Recently I read some articles about Pastry DHT.The articles said that Pastry DHT has an efficient routing.In Pastry's routing,each step's node ID has a longer common prefix with the destination node,but node IDs are assigned randomly so it is possible that message travel a very very long distance before it arrive the destination and as a result the routing is not efficient.
For example,a Pastry routing,the destination node ID is d467c4,the starting node ID is 65a1fc,the routing process is 65a1fc->d13da3->d4213f->d462ba->d46702->d467c4.It is possible that nodes on this routing are all over the world（IDs are assigned randomly）.The message will travel around the world before it arrive the final node.So this routing is not efficient.
So why Pastry DHT has an efficient routing?
That depends on your notion of efficiency. When designing overlay networks the first concern usually is to bound the total number of hops relative to the network size. In other words if there are n nodes you don't want O(n) routes, O(log n) is the usual goal because it can be achieved without total network awareness.
Route length in terms of latency, path cost or minimum bandwidth along the link are second-rank concerns. That are often achieved by adding some sort of locality-awareness or clustering after the hop-length has been optimized.
Pastry is efficient for the hop metric.