Tag Archives: fate

Constantly Compiling Compilers

At the outset of this FATE journey, I pondered how frequently I ought to update gcc from SVN and put new experimental gcc versions into service. At that point, I had some logistical difficulties in upgrading compilers due to my computer setup which made it a serious chore. But it’s much easier to do now– at least for x86_32 and x86_64, I know how to build one compiler that can build both binaries. Further, the C compiler only takes about 45 minutes to compile on my 2.13 GHz Core 2 Duo, leveraging both cores, and barring any other CPU activity.

So, if you know me, you should know that right about now I’m thinking… automated script! Here’s the pitch:

  • Write a new Python script that runs continuously which performs the following steps:
  • ‘svn update’ of my local copy of gcc-svn
  • If the SVN number is higher than the latest build, automatically build a new version and install it in its own directory
  • Wait 24 hours and repeat

This will run on my main FATE machine, so run the script at a low priority so it doesn’t take time away from the more important build/test operations.

So that seems straightforward enough, especially drawing on the experience I have with FATE already (this is basically a simpler version of the main FATE script). For bonus points, the script should also put the freshest compiler into service automatically so I don’t have to be bothered with the tedious, error-prone process. This will be a bit trickier, but it should be possible:

  • Contact the primary FATE MySQL database @ fate.multimedia.cx and update the relevant records in the configuration table.
  • Signal the FATE script to start using the new compiler versions. In the present arrangement, while I have 2 scripts in order to leverage 2 CPU cores, 1 of the scripts is in charge of the 2 gcc-svn configuration for x86_32 and x86_64. The compiler path is specified in a Python configuration file. I may have to modify FATE a little bit so that the script checks some independent file for the compiler location. Then, this new script can update that location.

There is also the matter of storing so many old gcc-svn compiler versions. I’ll likely clean that up manually at various intervals but its not terribly pressing; the main FATE build/test machine inherited a 400 GB hard drive when it went into service (which, you’ll be pleased to hear, is partially used in performing backup duty for the various multimedia.cx domains).

FATE on Twitter?

For the most part, I haven’t been able to abide the notion of Twitter, where people “micro-blog” little messages of up to 140 characters each. It just seems like a bunch of “look at me” nonsense with no real purpose. But then, I used to think the same about blogs in general until I found a few interesting blogs. Lately, I have finally found a few interesting people to follow on Twitter.

Twitter logo

Then I read mentions of “Twitter clients” and quickly realized that there is an entire software ecosystem around these little messages. Indeed, there is an officially sanctioned HTTP/REST-based API for writing your own client apps.

Naturally, where I’m going with this is: Would it be useful to adapt FATE to post, ahem, “tweets” regarding state transitions (something either broke or got fixed with an individual build)? Would anyone care, i.e., does anyone already actively follow Twitter? I’m getting close to the point where I believe I can implement an email notification system, most likely to a separate mailing list. But this new channel might not be too difficult to implement at the same time. (Actually, I’m still trying to figure out from the documentation whether or not it’s possible to post a new message through the API; I can’t find the right function, or perhaps I just don’t understand all the Twitter-specific jargon yet.)

This is a little more outlandish, but as I was looking at a list of tweets today, I suddenly wondered about the possibility of sending encrypted messages through such a channel. I’m not the only person who was curious. This person beat me to the brainstorm, even going so far as to hack up a proof of concept that encodes a message of arbitrary length into multiple tweets.

Archiving Binary Builds

Someone recently asked on the ffmpeg-devel mailing list for nightly binary builds of FFmpeg and its associated libraries to be made available for download. The idea seemed to gain traction on the list so I began thinking of how we can make this a reality. Since FATE clients are already building FFmpeg binaries day and night, it’s not a huge leap to consider extending the infrastructure slightly to include packaging the binaries and sending them to a central server. So let’s talk process:

