Thursday, December 31, 2020 [Tweets] [Favorites]

The Case of the Extra 40 ms

John Blair:

I reported my discovery to the integrator and the chip vendor (look, it’s the Android Thread scheduler!), but they continued to push back on the Netflix behavior. Why don’t you just copy more data each time the handler is called? This was a fair criticism, but changing this behavior involved deeper changes than I was prepared to make, and I continued my search for the root cause. I dove into the Android source code, and learned that Android Threads are a userspace construct, and the thread scheduler uses the epoll() system call for timing. I knew epoll() performance isn’t guaranteed, so I suspected something was affecting epoll() in a systematic way.

At this point I was saved by another engineer at the chip supplier, who discovered a bug that had already been fixed in the next version of Android, named Marshmallow. The Android thread scheduler changes the behavior of threads depending whether or not an application is running in the foreground or the background. Threads in the background are assigned an extra 40 ms (40000000 ns) of wait time.

A bug deep in the plumbing of Android itself meant this extra timer value was retained when the thread moved to the foreground. Usually the audio handler thread was created while the application was in the foreground, but sometimes the thread was created a little sooner, while Ninja was still in the background. When this happened, playback would stutter.

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