Don't permalloc the pkgimgs, but add option for PkgCacheInspector

[gbaraldi]

This saves some memory when loading the package images.
The option restores the current behavior for packages like https://github.com/timholy/PkgCacheInspector.jl to keep working.

This is a rebase of #48215

We might want to backport this to 1.9

[vchuravy]

Nominated for backport to 1.9 since it reduces memory usage significantly.

[giordano]

since it reduces memory usage significantly.

Do you have some numbers to share?

[vchuravy]

Omnipackage v1.9.0

Kbytes	RSS	Dirty
7984776	3494156	2072436

1.10 with this PR

Kbytes	RSS	Dirty
7194300	2713000	1619908

Just Trixi

1.8

Kbytes	RSS	Dirty
3438692	744456	584264

1.9

Kbytes	RSS	Dirty
2353464	1040784	613748

1.10 with Patch

Kbytes	RSS	Dirty
2112344	831728	429724

using pmap -x for the measurement and doing using OmniPackage / using Trixi respectivly.

[oscardssmith]

Can you add to the table 1.10 without the PR? (just for fairness)

[gbaraldi]

I think you start seeing benefits if you load the same package in multiple processes, because with this we can share

[ranocha]

Should this be backported to Julia v1.9.1?

[ranocha]

I think you start seeing benefits if you load the same package in multiple processes, because with this we can share

If this is the case, it would be great for our HPC applications using MPI

[gbaraldi]

It might be too late for 1.9.1. @KristofferC ?

[KristofferC]

For 1.9.1 yes.

[ranocha]

Thanks for the info - and for all the hard work on Julia, every new release, backports etc. I'm looking forward to v1.9.1 - and v1.9.2 including this fix 🙂