2. The handshake etc needs to be done on the handler side. So gevent solution is a better bet at the moment. The alternative is to have a separate Tornado/Node.js to handle websocket alongside Django/Pyramid app. Mongrel2 does look promising, although it seems zedshaw is pretty resource tide at the moment.

Don't run on your whole app on gevent, but only the part which needs streaming/long poll. By decreasing the surface area, you can pin point the bugs arising due to monkey patching.