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TriggerFd used a pipe on every platform without eventfd. On QNX, pipes are served by the single-threaded pipe resource manager, which makes them slow under load. A socketpair is pollable, nonblocking, and can be passed over SCM_RIGHTS, so it is a drop-in replacement for the pipe on macOS, QNX, and other POSIX systems. Linux keeps using eventfd. Add triggerfd_test covering create, trigger/clear, repeated triggers, non-blocking behaviour when the buffer is full, and move semantics.
QNX pipes go through a single-threaded pipe service. On macOS a socketpair was slower than a pipe, especially when a publish wakes another thread, and its read end always reports POLLOUT. Linux stays on eventfd and other POSIX systems stay on a pipe.
dallison
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Summary
TriggerFduses a Unix domain socketpair. QNX pipes go through a single-threaded pipe service.eventfd. macOS and other POSIX systems stay on a pipe. A socketpair was slower than a pipe on macOS, especially when a publish wakes another thread, and a socketpair read end always reportsPOLLOUT.Test plan
bazelisk test //toolbelt:triggerfd_teston macOS, which builds the pipe path. 1/1 passed.