Re: [PATCH] odb/files: be less aggressive with geometric repacking

[Patrick Steinhardt <ps@pks.im>]

On Tue, Aug 11, 2026 at 03:44:12PM -0500, Justin Tobler wrote:
> On 26/08/11 11:04AM, Patrick Steinhardt wrote:
> > When performing auto-maintenance with geometric repacking we have two
> > conditions that may trigger a repack:
> > 
> >   - Either the geometric sequence of packfiles is invalidated.
> > 
> >   - Or we have too many loose objects.
> > 
> > The first condition shouldn't trigger all that often: it may be hit when
> > we fetch a new packfile, but users tend to not do that all the time. The
> > second condition is what typically triggers more regularly though, as
> > every command that ends up writing new objects may cause us to cross the
> > threshold of loose objects. It is thus preferable to not be too
> > aggressive here, as otherwise we may end up repacking objects quite
> > often.
> > 
> > For the geometric-repacking strategy though we have a default of 100
> > objects, only. As we're approximating the count of objects by only
> > reading the "objects/17/" shared, we'd only need 2 objects in there
> > before we perform a repack by default, which is quite aggressive.
> > git-gc(1) on the other hand has a default of 6700, so it is quite a bit
> > more conservative here.
> 
> Ok IIUC, the reason two loose objects can potentially trigger repacking
> is because the heuristic used to estimate the number of loose objects
> only counts objects in "objects/17/" and multiples it by 256 (the
> maximum number of directories that are fanned-out). That makes sense and
> indeed seems like it could lead to repacking processes be spawned more
> frequently than desired.
> 
> My first thought is whether the heuristic itself should be updated to
> capture a more accurate estimate for the number of objects. That would
> of course require looking up more objects and thus be more expensive. If
> the goal here is just for a very rough estimate anyways, maybe it
> wouldn't be worth it though.

That wouldn't really solve the problem though. The problem is not really
that the estimation can be wrong, it's rather that even if it was always
correct we're still being too aggressive with packing the loose objects.
Because ultimately, a 100 objects is a comparatively small threshold,
and leads to 67 times more repacking compared to git-gc(1).

> Increasing the loose object threshold here to be more conservative seems
> like a reasonable approach. I'm not sure exactly why 6700 was chosen
> here. 6700 / 256 ~= 26.2 which means "objects/17/" would have to contain
> at least 27 objects before repacking is triggered. That is certainly
> much more conservative. I see that 6700 has also been chosen else where
> in the codebase as the threshold too. It might be nice to explain the
> reasoning a bit more in the commit message though.

Hmm, don't I already do that? In the paragraph you're responding to I'm
saying that git-gc(1) already had that default forever, so I'm adjusting
our heuristic to match that.

Thanks!

Patrick