Tweaking

Boost.Fiber enables synchronization of fibers running on different threads per default. This is accomplished by spinlocks (using atomics). (BOOST_FIBERS_SPINLOCK_STD_MUTEX defined at the compiler’s command line enables std::mutex instead of spinlocks).

disable synchronization

With BOOST_FIBERS_NO_ATOMICS defined at the compiler’s command line, synchronization between fibers (in different threads) is disabled. This is acceptable if the application is single threaded and/or fibers are not synchronized between threads.

TTAS locks

Spinlocks are implemented as TTAS locks, i.e. the spinlock tests the lock before calling an atomic exchange. This strategy helps to reduce the cache line invalidations triggered by acquiring/releasing the lock.

spin-wait loop

A lock is considered under high contention, if a thread repeatedly fails to acquire the lock because some other thread was faster. Waiting for a short time lets other threads finish before trying to enter the critical section again. While busy waiting on the lock, relaxing the CPU (via pause/yield memnonic) gives the CPU a hint that the code is in a spin-wait loop.

prevents expensive pipeline flushes (speculatively executed load and compare instructions are not pushed to pipeline)
another hardware thread (simultaneous multithreading) can get time slice
it does delay a few CPU cycles, but this is necessary to prevent starvation

It is obvious that this strategy is useless on single core systems because the lock can only released if the thread gives up its time slice in order to let other threads run. The macro BOOST_FIBERS_SPIN_SINGLE_CORE disables active spinning, in other words, the operating system is notified (via std::this_thread_yield()) that the thread gives up its time slice and the operating system switches to another thread.

exponential back-off

The macro BOOST_FIBERS_SPIN_MAX_TESTS determines how many times the CPU iterates in the spin-wait loop before yielding the thread or blocking in futex-wait. The spinlock tracks how many times the thread failed to acquire the lock. The higher the contention, the longer the thread should back-off. A “Binary Exponential Backoff” algorithm together with a randomized contention window is utilized for this purpose. BOOST_FIBERS_SPIN_MAX_COLLISIONS determines the upper limit of collisions between threads after the thread waits on a futex.

Table 1.4. macros for tweaking

Macro	Effect on Boost.Fiber
BOOST_FIBERS_NO_ATOMICS	no multithreading support, all atomics removed, no synchronization between fibers running in different threads
BOOST_FIBERS_SPINLOCK_STD_MUTEX	`std::mutex` used inside spinlock
BOOST_FIBERS_SPINLOCK_TTAS	spinlock with test-test-and-swap on shared variable
BOOST_FIBERS_SPINLOCK_TTAS_ADAPTIVE	spinlock with test-test-and-swap on shared variable, adaptive retries while busy waiting
BOOST_FIBERS_SPINLOCK_TTAS_FUTEX	spinlock with test-test-and-swap on shared variable, suspend on futex after certain number of retries
BOOST_FIBERS_SPINLOCK_TTAS_ADAPTIVE_FUTEX	spinlock with test-test-and-swap on shared variable, while busy waiting adaptive retries, suspend on futex certain amount of retries
BOOST_FIBERS_SPIN_SINGLE_CORE	on single core machines with multiple threads, yield thread (`std::this_thread::yield()`) after collisions
BOOST_FIBERS_SPIN_MAX_TESTS	max number of retries while busy spinning
BOOST_FIBERS_SPIN_MAX_COLLISIONS	max number of collisions between contending threads