Re: time needed to rebase shortend by using --onto?

> Hello,
> 
> I have a kernel topic branch containing 4 patches on top of Linux v5.4.
> (I didn't speak to the affected customer, so I cannot easily share the
> patch stack. If need be I can probably anonymize it or ask if I can
> publish the patches.)
> 
> It rebases clean on v5.10:
> 
> 	$ time git rebase v5.10
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Successfully rebased and updated detached HEAD.
> 
> 	real	3m47.841s
> 	user	1m25.706s
> 	sys	0m11.181s
> 
> If I start with the same rev checked out and explicitly specify the
> merge base, the rebase process is considerably faster:
> 
> 	$ time git rebase --onto v5.10 v5.4
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Performing inexact rename detection: 100% (36806539/36806539), done.
> 	Successfully rebased and updated detached HEAD.
> 
> 	real	1m20.588s
> 	user	1m12.645s
> 	sys	0m6.733s
> 
> Is there some relevant complexity in the first invocation I'm not seeing
> that explains it takes more than the double time? I would have expected
> that
> 
> 	git rebase v5.10
> 
> does the same as:
> 
> 	git rebase --onto v5.10 $(git merge-base HEAD v5.10)
> 
> . (FTR:
> 
> 	$ time git merge-base HEAD v5.10
> 	219d54332a09e8d8741c1e1982f5eae56099de85
> 
> 	real	0m0.158s
> 	user	0m0.105s
> 	sys	0m0.052s
> 
> , 219d5433 is v5.4 as expected.
> 
> 	$ git version
> 	git version 2.29.2
> 
> That's from the Debian package 1:2.29.2-1~bpo10+1 on a Debian 10 box.)
> 
> Best regards
> Uwe
>