Does long polling really work better than periodic ajax poll?

The overhead for long polling is http connection establishment, also on server side the 开发者_如何学运维Java EE server has to hold a lot of suspended requests.

For periodic polls with 10/20 seconds interval, it will work worse than long polling?

It depends on various things:

• How expensive is it on your client, server and network to make a new request? In some cases, the server may be able to handle "here's a new result - push it to anyone who cares" (from the backend) more easily than "have there been any results in the last 20 seconds" (from the client). Likewise new connections over a 3G network may be more expensive than keeping an existing one going.
• How important is it to get immediate results? (Think about a chat application, for example.)

(And no doubt there are others...)