问题
I just ran a load test against my app. I noticed some very large variability in latency for two identical requests: 3 s vs. 30 s. When I dug into the traces I found the following:
| | Traced (ms) | Untraced (ms) |
|----------------------+-------------+---------------|
| High-latency Request | 193 | 29948 |
| Low-latency Request | 305 | 2934 |
Here are screen shots for the traces:
Low overall latency
High overall latency
I cannot make sense of a 10-to-1 difference in runtime performance.
I only see these high-latency requests under load. Could something in my code account for this variability (assuming the same path through the code was followed for both requests)?
回答1:
In my experience, those extraordinarily high latencies are caused by a fresh instance starting up. There was a discussion in the AppEngine Google Group which is about similar issues: https://groups.google.com/d/msg/google-appengine/MBveo1KSTyY/mkYdyCmfAgAJ
Two things you can do:
- reduce time it takes an instance to start by reducing overall size of your application (code, libraries, assets) and heavy use of lazy initialization. Using a higher instance class might help too (stronger instances boot up faster)
- tweak your scaling options: always have one (or two) idle instances hanging around so there is no need to wait for an instance to boot during traffic peaks
Hope that helps. In my case, using a higher instance class helped a lot already!
回答2:
I had this problem too, google supports suggested to reduce max_concurrent_requests. The hypothesis at that time was the CPU was throttled. After reducing it, the latency gotten better in general. I still wasn't sure what's going on because I didn't expect the application to be using that much CPU. Unfortunately no tool to profile the CPU on GAE to pin point where exactly the CPU intensive part
来源：https://stackoverflow.com/questions/37307461/what-can-cause-high-variability-of-untraced-time-in-app-engine-requests