  • After building binaries as part of FATE, make a compressed bundle (.tar.bz2 for Unix, .zip for Windows) and ship the package right here to multimedia.cx. Hey, I have the space to spare (an alleged 500 GB with the current hosting plan, of which I am currently barely using 6 GB, and nearly half of that is FATE data). I also have FTP account options which should facilitate transfer.
  • The filename would follow the naming convention of “ffmpeg-bin-svn-OS-arch-compiler.ext”. So, for example, the build of SVN 18005 from Linux / x86_32 / gcc 4.2.4 would be: ffmpeg-bin-svn18005-linux-x86_32-gcc-4.2.4.tar.bz2.
  • What should be in the package? The FFmpeg binary programs and the associated libraries, of course. But in what hierarchy? A flat hierarchy doesn’t strike me as a good idea (unzip and potentially trash existing files in your current directory). Instead, package the files in, e.g., ffmpeg-svn18005-linux-x86_32-gcc-4.2.4/.
  • There should be a standard, auto-generated README.txt file in the same directory. Actually, perhaps it should be a simple HTML file since I envision it should contain — in addition to basic information such as its revision and the time it was built — a report of which tests passed and which failed, along with links to each report in the FATE database.
  • Create a web interface that allows users to browse among the latest FFmpeg binary builds. Out of everything presented in this brainstorm, this is the step that actually gives me some pause since I don’t exactly know how I would implement it off the top of my head.

Feedback welcome. Otherwise, these are likely to be the decisions I run with for the first iteration of this plan, when I get around to it (think of this as the public comment period).

Compiler Performance Profiling With FFmpeg

A Slashdot story named High Performance Linux Kernel Project discusses an effort — called LinuxDNA — to make the Linux kernel compilable with Intel’s C compiler (icc) in an effort to hopefully create a higher performing Linux kernel. The ensuing comments showcased a lot of back and forth about whether icc actually offers any performance gains over recent (or even ancient) gcc versions (“ICC really no longer has the performance lead that it once did over gcc.”). I was also curious about the claim that “it’s well known that gcc 2.95.3 generates much better code on a lot of platforms”; is that why we continually test 2.95.3 via FATE?

Yet another comment stated that, “We tried ICC on our simulator. The result: 8% slower than GCC. On intel chips.”. That’s when I realized that I’m in a position to offer some controlled testing using a CPU-intensive application: FFmpeg. At any given time, I have access to the latest builds compiled for 20 different configurations. This includes a copy built with icc 10.1.017.

So I ran some tests. Executive summary: icc finishes neck and neck with gcc 4.1.2 (a tiny bit ahead of gcc in my test), while both put most of the rest of the compilers in the test to shame, especially the latest gcc compilers. I have a chart to back up my claims, so there:

Followup: Be sure to see the results of this same exercise run without any manual ASM/SIMD optimizations.

Compiler performance when decoding MPEG-4 video and MP3 audio with FFmpeg

Small aside: I hope you appreciate that chart. You wouldn’t believe how long it took me to coerce OpenOffice.org to create it, nor how grotesquely volatile OOo 3 is on Mac OS X. In the end, the program didn’t play ball and I had to use Mac’s screenshot feature to capture the goods for publishing.

Methodology: I took a 104-minute movie that has been encoded with ISO MPEG-4 part 2 (a.k.a. DivX/XviD) video and MP3 audio and fed it through the following command:

$ time ffmpeg -i file.avi -f framecrc – > /dev/null

I used the ‘user’ output from the time prefix (out of the real, user, and sys times) which counts the approximate seconds that the process spent on the CPU. This should exclude I/O access and, really, probably just counts the number of 10ms time slices that the OS allocates to the process. I ran the test once for each compiler configuration, then ran through the configurations a second time and graphed the minimum time between the pair of runs for each configuration.

One day, I will have graphing working in FATE so that we can obtain continuous and historical performance data that will help us analyze trends, both in FFmpeg and in the compilers that build it.

Another comment from the Slashdot thread asserted that “it is simply healthy for the kernel to be compilable across more compilers,” to which another commenter challenged, “Prove it.” Again, I think I’m in a position to help here. While it may be more common for a test to break on all PowerPC configurations due to endian considerations, or for the build to break on the icc or gcc 2.95.3 configurations for reasons related to C99 arcana, there have been a few instances where FATE tests have inexplicably broken on very specific configurations. The latest example of this is when a recent code change in FFmpeg randomly caused the wc3movie-xan test spec to fail, but only on the Linux / x86_32 / gcc 4.2.4 configuration. Huh? Well, thanks to Vitor who promptly went to work with valgrind and found that the subsystem was doing some bad things in the first place and in a way that finally manifested on one configuration. (Incidentally, I’m pretty sure that the WC3 playback system was the first bit of code I ever contributed to FFmpeg.)

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