RE: [PATCH v2] checkout: eliminate unnecessary merge for trivial checkout

From: Ben Peart <peartben@gmail.com>
Date: Sep 13, 2016, 12:33 UTC
Message-ID: <001901d20dbb$0218f080$064ad180$@gmail.com>
In-Reply-To: <xmqqtwdkzwpp.fsf@gitster.mtv.corp.google.com>

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> -----Original Message-----
> From: Junio C Hamano [mailto:gitster@pobox.com]
> Sent: Monday, September 12, 2016 4:32 PM
> To: Ben Peart <peartben@gmail.com>
> Cc: git@vger.kernel.org; pclouds@gmail.com; 'Ben Peart'
> <benpeart@microsoft.com>
> Subject: Re: [PATCH v2] checkout: eliminate unnecessary merge for trivial
> checkout
> 
> "Ben Peart" <peartben@gmail.com> 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

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> 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.