[Ninja author here] Nice post, cool to see the deep dive! I also appreciate the details on how they produced their numbers.
As they observe, Ninja gets to be fast mostly by cheating: it avoids a lot of work by saying many things are just out of scope for Ninja to do, and that means it is a useful a target to race against. (Funny thing: when I wrote Ninja I was misremembering how fast an earlier build system was so I kept trying to make it faster. So don't treat it as a lower bound, I just made it up!)
I comment here to say I find the explanation for 'why' in this post unsatisfying. They mention three design decisions.
The first one is a criticism of CMake, not Ninja (?), so I don't think it can be why. I might have misunderstood?
The second reason given is doing some work like header dependencies in multiple threads. This is the most plausible reason to me but it still feels unlikely. It's a very small amount of work: the post mentions 300 compiles, so maybe parsing 300 small text files?
The third is that they run the compiler up front an additional time to gather headers, which is strictly more work than Ninja. There is some hand waving about file access patterns but I am skeptical; if the end-to-end build time is 3 seconds then the project is small enough to all fit in kernel caches. They also mention doing other things like invoking the compiler to get version information. This seems like it would dwarf any performance gain from number 2.
Maybe it's just my own curiosity, I think this post would be better if it had a better explanation for the reason. I'm not disputing the result, I just think the result should make you suspicious that something else is going on, and you might learn something from that! You could for example explore whether it's the header dependency thing by profiling the Ninja invocation and seeing if it's waiting for CPU or waiting for tasks to execute.
(If I had to guess without looking at any of the involved code, I would predict it's something about how CMake generates the build, like it introduces serialization in a place where build2 is parallel, or it adds some extra build steps like gathering the current git hash into a header file or something.)
[build2 author here] Thanks for the feedback! Some additional details:
> The first one is a criticism of CMake, not Ninja (?), so I don't think it can be why.
Fair enough. The point I was making is that if you want to compete with Ninja, you cannot leave any potential performance gains on the table.
> The second reason given is doing some work like header dependencies in multiple threads. This is the most plausible reason to me but it still feels unlikely.
We are talking about ~2% performance difference here. Parallelizing even a small amount of work across 24 threads rather that doing it serially saving a percent or two feels plausible to me.
> There is some hand waving about file access patterns but I am skeptical; if the end-to-end build time is 3 seconds then the project is small enough to all fit in kernel caches.
It fits into the system's file cache unless there is memory pressure, like one would expect from having 24 C++ compiler jobs running in parallel. We actually measured this in isolation (with more detailed results in the linked article) and it has a measurable effect.
> They also mention doing other things like invoking the compiler to get version information. This seems like it would dwarf any performance gain from number 2.
I measured this, it costs 70ms or ~2% of the overall time.
> It fits into the system's file cache unless there is memory pressure, like one would expect from having 24 C++ compiler jobs running in parallel. We actually measured this in isolation (with more detailed results in the linked article) and it has a measurable effect.
300 TUs is not much. If they build in 3 seconds then they are trivial (small). If the machine is 24-thread (I assume some sort of heterogeneous 12-core), how little RAM does the machine have for the kernel to start evicting page cache during the build?
> > They also mention doing other things like invoking the compiler to get version information. This seems like it would dwarf any performance gain from number 2. I measured this, it costs 70ms or ~2% of the overall time.
Couldn’t you amortize this to 0 by just caching the result and only changing it if the binary timestamp changed?
> Couldn’t you amortize this to 0 by just caching the result and only changing it if the binary timestamp changed?
Yes, that would be nice, but the tricky question is can any of this information change without the compiler binary mtime changing? First off, GCC's gcc/g++ binaries are drivers and are not what does the actual compilation, it's private cc1/cc1plus binaries that do the job. Can one of these change but not the driver? I think it's plausible (some package manager optimization where the file is not touched if it hasn't changed). So at a minimum we would need to discover where those are located (probably by invoking gcc/g++) and checking them as well. Could there be something else? Who knows. We value speed very much but we value correctness even more.
I think a more fruitful direction to explore is to improve GCC itself to dump all this information in a single invocation and in a machine-readable format (JSON). I think if we go from 70ms to 14ms (and perhaps even lower because this special GCC mode could conceivably do things faster than how we do it now), it would be good enough.
