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From:   Junio C Hamano <gitster@pobox.com>
To:     git@vger.kernel.org
Cc:     rsbecker@nexbridge.com, 'Jeff King' <peff@peff.net>,
        'Taylor Blau' <me@ttaylorr.com>,
        'Andy Koppe' <andy.koppe@gmail.com>,
        Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH] ci: avoid building from the same commit in parallel
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Date:   Mon, 21 Aug 2023 21:36:24 -0700
In-Reply-To: <xmqq1qfvor35.fsf_-_@gitster.g> (Junio C. Hamano's message of
        "Mon, 21 Aug 2023 17:31:26 -0700")
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Junio C Hamano <gitster@pobox.com> writes:

>  * There are tons of concurrency groups defined, but as a trial
>    change, here is to cover the "regular" matrix that consumes the
>    most resources (linux-asan-ubsan is the worst culprit, it seems).

Unfortunately, this did not work.

https://github.com/git/git/actions/runs/5933451874
https://github.com/git/git/actions/runs/5933451805

are trial runs that had the same commit with this patch pushed to
'seen' and 'pu'.  While one of them started the "regular" matrix,
the other one indeed went into paused state and waited.  But that is
way too late.  What happened was that their "config" (which everything
else depends on) started in parallel before the serialization at the
"regular" matrix kicked in.

So, one did wait before doing the "regular" matrix, until the other
one finished everything, and then kept going and did its own
"regular" matrix for the same commit.  It is because the avoidance
of "redundant build" was done at the "config" phase, which both of
them had already done X-<.

If we wanted to do this, I suspect that we need to serialize the
entire thing, not at the individual level where we currently define
the "concurrency" thing.