Re: [PATCH v2] checkout: optimize "git checkout -b <new_branch>"

On Tue, Jul 31, 2018 at 7:03 PM Ben Peart <Ben.Peart@microsoft.com> wrote:

I'd like to see this giant list of checks broken down and pushed down to smaller areas so that chances of new things being added but checks not updated become much smaller. And ideally there should just be no behavior change (I think with your change, "checkout -b" will not report local changes, but it's not mentioned in the config document; more things like that can easily slip).

So. I assume this reason for this patch is because on super large worktrees

 - 2-way merge is too slow
 - applying spare checkout patterns on a huge worktree is also slow
 - writing index is, again, slow
 - show_local_changes() slow

For 2-way merge, I believe we can detect inside unpack_trees() that it's a 2-way merge (fn == twoway_merge), from HEAD to HEAD (simple enough check), then from the 2-way merge table we know for sure nothing is going to change and we can just skip traverse_trees() call in unpack_trees().

On the sparse checkout application. This only needs to be done when there are new files added, or the spare-checkout file has been updated since the last time it's been used. We can keep track of these things (sparse-checkout file change could be kept track with just stat info maybe as an index extension) then we can skip applying sparse checkout not for this particular case for but general checkouts as well. Spare checkout file rarely changes. Big win overall.

And if all go according to plan, there will be no changes made in the index (by either 2-way merge or sparse checkout stuff) we should be able to just skip writing down the index, if we haven't done that already.

show_local_changes() should be sped up significantly with the new cache-tree optimization I'm working on in another thread.

If I have not made any mistake in my analysis so far, we achieve a big speedup without adding a new config knob and can still fall back to slower-but-same-behavior when things are not in the right condition. I know it will not be the same speedup as this patch because when facing thousands of items, even counting them takes time. But I think it's a reasonable speedup without making the code base more fragile.