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
理解：
TP50 表示4次请求，响应时间[2s, 10s, 100s, 1000s] 中，4*0.5 =2前2次请求取top，也就是[2s,10s]里面取最大值为10s。
TP90 表示4次请求，响应时间[2s, 10s, 100s, 1000s] 中，4*0.9 约等于4次请求中取top，也就是说从[2s, 10s, 100s, 1000s]里面取响应时间最大的值为1000s。