Feature: Reduce Mypy's Cache Size

Reducing Mypy's Cache Size

This is a meta-issue talking about different ways to reduce Mypy's cache size. I've been working on a branch in my free time, though it is probably too big for a PR, so I thought it would be best to get everyone's opinion on which optimizations (if any) would be worth-while.

In short, I've reduced Mypy's filesystem cache by about 27% using a few different techniques.

Here is a breakdown of each of the commits, what I did to reduce the cache size, and by how much. We probably don't need to include all of these techniques since a lot of them only marginally reduce the cache size. The cache I've been using as a comparison is Mypy's own cache when checking itself.

The Numbers

* Percent is calculated based on the original cache size of the master branch

The best techniques based on total savings are:

• Don't store fields that are nullable (8.4%)
• Don't store empty list/tuples in TypeInfo classes (4.5%)
• Replace .class key with empty string (4.4%)
• Drop builtins. prefix for common types (3.0%)
• Don't store arg_names and arg_kinds for func defs with type info (3.0%)

After that point everything starts to drop off, though they still might be worth including.

Backwards Compatibility

I made sure to check that my changes would not break backwards compatibility. These new techniques allow for loading of both old and new caches, but of course the new cache format will be used when cached files needs to be rebuild.

Why?

I was poking around the cache folder to see why the cache was the size it was, and I noticed that there was a lot of optimizations that could be made. In theory, smaller caches are quicker to save and load, while taking up less space on the users computer. In CI systems where storage space is metered, having a smaller cache size will speed up CI workflows and use up less cloud storage.

Let me know if this is something you guys would be interested in! If so I'll start splitting this into separate PR's. Thanks!

Inizia esaminando il branch shrink-cache e la suddivisione delle ottimizzazioni in questo issue, in particolare i maggiori risparmi proposti e la compatibilità dichiarata tra le cache vecchie e nuove. L'ambito non è ancora stato selezionato; il lavoro sarebbe considerato completato una volta concordate le tecniche che vale la pena adottare e suddiviso il lavoro scelto in pull request separate.