On Tue, Sep 20, 2022 at 04:06:26PM -0400, Jeff King wrote: > On Tue, Sep 20, 2022 at 03:49:07PM -0400, Taylor Blau wrote: > > > > Is that true also of "multi-pack-index repack"? I guess it would depend > > > on how you invoke it. I admit I don't think I've ever used it myself, > > > since the new "repack --geometric --write-midx" approach matches my > > > mental model. I'm not sure when you'd actually run the "multi-pack-index > > > repack" command. But if you did it with --batch-size=0 (the default), I > > > think we'd end up traversing every object in history. > > > > We could probably benefit from it, but only if there is a MIDX bitmap > > around to begin with. For instance, you could first try and lookup each > > object you're missing a namehash for and then read its value out of the > > hashcache extension in the MIDX bitmap (assuming the MIDX bitmap exists, > > and has a hashcache). > > > > But if you don't have a MIDX bitmap, or it has a poor selection of > > commits, then you're out of luck. > > You could also use a pack bitmap if there is one (and it's one of the > included packs). But yes, if you have neither, it's no help. Good point. But, yeah, you have to have them to begin with. > Mostly I'm just concerned that this could have an outsized negative > performance effect if you have a setup like: > > - you have a gigantic repository, say that takes 15 minutes to do a > full "rev-list --objects" on (like linux.git with all its forks) > > - most of that is in one big pack, but you acquire new packs > occasionally via pushes, etc > > - doing "git repack --geometric" rolls up the new packs, nicely > traversing just the new objects > > - doing "git multi-pack-index repack" before your patch is fast-ish. > It stuffs all the objects into a new pack. But after your patch, it > does that 15-minute traversal. > > But I don't know if that's even realistic, because I'm still wondering > why somebody would run "git multi-pack-index repack" in the first place. > And if they'd always do so with --batch-size anyway, which would > mitigate this (because it gives a geometric-ish approach where we leave > the huge pack untouched). Yeah, the `--geometric` path(s) don't have this problem, because the big pack will already be covered by either a pack or MIDX bitmap, and we can read out all of the namehash values from there. But I tend to agree that this is pretty unrealistic, so I'm hopeful that it isn't a huge deal. If it is, though, we can always just "turn off" the traversal parts. (Though I have to imagine that a repository large enough to care about the existence of namehash values probably isn't using `git multi-pack-index repack` anyway). > Yeah, sorting the packs by mtime might be sensible. I know in the final > midx, we use object order to find the "preferred" pack. And you could > iterate the objects here, passing along their de-duped pack name. But I > don't think we have the objects here in that useful order; that is > really the order of the midx's .rev file, IIRC, and this is probably the > actual sha1 order. We already need the sorted order in order to compute the rollup for non-zero `--batch-size` arguments, so using that to construct the pack is just a matter of dragging the sort out of the function to compute the rollup itself (and into `midx_repack()` instead). Patches incoming... ;-) Thanks, Taylor
