"Ben Peart" [off-list ref] writes:
I completely agree that optimizing within merge_working_tree would provide
more opportunities for optimization. I can certainly move the test into
that function as a first step.
Note that "optimizing more" was not the primary point of my
response.
Quite honestly, I'd rather see us speed up _ONLY_ obviously correct
and commonly used cases, while leaving most cases that _MAY_ turn
out to be optimizable (if we did careful analysis) unoptimized, and
instead have them handled by generic but known to be correct
codepath, if it means we do NOT to have to spend mental bandwidth to
analyze not-common case--that is a much better tradeoff.
The suggestion to move the check one level down in the callchain was
primarily to avoid the proposed optimization from being overly eager
and ending up skipping necessary parts of what merge_working_tree()
does (e.g. like I suspected in the review that the proposed patch
skips the check for "you have unmerged entries" situation).
-----Original Message-----
From: Junio C Hamano [mailto:gitster@pobox.com]
Sent: Monday, September 12, 2016 4:32 PM
To: Ben Peart <redacted>
Cc: git@vger.kernel.org; pclouds@gmail.com; 'Ben Peart'
[off-list ref]
Subject: Re: [PATCH v2] checkout: eliminate unnecessary merge for trivial
checkout
"Ben Peart" [off-list ref] writes:
quoted
I completely agree that optimizing within merge_working_tree would
provide more opportunities for optimization. I can certainly move the
test into that function as a first step.
Note that "optimizing more" was not the primary point of my response.
Quite honestly, I'd rather see us speed up _ONLY_ obviously correct and
commonly used cases, while leaving most cases that _MAY_ turn out to be
optimizable (if we did careful analysis) unoptimized, and instead have
them
handled by generic but known to be correct codepath, if it means we do NOT
to have to spend mental bandwidth to analyze not-common case--that is a
much better tradeoff.
The suggestion to move the check one level down in the callchain was
primarily to avoid the proposed optimization from being overly eager and
ending up skipping necessary parts of what merge_working_tree() does (e.g.
like I suspected in the review that the proposed patch skips the check for
"you have unmerged entries" situation).
The check for unmerged entries makes complete sense when you are about
to attempt to merge different commit trees and generate an updated index
and working directory. This optimization however is trying to skip
those expensive steps for the specific case of creating a new branch and
switching to it. In this narrow (but common) case, all that needs to
happen is that a new ref is created and HEAD switched to it. Since
we're not doing a merge, I don't believe the check is necessary.