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On Apple Silicon Macs, using a Darwin thread priority of PRIO_DARWIN_BG seems to map directly to the QoS class Background. With this priority, the thread is confined to efficiency cores only, which makes background indexing take forever. Introduce a new ThreadPriority "Low" that sits in the middle between Background and Default, and maps to QoS class "Utility" on Mac. Make this new priority the default for indexing. This makes the thread run on all cores, but still lowers priority enough to keep the machine responsive, and not interfere with user-initiated actions. I didn't change the implementations for Windows and Linux; on these systems, both ThreadPriority::Background and ThreadPriority::Low map to the same thread priority. This could be changed as a followup (e.g. by using SCHED_BATCH for Low on Linux). See also https://github.com/clangd/clangd/issues/1119. Reviewed By: sammccall, dgoldman Differential Revision: https://reviews.llvm.org/D124715
llvm/lib/Support/Unix README =========================== This directory provides implementations of the lib/System classes that are common to two or more variants of UNIX. For example, the directory structure underneath this directory could look like this: Unix - only code that is truly generic to all UNIX platforms Posix - code that is specific to Posix variants of UNIX SUS - code that is specific to the Single Unix Specification SysV - code that is specific to System V variants of UNIX As a rule, only those directories actually needing to be created should be created. Also, further subdirectories could be created to reflect versions of the various standards. For example, under SUS there could be v1, v2, and v3 subdirectories to reflect the three major versions of SUS.