Re: [PATCH 1/6] t1092: use ORT merge strategy

From: Derrick Stolee <stolee@gmail.com>
Date: Aug 18, 2021, 18:42 UTC
Message-ID: <b3c7e700-f36c-d58f-0c49-450444df8c63@gmail.com>
In-Reply-To: <xmqqzgte62ud.fsf@gitster.g>

On 8/18/2021 2:10 PM, Junio C Hamano wrote:

Show 11 quoted lines

> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> From: Derrick Stolee <dstolee@microsoft.com>
>>
>> The sparse index will be compatible with the ORT merge strategy, so
>> let's use it explicitly in our tests.
> 
> Unless you mean that the sparse index will only be compatible with
> ort, but will never be with recursive, I do not quite see why this
> is taking us into a good direction.  Is this because we want to gain
> test coverage for ort early, before we flip the default to ort [*1*]?

The sparse index will _work_ with the recursive merge strategy, it will just continue to be slow, and likely slower than if we had a full index. This is because the recursive merge algorithm will expand a sparse index into a full one before doing any of its logic (hence my confidence that it will work).

The main point why ORT is the focus is that the ORT strategy is required so the sparse index can get the intended performance gains (i.e. it does not expand in most cases). The ORT algorithm can resolve conflicts outside the sparse-checkout cone without needing the index as a data structure and instead the resulting tree is recorded in the correct sparse directory entry.

Show 14 quoted lines

> [Footnote]
> 
> *1* If the answer is "no, it is because sparse index will not work
>     with recursive", the please disregard the rest, but just in
>     case it is not...
> 
>     It seems to me that it would let us live in the future in a more
>     comprehensive way if we tweaked merge_recursive() and/or
>     merge_recursive_generic() so that all internal callers, not just
>     builtin/merge.c, would redirect to the ort machinery when say
>     GIT_TEST_REPLACE_RECURSIVE_WITH_ORT environment exists, and
>     doing it that way we do not need to sprinkle "-srecursive" and
>     "-sort" everywhere in our tests at randomly chosen places to
>     give test coverage to both strategies.

I could also change this patch to stop using ORT _all the time_ and instead let the GIT_TEST_MERGE_ALGORITHM decide which is tested.

That is, except for the final tests that check that the index is not expanded. Those tests must specify the ORT strategy explicitly.

I think I started playing with the GIT_TEST_MERGE_ALGORITHM because it appears to override the command-line option, but I will need to double-check that.

Thanks, -Stolee