UPDATE: SCons is Still Really Slow

A while back I posted a series of articles exploring the scalability of SCons, a popular Python-based build tool. In a nutshell, my experiments showed that SCons exhibits roughly quadratic growth in build runtimes as the number of targets increases:

Recently Dirk Baechle attempted to rebut my findings in an entry on the SCons wiki: Why SCons is not slow. I thought Dirk made some credible suggestions that could explain my results, and he did some smart things in his effort to invalidate my results. Unfortunately, his methods were flawed and his conclusions are invalid. My original results still stand: SCons really is slow. In the sections that follow I’ll share my own updated benchmarks and show where Dirk’s analysis went wrong.

Test setup

As before, I used genscons.pl to generate sample builds ranging from 2,000 to 50,000 targets. However, my test system was much beefier this time:

2013 2010
OS Linux Mint 14 (kernel version 3.5.0-17-generic) RedHat Desktop 3 (kernel version 2.4.21-58.ELsmp)
CPU Quad 1.7GHz Intel Core i7, hyperthreaded Dual 2.4GHz Intel Xeon, hyperthreaded
RAM 16 GB 2 GB
HD SSD (unknown)
SCons 2.3.0 1.2.0.r3842
Python 2.7.3 (system default) 2.6.2

Before running the tests, I rebooted the system to ensure there were no rogue processes consuming memory or CPU. I also forced the CPU cores into “performance” mode to ensure that they ran at their full 1.7GHz speed, rather than at the lower 933MHz they switch to when idle.

Revisiting the original benchmark

I think Dirk had two credible theories to explain the results I obtained in my original tests. First, Dirk wondered if those results may have been the result of virtual memory swapping — my original test system had relatively little RAM, and SCons itself uses a lot of memory. It’s plausible that physical memory was exhausted, forcing the OS to swap memory to disk. As Dirk said, “this would explain the increase of build times” — you bet it would! I don’t remember seeing any indication of memory swapping when I ran these tests originally, but to be honest it was nearly 4 years ago and perhaps my memory is not reliable. To eliminate this possibility, I ran the tests on a system with 16 GB RAM this time. During the tests I ran vmstat 5, which collects memory and swap usage information at five second intervals, and captured the result in a log.

Next, he suggested that I skewed the results by directing SCons to inherit the ambient environment, rather than using SCons’ default “sanitized” environment. That is, he felt I should have used env = Environment() rather than env = Environment(ENV = os.environ). To ensure that this was not a factor, I modified the tests so that they did not inherit the environment. At the same time, I substituted echo for the compiler and other commands, in order to make the tests faster. Besides, I’m not interested in benchmarking the compiler — just SCons! Here’s what my Environment declaration looks like now:

env = Environment(CC = 'echo', AR = 'echo', RANLIB = 'echo')

With these changes in place I reran my benchmarks. As expected, there was no change in the outcome. There is no doubt: SCons does not scale linearly. Instead the growth is polynomial, following an n1.85 curve. And thanks to the the vmstat output we can be certain that there was absolutely no swapping affecting the benchmarks. Here’s a graph of the results, including an n1.85 curve for comparison — notice that you can barely see that curve because it matches the observed data so well!

SCons full build runtime - click for larger view

For comparison, I used the SCons build log to make a shell script that executes the same series of echo commands. At 50,000 targets, the shell script ran in 1.097s. You read that right: 1.097s. Granted, the shell script doesn’t do stuff like up-to-date checks, etc., but still — of the 3,759s average SCons runtime, 3,758s — 99.97% — is SCons overhead.

I also created a non-recursive Makefile that “builds” the same targets with the same echo commands. This is a more realistic comparison to SCons — after all, nobody would dream of actually controlling a build with a straight-line shell script, but lots of people would use GNU make to do it. With 50,000 targets, GNU make ran for 82.469s — more than 45 times faster than SCons.

What is linear scaling?

If the performance problems are so obvious, why did Dirk fail to see them? Here’s a graph made from his test results:

SCons full build runtime, via D. Baechle - click for full size

Dirk says that this demonstrates “SCons’ linear scaling”. I find this statement baffling, because his data clearly shows that SCons does not scale linearly. It’s simple, really: linear scaling just means that the build time increases by the same amount for each new target you add, regardless of how many targets you already have. Put another way, it means that the difference in build time between 1,000 targets and 2,000 targets is exactly the same as the difference between 10,000 and 11,000 targets, or between 30,000 and 31,000 targets. Or, put yet another way, it means that when you plot the build time versus the number of targets, you should get a straight line with no change in slope at any point. Now you tell me: does that describe Dirk’s graph?

