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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 w...
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