Practical parallelization of D compilation

[Chris Katko]

On Wednesday, 8 January 2020 at 06:51:57 UTC, H. S. Teoh wrote:
> On Wed, Jan 08, 2020 at 04:40:02AM +0000, Guillaume Lathoud via 
> Digitalmars-d-learn wrote:
>> [...]
> [...]
>
> Generally, the recommendation is to separately compile each 
> package. E.g., if you have a source tree of the form:
>
> 	src/
> 	src/main.d
> 	src/pkg1/mod1.d
> 	src/pkg1/mod2.d
> 	src/pkg2/mod3.d
> 	src/pkg2/mod4.d
>
> then you'd have 3 separate compilations:
>
> 	dmd -ofpkg1.o src/pkg1/mod1.d src/pkg1/mod2.d
> 	dmd -ofpkg2.o src/pkg2/mod3.d src/pkg2/mod4.d
> 	dmd -ofmyprogram src/main.d pkg1.o pkg2.o
>
> The first two can be done in parallel, since they are 
> independent of each other.
>
> The reason per-package granularity is suggested is because the 
> accumulated overhead of separately compiling every file makes 
> it generally not worth the effort.  D compiles fast enough that 
> per-package compilation is still reasonably fast, but you no 
> longer incur as much overhead from separately compiling every 
> file, yet you still retain the advantage of not recompiling the 
> entire program after every change.
>
> (Of course, the above example is greatly simplified; generally 
> you'd have about 10 or more files per package, and many more 
> packages, so the savings can be quite significant.)
>
>
> T

What's the downsides / difficulties / "hoops to jump through" 
penalty for putting code into modules instead of one massive 
project? Is it just a little extra handwriting/boilerplate, or is 
there a performance impact talking to other modules vs keeping it 
all in one?