Understanding git gc --auto
garbage-collection, git
Solution
One of the main points of `gc --auto` is that it should be very quick, so other commands can frequently call it “just in case”. To achieve that, the object count is only guessed. As `git help config` says under `gc.auto`:
When there are approximately more than this many loose objects in the repository […]
Looking at the code (`too_many_loose_objects()` in `buildin/gc.c`), here’s what happens:
- The gc.auto is divided by 256 and rounded up
- The folder that contains all the objects that start with `17` is opened
- It is checked if the folder contains more objects than the result of step 1
This works fine, since SHA-1 is evenly distributed, so “all the objects that start with X” is representative for the whole set. But of course this only works for a big big amount of objects. To lazy to do the maths, I would guess at least >3000. With 6700 (the default value of `gc.auto`), this should already work quite reliably.
The core question for me is why you need such a low setting and whether it is important that this really runs at 250 objects. With a setting of 250, `gc` will run as soon as you have 2 loose objects that start with `17`. The chance that this happens is `> 80%` for 600 objects and `> 90%` for 800 objects.
Update: Couldn’t help it – had to do the math :). I was wondering how well that estimation system would work. Here’s a plot of the results. For any given `gc.auto`, how high is the probability that `gc` will start when there are `gc.auto` (red) / `gc.auto * 1.1` (green) / `gc.auto * 1.2` (orange) / `gc.auto * 1.5` (blue) / `gc.auto * 2` (purple) loose objects in the repo?
Problem
I'm experimenting with fairly aggressive auto gc in Git, mainly for packing purposes. In my repos if I do `git config --list` I have setup ``` ... gc.auto=250 gc.autopacklimit=30 ... ``` If I do `git count-objects -v` I get ``` count: 376 size: 1251 in-pack: 2776 packs: 1 size-pack: 2697 prune-packable: 0 garbage: 0 ``` But `git gc --auto` doesn't change these figures, nothing is being packed! shouldn't the loose objects get packed since I'm 126 objects over the gc.auto limit?