Re: [PATCH v3] checkout: optimize "git checkout -b <new_branch>"

On 8/16/2018 2:37 PM, Duy Nguyen wrote:

On Thu, Aug 16, 2018 at 8:27 PM Ben Peart [off-list ref] wrote:

quoted

From: Ben Peart <redacted>

Skip merging the commit, updating the index and working directory if and
only if we are creating a new branch via "git checkout -b <new_branch>."
Any other checkout options will still go through the former code path.

If sparse_checkout is on, require the user to manually opt in to this
optimzed behavior by setting the config setting checkout.optimizeNewBranch
to true as we will no longer update the skip-worktree bit in the index, nor
add/remove files in the working directory to reflect the current sparse
checkout settings.

For comparison, running "git checkout -b <new_branch>" on a large repo takes:

14.6 seconds - without this patch
0.3 seconds - with this patch

I still don't think we should do this. If you want lightning fast
branch creation, just use 'git branch'. From the timing breakdown you
shown in the other thread it looks like sparse checkout still takes
seconds, which could be optimized (or even excluded, I mentioned this
too). And split index (or something similar if you can't use it) would
give you saving across the board. There is still one idea Elijah gave
me that should further lower traverse_trees()  cost.

We have investigated some of these already - split index ended up 
slowing things down more than it sped them up do to the higher compute 
costs.  Sparse checkout we've already optimized significantly - limiting 
the patterns we accept so that we can do the lookup via a hashmap 
instead of the robust pattern matching.  We will continue to look for 
other optimizations and appreciate any and all ideas!

In the end, this optimization makes a huge performance improvement by 
avoiding doing a lot of work that isn't necessary.  Taking a command 
from 14+ seconds to sub-second is just too much of a win for us to ignore.

But anyway, it's not my call. I'll stop here.