ogiroux / atomic_wait

Sample implementation of C++20 atomic_wait/notify

Sample implementation of C++20 synchronization facilities

This repository contains a sample implementation of these http://wg21.link/p1135 facilities:

• atomic_wait / _notify free-functions (not the member functions)
• counting_ / binary_semaphore
• latch and barrier

How do I build the sample?

This project is self-contained.

git clone https://github.com/ogiroux/atomic_wait
build.sh

What platforms are supported?

Linux, Mac and Windows.

How are atomic_wait / _notify composed?

The implementation has a variety of strategies that it selects by platform:

• Contention state table. Optimizes futex usage, or holds CVs, unless -D__NO_TABLE.
• Futex. Supported on Linux and Windows, unless -D__NO_FUTEX. Requires a table on Linux.
• Condition variables. Supported on Linux and Mac, unless -D__NO_CONDVAR. Requires a table.
• Timed back-off. Supported on everything, unless -D__NO_SLEEP.
• Spinlock. Supported on everything, only used last unless -D__NO_IDENT.

These strategies are selected for each platform, in the order written, based on what's disabled with the macros:

• Linux: futex + table -> CVs + table -> timed backoff -> spin.
• Mac: CVs + table -> timed backoff -> spin.
• Windows: futex -> timed backoff -> spin.
• CUDA: timed backoff -> spin.
• Unidentified platform: spin.

How do counting_ / binary_semaphore work?

The implementation has these specializations:

• The fully general case, for counting_semaphore instantiated for huge numbers. This is implemented in terms of atomic<ptrdiff_t>, atomic_wait / _notify. This path is always enabled.
• The constrained case, the default range, for numbers supported by the underlying platform semaphore (typically a long). This is implemented in terms of POSIX, Dispatch and Win32 semaphores, with optimizations below. Disable this path with -D__NO_SEM.
• The case of a unit range, such as the alias binary_semaphore. This is specialized only when platform semaphores are disabled. This path uses atomic<ptrdiff_t>, atomic_wait / _notify.

Platform semaphores get (because they need) some additional optimizations, in up two orthogonal directions:

• Front buffering: an atomic object models the semaphore's conceptual count (incl. negative values). Operations to the platform semaphore are avoided as long as the modeled count stays positive. This is enabled by default on all platforms, but can be disabled with -D__NO_SEM_FRONT.
• Back buffering: when the platform semaphore does not natively support the release( count ) operation, an atomic object distributes the release(1) cooperatively among all threads waiting on the semaphore, as in a binary tree. This is used by default on Linux and Mac OS X, and can be disabled with -D__NO_SEM_BACK.

What about latch and barrier?

At the moment, they are only implemented in terms of atomic operations. These aren't ready for review.