首先给出Google到的答案：
The tp90 is a minimum time under which 90% of requests have been served.
tp90 = top percentile 90
Imagine you have response times:
10s
1000s
100s
2s
Calculating TP is very simple:
1. Sort all times in ascending order: [2s, 10s, 100s, 1000s]
2. find latest item in portion you need to calculate.
2.1 For TP50 it will be ceil(4*0.5) = 2 requests. You need 2nd request.
2.2 For TP90 it will be ceil(4*0.9) = 4. You need 4th request.
3. We get time for the item found above. TP50=10s. TP90=1000s
依此，翻译过来，TP99就是满足百分之九十九的网络请求所需要的最高耗时。同理TP999就是满足千分之九百九十九的网络请求所需要的最高耗时。
举个例子：有四次请求耗时分别为：
10ms，1000ms，100ms，2ms
那么我们可以这样计算TP99：4次请求中，99%的请求数为4*0.99，进位取整也就是4次，满足这全部4次请求的的最高耗时为1000ms，也就是TP99的答案是1000ms。