I was monitoring the processes on a build machine via ‘top’ during the testing phase of a FATE build/test cycle. At the top of the list was ‘ffmpeg <defunct>’. I was a bit concerned about FFmpeg zombie processes until I noticed that the PID attached to the zombie was steadily increasing at each refresh.

It turns out that these zombies are merely an artifact of the current infrastructure. According to the profiling information from my build/test script, the ‘test’ phase always seems to take 71 seconds to execute, give or take a second, regardless of platform. Incidentally, there are presently 71 active tests in the FATE suite. This led me to recognize that the build/test script is comically inefficient in this respect and that it should be possible to blaze through the tests much more quickly, and perhaps during the build phase as well, provided that not much has changed in the source (the build machines leverage ccache).

At issue is the way in which the script runs commands. It uses the Python subprocess module to spin off a process, monitor the stdout and stderr on separate pipes, and also kill the process if it runs too long. The upshot of the current method is that the script always waits at least 1 second before first checking if the child has finished. This leads to the zombies since the child FFmpeg process has finished but is waiting for its parent to wait for its final status code. I am working on revising this algorithm to be considerably more efficient, particularly since I anticipate eventually having many hundreds of individual tests in the suite.

Here’s another curious artifact I have observed regarding profiling. Python’s os module provides a nifty times() function that returns a bunch of useful timing data. Among the 5 values returned is the cumulative time that child processes of the main process have spent running on the CPU. I thought this would be perfect for profiling since it only accounts for CPU runtime, no I/O time. In reality, I am thinking that the OS simply counts the number of times that a process gets to run on the CPU and multiplies that by 10 milliseconds. At least, empiricial evidence suggests that to be the case since every test seems to complete in a time evenly divisible by 10 msec. I suppose this is good enough for the time being. Fortunately, there are some tests that run long enough for substantial differences to be observed between platforms. For example, the test designated h264-conformance-ba1_ft_c takes on the order of 1280 ms on PPC, 160 ms on x86_32, and 400 ms on x86_64 (all with gcc 4.2.2 compilation on Linux). Of course, those numbers should not be compared with each other, but with the same test run over time on the same CPU.

I’m open to more profiling ideas. Perhaps FFmpeg could include new command line options for fine grain testing of certain modules, or come with separate test programs to achieve the same. E.g., push a few hundred test vectors through DCT/IDCT and log the nominal timing from the timestamp counter for later graphing. For all I know, FFmpeg already has some options to achieve this (usually when I propose a new FFmpeg testing feature to Michael, he helpfully advises that said feature has been in the codebase for years).