[speculative] Store cache files in bundle

[pfitzseb]

Much like the registry is nowadays stored in a single tar file.

This will help with performance on file systems that are slow when accessing many small files and also with overeager antivir software.

[davidanthoff]

Is that really a problem? I would have thought that the number of files we typically open is still small enough that this all works? Plus, not clear to me what the "bundle" unit would be? One gigantic file for all cached stuff? Then we need selective loads, which seems very involved...

[pfitzseb]

Apparently it can be, yes. I don't have a machine I can repro this on, but we got reports of a ~3x slowdown.

One gigantic file for all cached stuff? Then we need selective loads, which seems very involved...

Yes, it's not entirely clear whether this is worth the effort. Appending to a binary (e.g. a tarball) isn't very tricky though -- it's just a file system, basically.

[davidanthoff]

Hm, I still find this weird :) Essentially the number of files we need to load is equal to the number of precompile files that Julia needs to load, right? And that doesn't seem like a bottleneck?

The registry scenario seems like a completely different beast, I think? Wasn't the problem there updating it, because one had to read so many different files? But our update story is quite different, I think?

One thing we could do is use telemetry to get a sense how many deps a Manifest.toml typically has, that might be useful to think this through.