[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.)

Cmake spend 15 seconds to generate this project? I find it unlikely build2 is doing the same work as cmake is. Now I will grant cmake is single threaded and slow, so there is a lot of room to do the things it does better (the language sucks, and is part of what forces single threaded). It also wouldn't surprise me if cmake is doing things that are not really needed (odds are the default compiler works - most of the time it isn't valuable to check the version)

> Let's see if we can go even faster. Next, we disable compression in the file cache. We will discuss the file cache in more detail a bit later but for now let's just say that by disabling compression we trade temporary disk space usage for speed:

Something is wrong here. Which compression algorithm is being used here and how much has it been tuned? A core hypothesis of the likes of zram is that disk access is so slow (even NVME), that you can often beat it with the bit-rate of decompression.

1. Is something slow like gzip being used?

2. Is the compression effort over-tuned for size? Do some space benchmarks and make sure that you aren't saving a few dozen MB on GBs of data.

zstd, with 1-3 effort (you may even find negative is a overall win), and a trained dictionary (your data does all look identical) is probably a good start.

I wonder how it compares to Tup ( https://gittup.org/tup/ ).

Great that there are performance benefits, but one thing that I think both meson/ninja and cmake (or cmake/ninja) got wrong, and GNU configure got right is "./configure --help". Why do neither of these tools simply add --help here? Yes, their syntax is different, but the issue is not only about --help. Often I could disable documentation or man page via --disable-man or --disable-doc or something like that. I recently had a discussion with a guy who transitioned into ninja, and I reasoned that there should be an option to skip installing man-pages. He thinks everyone needs manpages. I told him I never look at any local man page ever; I only look online for help. And have been doing so for almost 30 years. I understand the 1970s era of man-pages, but I have no use for them (I do gather local documentation, just not via manpages). He did not want to add a way to install his software without having available xmlto, docbook variants (which are a pain to install, just look at the LFS/BLFS instructions and even then they do not work cleanly). Meson and cmake are better than the GNU autotools, but the few things GNU autotools got right, were lost in both meson and cmake, which is interesting. It seems we can only select between different trade-offs here. New is not automatically better. And being 5% faster really is pointless if functionality is removed or not available anymore.

> And now we are 2.2% faster than Ninja!

This claim isn't supported by the author's measurements?

I think what you're talking about is part of a much larger pattern. Autotools imagines that the most important thing is the "user" in a world where people want to download your software and build it on their machine. They don't care (as much) about fast build times, the semantic clarity of the build files themselves, the ease with which one may change or extend the build sequence, test harnesses . . . Those are all developer worries.

They do care that the code can compile on their (possibly old) machine. They do care about the ability to include some features and not others (including their ability to not have to make decisions on this if they don't want to). And they certainly want documentation that helps them to achieve their goal (which, again, is to build the software).

The newer tools make substantial inroads in making the developer's life easier. If your software is not of the sort where people download it and try to build it (because it only ships as a closed binary, is entirely internal, etc.) then maybe that makes sense. But until all these fancy gadgets start seriously caring about the use case above, I'm going to stick with autotools.

I wish that when people posted a performance win, they would include a theory explaining it. Unexpected speedups are usually noise, bugs, or harness issues. No program is faster due to the superior manly virtue of its programmer. Every effect has a cause.

When, from first principles, something shouldn't be faster, yet it is, you dig and dig until you understand. You don't just say your shit rules and the other thing sux0rz. If you do, you'll regret it: if you don't understand how you got a win, you don't understand how to keep it.