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低耗时。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低耗时。

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次请求的的最低耗时为1000ms，也就是TP99的答案是1000ms。

如有纰漏，欢迎斧正。