就某種演算法而言：
performance 效能，可能是某演算法的預測（分類）精度、耗時等指標。
scalability 延展性，即規模可調整。 應用程式應對負載改變的能力，比如，一個系統在伺服器於100個users和10000個users時的效能差別，如果這種差別在可以接收範圍內，就說明這種系統延展性強。或者系統的效能隨硬體條件的增加而同步增加，基本成比例增長。如一個伺服器的回應時間是100ms，四個是25ms，當然這是理想情況。它有horizontal 和vertical之分，如cluster。
衡量某演算法的scalability的時候，應該是指演算法應對資料樣本劇增時的穩定性吧。
Scalability
In some large-scale systems, it is hard to predict the number and behavior of end users. Scalability refers to a system’s ability to support fast increasing numbers of users. The intuitive way to scale up the number of concurrent sessions handled by a server is to add resources (memory, CPU or hard disk) to it. Clustering is an alternative way to resolve the scalability issue. It allows a group of servers to share the heavy tasks, and operate as a single server logically.
from:http://hi.baidu.com/wmyqzq/blog/item/f62e398bf507201fc8fc7a00.html
