Re: [PATCH 1/1] commit-reach: fix first-parent heuristic

Derrick Stolee [off-list ref] writes:

On 10/19/2018 1:24 AM, Junio C Hamano wrote:

quoted

"Derrick Stolee via GitGitGadget" [off-list ref] writes:

quoted

We can also re-run the performance tests from commit 4fbcca4e
"commit-reach: make can_all_from_reach... linear".

Performance was measured on the Linux repository using
'test-tool reach can_all_from_reach'. The input included rows seeded by
tag values. The "small" case included X-rows as v4.[0-9]* and Y-rows as
v3.[0-9]*. This mimics a (very large) fetch that says "I have all major
v3 releases and want all major v4 releases." The "large" case included
X-rows as "v4.*" and Y-rows as "v3.*". This adds all release-candidate
tags to the set, which does not greatly increase the number of objects
that are considered, but does increase the number of 'from' commits,
demonstrating the quadratic nature of the previous code.

These micro-benchmarks are interesting, but we should also remember
to describe the impact of the bug being fixed in the larger picture.
What end-user visible operations are affected?  Computing merge-base?
Finding if a push fast-forwards?  Something else?

Sorry about that. Here is a description that could be inserted into
the commit message:

The can_all_from_reach() method synthesizes the logic for one
iteration of can_all_from_reach_with_flags() which is used by the
server during fetch negotiation to determine if more haves/wants are
needed. The logic is also used by is_descendant_of() which is used to
check if a force-push is required and in 'git branch --contains'.

I am afraid that it is still not end-user serving enough.  The level
of "larger picture" I had in mind was those that would appear as an
entry in the release notes, e.g. (this is for illustration purposes
only; I do not claim its actual contents is correct).

	We started using commit generation numbers in various
	reachability computations, but due to a bug, negitiation
	between the "git fetch" and the server started to require
	30% more roundtrips than necessary, and it has become less
	efficient to see if a commit is a descendant of another
	commit in certain cases, which has been corrected in this
	release.

Thanks.