Performance hitch when typing in the search box while versions tab is open

[JonnyOThan]

I'm pretty sure the search box isn't the only way to trigger this but I found a consistent repro:

1. open the versions tab
2. in the search box, start typing "ksp community fixes"
3. When you hit the second 'm', CKAN will hang for a few seconds

The first time I did this the hang was probably ~20 seconds. Doing it again resulted in a shorter hang, but it's still several seconds long.

I'm running this on the latest dev version so I don't have symbols (huh apparently there are some symbols in here; just not jitted code maybe) but maybe this will help:

The thread that eventually unblocked the main thread is here:

Kinda hard to tell, but maybe calling ToList on a parallel enumerator is doing some bad stuff?

[HebaruSan]

I think we have over-parallelization here. RelationshipResolver.ModList is parallel (for speeding up very large changesets), and Versions.checkInstallable calls it in parallel (via ModuleInstaller.CanInstall; that's what feeds the ToList call highlighted above). This nesting can do wacky things to threads; I hit this a couple of times while parallelizing other stuff. This probably also explains why the Versions tab doesn't start filling in right away.

That link recommends only parallelizing the outer loop.

[HebaruSan]

... confirmed (more or less). Commenting this line makes the inner loop non-parallel, which makes the response to color the list instantaneous and fixes the hang-up issue in the OP:

CKAN/Core/Relationships/RelationshipResolver.cs

Line 525 in 3b16d38

.AsParallel()

... but I think we'll want to be able to make that conditional somehow in case there are other places that would still benefit from it being parallel...