Simple CMake projects using the Ninja generator are very efficient, unless you create generated files.
And if they are in their own targets, they are not really an issue (they would serialize everything that depends on them as you'd expect), but if you have them in a library grouped with other files to compile, then the whole library compilation is serialized.
And obviously worse if you also have to build the generator for the generated files, but that's not a big surprise, you can observe that in full builds of Chromium or its libraries too waiting for protoc if you crank the parallelization a lot.
Yes, I don’t remember the details but vaguely remember that CMake tends to group things together that could in principle be made more parallel, as you mention with generates files in a library. On the other hand if build2 makes it easier for authors to express these kinds of patterns without the serialization then I count that as a win for build2!
I feel like make/ninja suffer from the same output modstamp > input modstamps to rebuild. Really a build system should track modstmaps on all inputs (apps included), and the rebuild on not-equal.
Would be nice to see build2 go this route.
I did some exploration of this idea in a followup build system! See https://neugierig.org/software/blog/2022/03/n2.html . (It's not really production-ready.)
Reading about manifests immediately reminded be of Clearcase/Clearmake. My memory may be not correct after all the years passed, but configuration records for derived objects contained the same information as proposed manifests + some more metadata about source (versions, at least). If I remember correctly SCons allows to implement a custom "checker" (or whatever it is called there) to compare files based on any criteria that are useful, not just timestamps. But this is about the only visible advantage of SCons over Make, at least for our case.
I really like the ideas you have in n2. Hashing the entire build string is a great idea. And it really is as simple as just having all the timestamps of the inputs in the hash. And you have the output timestamp as a quick test.
Why not just embrace Content Based Addressing and the Bazel action cache? Not necessarily adopt the RBE protos, but you could have on-disk cache that are similar or even identical.
Nowadays, I think a build tool that doesn't natively support distributed caching (and possibly remote execution) is a weird choice. And I don't think that spawning layers of processes allows for good parallelization as you don't know if an action is going to be network bound or compute bound and the job slot is then spent. So you either oversubscribe or undersubscribe.
Sadly, I think things like distcc or ccache have mostly gone out of mode. Vertically scaling (multi core, multi thread, multi TB RAM build machines) and simple artifact caching have won for now. Artifact caching with dependency tracking in the midst of job scheduling was solved “good enough” by OS based mechanisms. Having used and managed the basics of a build farm, removing the need for networking, build farm management and build job coordination is a huge burden relief when your source tree doesn’t require it.
Part of the motivation to use processes is because their structure helps to keep the job generic, uni
I stumbled upon the very experimental nix-ninja [0] recently.
They seem to create a dynamic nix derivation per compilation unit, which would be very similar to what you describe as manifests in your post as it also creates a hash of all inputs.
Would be interesting to here your opinion on that approach
[0]: https://github.com/pdtpartners/nix-ninja
Thank you for Ninja!
The speed (reading the build.ninja file) was never a concern for us. If I could share a wish-list, it will be:
- Fix the possibility of a segmentation fault when the build file is damaged;
- Use a better order of execution: https://github.com/ninja-build/ninja/issues/2157
Thank you for your work on Ninja. It's just really good.
Thanks for saying this! I am close enough to it that I mostly remember all of the bad decisions I made that are now unfixable, haha.
I would like to read blogpost about this and what could be done better.
Better to have made imperfectly and learned than to not have made at all. :)
Always glad to see authors on here. Thanks for your insight!
Since they had to rewrite the build file for their program I also assume that something is missing. Didn’t see any mention of verifying that.
I also really didn’t like their denigrating tone. It totally turns me off trying build2, because it seems they don’t understand the point of separating build stages like environment setup (getting dependencies), configure, native build, cross build, packaging. I am a very happy ninja user instead of a batteries-included solution because it does its one job well and can be used very flexibly. I personally detest cmake, so I use nix + own configure script + ninja.
The blog is also wrong about ninja being unable to call configure, but I intentionally don’t want that (I want the build stages to communicate in one direction for sanity).
> I want the build stages to communicate in one direction for sanity
That's easy to do if you control the whole pipeline and can integrate all the features together, not so much with the CMake model unfortunately. I think it would be nice if CMake had Ninja integrated as a library, it could lead to some nice optimizations later.