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Subject: bug#63412: Topological sorting in cuirass
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From: Ludovic =?UTF-8?Q?Court=C3=A8s?= <ludo@gnu.org>
In-Reply-To: <ZFtspPexmg3YM/ug@jurong> (Andreas Enge's message of "Wed, 10 May
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Hi,

Andreas Enge <andreas@enge.fr> skribis:

> This is a wishlist bug, but it is important for architectures where we
> are currently short on build power, and where this issue can stall builds
> and waste an arbitrary amount of build power.
>
> Cuirass should sort builds and only offload derivations for which all
> inputs are available.

Cuirass has two build backends: talking to the build daemon, and using
the ZeroMQ-based =E2=80=9Cremote worker=E2=80=9D protocol.  In practice we =
use the
latter, which fixes scalability issues with the former.

The worker protocol implements work stealing: workers periodically send
messages to =E2=80=9Cremote server=E2=80=9D asking for work.  Said server r=
eplies with a
derivation for one of the systems the worker supports; that derivation
is chosen among the =E2=80=9Cbuilds=E2=80=9D whose dependencies have all be=
en
successfully built.

And here=E2=80=99s the trick: the server is doing the right thing, but it h=
as a
partial view.  Namely, the server sees =E2=80=9Cbuilds=E2=80=9D rather than
=E2=80=9Cderivations=E2=80=9D.  =E2=80=9CBuilds=E2=80=9D are the things exp=
licitly declared in the
jobset.  If you have a declared build for GCC, but no build for MPC and
MPFR, then the server will consider that GCC has zero non-built
dependencies, even though MPFR and MPC may still need to be built.

Having =E2=80=98remote-worker=E2=80=99 operate on derivations rather than b=
uilds would
address this impedance mismatch, though there are complications (there
are bits of the database schema that amalgamate build/derivation).

Food for thought!

Ludo=E2=80=99.