1.Make sure that your Apache is configured to accept as many as 400+ concurrent users. Here are instructions on calculating and setting.
2.Make sure that your JMeter is configured to produce as many as 400+ concurrent users. Here is the guide on proper JMeter tuning.
3.Make sure that your Apache and JMeter systems are not overloaded and have enough spare CPU, RAM, Network and Disk IO, disk space, swap, etc. You can use PerfMon JMeter Plugin for monitoring systems health.
以前用 JMeter 的經驗是
Server 還沒掛，client 自己先爆掉
不過原因通常是 Out of memory