Here’s another version of that graph, this time augmented with a couple additional lines that show what the plot would look like if SCons were truly scaling linearly. The first projection is based on the original graph from 2,500 to 4,500 targets — that is, if we assume that SCons scales linearly and that the increase in build time between 2,500 and 4,500 targets is representative of the cost to add 2,000 more targets, then this line shows us how we should expect the build time to increase. Similarly, the second projection is based on the original graph between 4,500 and 8,500 targets. You can easily see that the actual data does not match either projection. Furthermore you can see that the slope of these projections is increasing:

SCons full build runtime with linear projections, via D. Baechle - click for full size

This shows the importance of testing at large scale when you’re trying to characterize the scalability of a system from empirical data. It can be difficult to differentiate polynomial from logarithmic or linear at low scales, especially once you incorporate the constant factors — polynomial algorithms can sometimes even give better absolute performance for small inputs than linear algorithms! It’s not until you plot enough data points at large enough values, as I’ve done, that it becomes easy to see and identify the curve.

What does profiling tell us?

Next, Dirk reran some of his tests under a profiler, on the very reasonable assumption that if there was a performance problem to be found, it would manifest in the profiling data — surely at least one function would demonstrate a larger-than-expected growth in runtime. Dirk only shared profiling data for two runs, both incremental builds, at 8,500 and 16,500 targets. That’s unfortunate for a couple reasons. First, the performance problem is less apparent on incremental builds than on full builds. Second, with only two datapoints it is literally not possible to determine whether growth is linear or polynomial. The results of Dirk’s profiling was negative: he found no “significant difference or increase” in any function.

Fortunately it’s easy to run this experiment myself. Dirk used cProfile, which is built-in to Python. To profile a Python script you can inject cProfile from the command-line, like this: python -m cProfile scons. Just before Python exits, cProfile dumps timing data for every function invoked during the run. I ran several full builds with the profiler enabled, from 2,000 to 20,000 targets. Then I sorted the profiling data by function internal time (time spent in the function exclusively, not in its descendents). In every run, the same two functions appeared at the top of the list: posix.waitpid and posix.fork. To be honest this was a surprise to me — previously I believed the problem was in SCons’ Taskmaster implementation. But I can’t really argue with the data. It makes sense that SCons would spend most of its time running and waiting for child processes to execute, and even that the amount of time spent in these functions would increase as the number of child processes increases. But look at the growth in runtimes in these two functions:

SCons full build function time, top two functions - click for full size

Like the overall build time, these curves are obviously non-linear. Armed with this knowledge, I went back to Dirk’s profiling data. To my surprise, posix.waitpid and posix.fork don’t even appear in Dirk’s data. On closer inspection, his data seems to include only a subset of all functions — about 600 functions, whereas my profiling data contains more than 1,500. I cannot explain this — perhaps Dirk filtered the results to exclude functions that are part of the Python library, assuming that the problem must be in SCons’ own code rather than in the library on which it is built.

This demonstrates a second fundamental principle of performance analysis: make sure that you consider all the data. Programmers’ intuition about performance problems is notoriously bad — even mine! — which is why it’s important to measure before acting. But measuring won’t help if you’re missing critical data or if you discard part of the data before doing any analysis.


On the surface, performance analysis seems like it should be simple: start a timer, run some code, stop the timer. Done correctly, performance analysis can illuminate the dark corners of your application’s performance. Done incorrectly — and there are many ways to do it incorrectly — it can lead you on a wild goose chase and cause you to squander resources fixing the wrong problems.

Dirk Baechle had good intentions when he set out to analyze SCons performance, but he made some mistakes in his process that led him to an erroneous conclusion. First, he didn’t run enough large-scale tests to really see the performance problem. Second, he filtered his experimental data in a way that obscured the existence of the problem. But perhaps his worst mistake was to start with a conclusion — that there is no performance problem — and then look for data to support it, rather than starting with the data and letting it impartially guide him to an evidence-based conclusion.

To me the evidence seems indisputable: SCons exhibits roughly quadratic growth in runtimes as the number of build targets increases, rendering it unusable for large-scale software development (tens of thousands of build outputs). There is no evidence that this is a result of virtual memory swapping. Profiling suggests a possible pair of culprits in posix.waitpid and posix.fork. I leave it to Dirk and the SCons team to investigate further; in the meantime, you can find my test harness and test results in my GitHub repo. If you can see a flaw in my methodology, sound off in the comments!

8 thoughts on “UPDATE: SCons is Still Really Slow

  1. Hi Eric,

    thank you very much for taking another close look at SCons’ performance, and providing additional test data and results.

    You were right about the term “linear scaling”, and I rewrote the offending sentence. I didn’t use it correctly, trying to give it the meaning of something like “less than quadratic scaling”. Well spotted, and if you see more errors like this one, tell me please. I’m not a native speaker, so sometimes my language “slips”…

    The Wiki page you are refering to above, isn’t meant to be a qualified answer on behalf of the SCons project to your blog articles. Else, I (or some other dev) would have replied to one of them directly, and pointed you and other users to the page. I published my intermediate results for other SCons developers, in order to discuss my work done so far, its results and the next steps. That’s also why it’s listed/linked under the developer zone only…
    Please also bear in mind that it’s still a work in progress, as the last paragraphs imply.

    What I’ve been aiming at so far, is not to invalidate your results. I’m simply interested in improving SCons’ performance, and in “invalidating” the claim that the Taskmaster has a design flaw, which allegedly creates the quadratic behaviour. I’m actually trying to reproduce your results on my side, such that I can then analyse where the real problem lies.

