Re: [PATCH v4 0/8] repack: support repacking into a geometric sequence

On Tue, Feb 23, 2021 at 12:54:56PM -0700, Martin Fick wrote:

quoted

Yeah, this is definitely a heuristic that can get out of sync with
reality. I think in general if you have base pack A and somebody pushes
up B, C, and D in sequence, we're likely to roll up a single DBC (in
that order) pack. Further pushes E, F, G would have newer mtimes. So we
might get GFEDBC directly. Or we might get GFE and DBC, but the former
would still have a newer mtime, so we'd create GFEDBC on the next run.

The issues come from:

  - we are deciding what to roll up based on size. A big push might not
    get rolled up immediately, putting it out-of-sync with the rest of
    the rollups.

Would it make sense to somehow detect all new packs since the last rollup and 
always include them in the rollup no matter what their size? That is one thing 
that my git-exproll script did. One of the main reasons to do this was because 
newer packs tended to look big (I was using bytes to determine size), and 
newer packs were often bigger on disk compared to other packs with similar 
objects in them (I think you suggested this was due to the thickening of packs 
on receipt). Maybe roll up all packs with a timestamp "new enough", no matter 
how big they are?

That works against the "geometric" part of the strategy, which is trying
to roll up in a sequence that is amortized-linear. I.e., we are not
always rolling up everything outside of the base pack, but trying to
roll up little into medium, and then eventually medium into large. If
you roll up things that are "too big", then you end up rewriting the
bytes more often, and your amount of work becomes super-linear.

Now whether that matters all that much or not is perhaps another
discussion. The current strategy is mostly to repack all-into-one with
no base, which is the worst possible case. So just about any rollup
strategy will be an improvement. ;)

-Peff