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author | sheaf <sam.derbyshire@gmail.com> | 2022-09-07 14:22:34 +0200 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2023-04-20 18:33:34 -0400 |
commit | 5c8731244bc13a3d813d2a4d53b3188b28dc8355 (patch) | |
tree | 49561b32034c2f4199479290975b2e624c8ffab1 /utils | |
parent | 7012ec2facc632fe4966916f797e4d1f612d7318 (diff) | |
download | haskell-5c8731244bc13a3d813d2a4d53b3188b28dc8355.tar.gz |
Implement -jsem: parallelism controlled by semaphores
See https://github.com/ghc-proposals/ghc-proposals/pull/540/ for a
complete description for the motivation for this feature.
The `-jsem` option allows a build tool to pass a semaphore to GHC which
GHC can use in order to control how much parallelism it requests.
GHC itself acts as a client in the GHC jobserver protocol.
```
GHC Jobserver Protocol
~~~~~~~~~~~~~~~~~~~~~~
This proposal introduces the GHC Jobserver Protocol. This protocol allows
a server to dynamically invoke many instances of a client process,
while restricting all of those instances to use no more than <n> capabilities.
This is achieved by coordination over a system semaphore (either a POSIX
semaphore [6]_ in the case of Linux and Darwin, or a Win32 semaphore [7]_
in the case of Windows platforms).
There are two kinds of participants in the GHC Jobserver protocol:
- The *jobserver* creates a system semaphore with a certain number of
available tokens.
Each time the jobserver wants to spawn a new jobclient subprocess, it **must**
first acquire a single token from the semaphore, before spawning
the subprocess. This token **must** be released once the subprocess terminates.
Once work is finished, the jobserver **must** destroy the semaphore it created.
- A *jobclient* is a subprocess spawned by the jobserver or another jobclient.
Each jobclient starts with one available token (its *implicit token*,
which was acquired by the parent which spawned it), and can request more
tokens through the Jobserver Protocol by waiting on the semaphore.
Each time a jobclient wants to spawn a new jobclient subprocess, it **must**
pass on a single token to the child jobclient. This token can either be the
jobclient's implicit token, or another token which the jobclient acquired
from the semaphore.
Each jobclient **must** release exactly as many tokens as it has acquired from
the semaphore (this does not include the implicit tokens).
```
Build tools such as cabal act as jobservers in the protocol and are
responsibile for correctly creating, cleaning up and managing the
semaphore.
Adds a new submodule (semaphore-compat) for managing and interacting
with semaphores in a cross-platform way.
Fixes #19349
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