I think it'll be faster for my foreign friends to connect to..

(sorry it seems that I have mistaken a forum..

You can add another Linode in another datacenter, sure.

There is no magic zero-latency connectivity between Linode datacenters, however. They connect to the Internet independently and are beholden to the laws of physics.

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I see, but the fact is that some places around the world even get "request time out" when ping my linode..
Will an extra IP take effect..?

Probably those users are not able to reach your Linode at all. Based on what others have posted, you can probably reduce ping time by 150 ms at most by choosing a different datacenter.

If you are able to communicate with the users that are having problems, ask if they can provide traceroute output. This will show you the path that packets are taking. (In Windows, the command is called tracert.)