    While doing my analysis I don’t hide any data or filter it. As the paragraph above the SVG graphs (“melski_update_d.svg” and “melski_update_e.svg”, mentioned in the “What does profiling tell us?” section) clearly states, all results are available in the “scons_testresults” repository. If you take yourself the time and look into the folders “scons120_vs_make/results/d” and “e”, you’ll find the series of “prof1*” files with the profiling results and graphs of the clean run builds…and they also list “waitpid” and “fork”, as expected.

    Up to now, the Taskmaster and its “design problem” were my priority concerns. That’s why I used the cumulative times of the profiling output and simply compared the ratio of “build time” vs “time to find tasks”…expecting the tasking times to explode when doubling the number of files from project size “d” to “e”. I still think that’s a reasonable approach for this specific purpose.

    Your latest profilings indicate that the “waitpid” and “fork” methods do behave badly instead. I now have (thanks to christmas!) an 8GByte machine at home, such that I can finally run larger build jobs. Combined with a “reducing memory” patch, that I got into the default branch a few days ago, I’ll hopefully be able to reproduce your findings and then track the error down further.
    However, doing this will probably need some time…so please don’t take any additional data published on the Wiki page too serious, or even as an offense. I can keep you posted about the final results, if you like.

    Thanks a lot in advance for your patience and understanding.

    Regards and all the best for 2014,


    • @Dirk Thanks for taking the time to comment here, and for exploring the boundaries of SCons performance! I’m sure that many people will benefit from your efforts to make SCons faster and more scalable. However, I think your statement that your results are preliminary or a “work in progress” is lost on most of the people who read your wiki page. At least, the comments on the SCons dev mailing list suggest that your results have been taken as conclusive evidence that there is not a quadratic growth in runtimes, when the data — even yours! — clearly shows otherwise. Surely nobody benefits by pretending that this problem does not exist.

      Regarding filtering of profiling data: it’s true that waitpid and fork show up in your complete profiling data, when you include both the full and incremental builds. But you only linked to the incremental build data, and you stated unequivocally that the results “don’t show any significant difference or increase for the percentage of runtime in each function”. I think this presentation will mislead many readers to think that there is no performance problem evident in any of the profiling data. In fact it’s only on a careful rereading that I see you meant that statement to apply only to the incremental build. But if your data revealed a problem in the full builds, why not include that fact in your analysis? Your statements and the way you have presented the data obscure the existence of a problem which is quite evident from the actual data.

      Regarding Taskmaster being the root problem: based on the data I have now, I admit that I was wrong on that point. As I stated in this article, programmers’ intuition about performance problems is notoriously bad, and I’m no exception to that rule! This just highlights the fact that we must always measure when dealing with performance, and we must remain impartial in our analysis.

      Again thank you for your comments and I wish you good luck in fixing SCons!

    • There appears to be a fundamental problem with the way python spawns processes with respect to performance. When a scons action is run fork is called and then target action is performed from the new process. Performance of the fork system call directly depends on the resources allocated by the calling process. In the case of these performance tests scons uses more resources as the number of build targets increase. This increases the amount of work required to perform a fork which explains why fork doesn’t scale linearly with the number of build targets.

      Other tools like make utilize vfork followed by a call to execve which is a much lighter weight way to spawn a process. AFAIK python doesn’t natively support a lighter weight mechanism for spawning tasks. With the following code you can see that as you change the amount of memory held by lots_of_memory the runtime of the entire program significantly changes due to the varying cost of performing the fork system call.

      import os
      lots_of_memory =’A’*5000000000
      for i in range(400):
      args = [‘echo’, ‘-n’, ‘1’]
      os.spawnvpe(os.P_WAIT, args[0], args, os.environ)

      • @Trevor Thanks for the insightful comments. I think you’ve hit the nail on the head here. It would be an interesting experiment to try rebuilding Python to use vfork instead of fork. For a proof-of-concept that’s probably a one-character patch, but for production use I think more work would be required to bring Python’s implementation into compliance with the strict constraints imposed by vfork (for example, the child process should not modify any data before calling exec).

      • In the meantime, I finished my investigations and profilings about SCons’ bad performance. Turns out that the “fork” call really is the bad guy here, as you suspected. I updated the Wiki page http://scons.org/wiki/WhySconsIsNotSlow with my latest findings and results. Some members of the SCons team have developed a wrapper, which redirects the subprocess calls to the more lightweight posix_spawn internally. Unfortunately, this is for Linux (Posix) systems only…but it will definitely help a lot of users when we integrate this module to our next release.

  2. Thanks for your fast reply. I don’t deny that there might be a problem with SCons’ performance. As I stated above, your observations appear to show where the culprit lies, and I will have a look at it.

    If you feel that the Wiki page in question needs to be amended immediately, please add your comments, disclaimers, analysis and concerns right there. Our Wiki can be edited freely and is open to anyone (except spambots šŸ˜‰ ). It’s not MY page, it’s A page. You seem to know exactly what needs to be done to remove any ambiguity or misleading conclusions…so be my guest.



  3. Pingback: SCons | gumdaeng

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