On Fri, Jun 17, 2016 at 09:00:45AM +0000, Atila Neves via 
Digitalmars-d-announce wrote:
> On Friday, 17 June 2016 at 06:18:28 UTC, H. S. Teoh wrote:
> > On Fri, Jun 17, 2016 at 05:41:30AM +0000, Jason White via
> > Digitalmars-d-announce wrote: [...]
> > > Where Make gets slow is when checking for changes on a ton of
> > > files.  I haven't tested it, but I'm sure Button is faster than
> > > Make in this case because it checks for changed files using
> > > multiple threads.  Using the file system watcher can also bring
> > > this down to a near-zero time.
> > 
> > IMO using the file system watcher is the way to go. It's the only
> > way to beat the O(n) pause at the beginning of a build as the build
> > system scans for what has changed.
> See, I used to think that, then I measured. tup uses fuse for this and
> that's exactly why it's fast. I was considering a similar approach
> with the reggae binary backend, and so I went and timed make, tup,
> ninja and itself on a synthetic project. Basically I wrote a program
> to write out source files to be compiled, with a runtime parameter
> indicating how many source files to write.
> The most extensive tests I did was on a synthetic project of 30k
> source files. That's a lot bigger than the vast majority of developers
> are ever likely to work on. As a comparison, the 2.6.11 version of the
> Linux kernel had 17k files.

Today's software projects are much bigger than you seem to imply. For
example, my work project *includes* the entire Linux kernel as part of
its build process, and the size of the workspace is dominated by the
non-Linux components. So 30k source files isn't exactly something
totally far out.

> A no-op build on my laptop was about (from memory):
> tup: <1s
> ninja, binary: 1.3s
> make: >20s
> It turns out that just stat'ing everything is fast enough for pretty
> much everybody, so I just kept the simple algorithm. Bear in mind the
> Makefiles here were the simplest possible - doing anything that
> usually goes on in Makefileland would have made it far, far slower. I
> know: I converted a build system at work from make to hand-written
> ninja and it no-op builds went from nearly 2 minutes to 1s.

Problem: stat() isn't good enough when network file sharing is involved.
It breaks correctness by introducing heisenbugs caused by (sometimes
tiny) differences in local hardware clocks. It also may break if two
versions of the same file share the same timestamp (often thought
impossible, but quite possible with machine-generated files and a
filesystem that doesn't have subsecond resolution -- and it's rare
enough that when it does happen people are left scratching their heads
for many wasted hours).   To guarantee correctness you need to compute a
digest of file contents, not just timestamp.

> If you happen to be unlucky enough to work on a project so large you
> need to watch the file system, then use the tup backend I guess.

Yes, I'm pretty sure that describes a lot of software projects out there
today. The scale of software these days is growing exponentially, and
there's no sign of it slowing down.  Or maybe that's just an artifact of
the field I work in? :-P


Never step over a puddle, always step around it. Chances are that whatever made 
it is still dripping.

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