Pursuant to yesterday’s conundrum of creating a portable process runner in Python for FATE that can be reliably killed when exceeding time constraints, I settled on a solution. As Raymond Tau reminded us in the ensuing discussion, Python won’t use a shell to launch the process if the program can supply the command and its arguments as a sequence data structure. I knew this but was intentionally avoiding it. It seems like a simple problem to break up a command line into a sequence of arguments– just split on spaces. However, I hope to test metadata options eventually which could include arguments such as ‘-title “Hey, is this thing on?”‘ where splitting on spaces clearly isn’t the right solution.

I got frustrated enough with the problem that I decided to split on spaces anyway. Hey, I control this system from top to bottom, so new rule: No command line arguments in test specs will have spaces with quotes around them. I already enforce the rule that no sample files can have spaces in their filenames since that causes trouble with remote testing. When I get to the part about testing metadata, said metadata will take the form of ‘-title “HeyIsThisThingOn?”‘ (which will then fail to catch myriad bugs related to FFmpeg’s incorrect handling of whitespace in metadata arguments, but this is all about trade-offs).

So the revised Python process runner seems to work correctly on Linux. The hangaround.c program simulates a badly misbehaving program by eating the TERM signal and must be dealt with using the KILL signal. The last line in these examples is a tuple containing return code, stdout, stderr, and CPU time. For Linux:

$ ./upr.py 
['./hangaround', '40']
process ID = 2645
timeout, sending TERM
timeout, really killing
[-9, '', '', 0]

The unmodified code works the same on Mac OS X:

$ ./upr.py
['./hangaround', '40']
process ID = 94866
timeout, sending TERM
timeout, really killing
[-9, '', '', 0]

Now a bigger test: Running the upr.py script on Linux in order to launch the hangaround process remotely on Mac OS X via SSH:

$ ./upr.py 
['/usr/bin/ssh', 'foster-home', './hangaround', '40']
process ID = 2673
timeout, sending TERM
[143, '', '', 50]

So that’s good… sort of. Monitoring the process on the other end reveals that hangaround is still doing just that, even after SSH goes away. This occurs whether or not hangaround is ignoring the TERM signal. This is still suboptimal.

It would be possible to open a separate SSH session to send a TERM or KILL signal to the original process… except that I wouldn’t know the PID of the remote process. Or could I? I’m open to Unix shell magic tricks on this problem since anything responding to SSH requests is probably going to be acceptably Unix-like. I would rather not go the ‘killall ffmpeg’ route because that could interfere with some multiprocessing ideas I’m working on.

Here’s a brute force brainstorm: When operating in remote-SSH mode, prefix the command with ‘ln -s ffmpeg ffmpeg-<unique-key>’ and then execute the symbolic link instead of the main binary. Then the script should be able to open a separate SSH session and execute ‘killall ffmpeg-<unique-key>’ without interfering with other processes. Outlandish but possibly workable.

I am working hard at designing a better FATE right now. But first thing’s first: I’m revisiting an old problem and hoping to conclusively determine certain process-related behavior.

I first described the problem in this post and claimed in this post that I had hacked around the problem. Here’s the thing: When I spin off a new process to run an FFmpeg command line, Python’s process object specifies a PID. Who does this PID belong to? The natural assumption would be that it belongs to FFmpeg. However, I learned empirically that it actually belongs to a shell interpreter that is launching the FFmpeg command line, which has a PID 1 greater than the shell interpreter. So my quick and dirty solution was to assume that the actual FFmpeg PID was 1 greater than the PID returned from Python’s subprocess.Popen() call.

Bad assumption. The above holds true for Linux but not for Mac OS X, where the FFmpeg command line has the returned PID. I’m not sure what Windows does.

This all matters for the timeout killer. FATE guards against the possibility of infinite loops by specifying a timeout for each test. Timeouts don’t do much good when they trigger TERM and KILL signals to the wrong PID. I tested my process runner carefully when first writing FATE (on Linux) and everything worked okay with using the same PID returned by the API. I think that was because I was testing the process runner using the built-in ‘sleep’ shell command. This time, I wrote a separate program called ‘hangaround’ that takes a number of seconds to hang around before exiting. This is my testing methodology:

From another command line:

$ ps ax|grep hangaround
21433 pts/2    S+     0:00 /bin/sh -c ./hangaround 30
21434 pts/2    S+     0:00 ./hangaround 30
21436 pts/0    R+     0:00 grep hangaround

That’s Linux; for Mac OS X:

>>> process.pid
82079

$ ps ax|grep hangaround
82079 s005  S+     0:00.01 ./hangaround 30
82084 s006  R+     0:00.00 grep hangaround

So, the upshot is that I’m a little confused about how I’m going to create a general solution to work around this problem– a problem that doesn’t occur very often but makes FATE fail hard when it does show up.

Followup:

• Process Runner Redux: Making some hard decisions regarding these problems

Felix von Leitner delivered a talk at the 2009 Linux Kongress about the state of the art in compiler optimization (link to PDF slides). Presentation slides by themselves are not a good way to understand a talk and it would be better to learn if video for the actual talk is posted somewhere. Compiler optimization (or lack thereof) is fairly important to FFmpeg developers.

The talk analyzes how LLVM, icc, MSVC, Sun C, and gcc generate fast code in this day and age. One basic theme I gathered is that coders should forgo clever C optimizations as they tend to be counterproductive. I wish I could believe that, but there was that recent episode where I optimized FFmpeg’s Theora decoder by removing structure dereferences. I’m sure that other performance-minded multimedia hackers will have other nits to pick with the broad generalizations in the presentation. I call your attention to the fighting words (which I have taken out of context since it’s such a fun quote) on slide 41: “Note: gcc is smarter than the video codec programmer on all platforms.” Further, slides 53-55 specifically call out madplay for inline ASM that allegedly didn’t improve efficiency vs. what the compiler could achieve with the raw C code.

On the whole, the findings are probably quite accurate for the kind of C code that most people need to write (e.g., “No need to write a >> 2 when you mean a/4!”).

Speaking of compilers, FATE now covers Intel’s 11.1 series C compiler for both 32- and 64-bit icc. I have also updated the stale snapshots of the gcc-svn for my machines (I still need to write a tool to do that for me automatically and continuously).

I’ve been on a gaming kick lately. I found a game in my collection called Roketz; the full DOS game can be downloaded from the publisher. The game has 2 files bearing the extension VQM that appear to be FMV files. Wiki and samples.

Roketz comes from a company called Bluemoon. According to their website, they’re also responsible for building the technological foundations for 2 well-known pieces of software: Kazaa and Skype. I’ve never used either, personally, but I understand that Skype uses a custom vocoder called SILK. Maybe Roketz and VQM is where the team got their start in codec technology?