113 lines
5.0 KiB
C++
113 lines
5.0 KiB
C++
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// Copyright Oliver Kowalke 2016.
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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#ifndef BOOST_FIBERS_SPINLOCK_TTAS_H
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#define BOOST_FIBERS_SPINLOCK_TTAS_H
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#include <atomic>
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#include <chrono>
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#include <random>
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#include <thread>
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#include <boost/fiber/detail/config.hpp>
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#include <boost/fiber/detail/cpu_relax.hpp>
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// based on informations from:
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// https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
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// https://software.intel.com/en-us/articles/long-duration-spin-wait-loops-on-hyper-threading-technology-enabled-intel-processors
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namespace boost {
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namespace fibers {
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namespace detail {
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class spinlock_ttas {
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private:
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enum class spinlock_status {
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locked = 0,
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unlocked
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};
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// align shared variable 'state_' at cache line to prevent false sharing
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alignas(cache_alignment) std::atomic< spinlock_status > state_{ spinlock_status::unlocked };
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// padding to avoid other data one the cacheline of shared variable 'state_'
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char pad[cacheline_length];
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public:
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spinlock_ttas() noexcept = default;
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spinlock_ttas( spinlock_ttas const&) = delete;
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spinlock_ttas & operator=( spinlock_ttas const&) = delete;
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void lock() noexcept {
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std::size_t collisions = 0 ;
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for (;;) {
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// avoid using multiple pause instructions for a delay of a specific cycle count
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// the delay of cpu_relax() (pause on Intel) depends on the processor family
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// the cycle count can not guaranteed from one system to the next
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// -> check the shared variable 'state_' in between each cpu_relax() to prevent
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// unnecessarily long delays on some systems
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std::size_t tests = 0;
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// test shared variable 'status_'
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// first access to 'state_' -> chache miss
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// sucessive acccess to 'state_' -> cache hit
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// if 'state_' was released by other fiber
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// cached 'state_' is invalidated -> cache miss
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while ( spinlock_status::locked == state_.load( std::memory_order_relaxed) ) {
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#if !defined(BOOST_FIBERS_SPIN_SINGLE_CORE)
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if ( BOOST_FIBERS_SPIN_MAX_TESTS > tests) {
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++tests;
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// give CPU a hint that this thread is in a "spin-wait" loop
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// delays the next instruction's execution for a finite period of time (depends on processor family)
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// the CPU is not under demand, parts of the pipeline are no longer being used
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// -> reduces the power consumed by the CPU
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cpu_relax();
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} else if ( BOOST_FIBERS_SPIN_MAX_TESTS + 20 > tests) {
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++tests;
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// std::this_thread::sleep_for( 0us) has a fairly long instruction path length,
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// combined with an expensive ring3 to ring 0 transition costing about 1000 cycles
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// std::this_thread::sleep_for( 0us) lets give up this_thread the remaining part of its time slice
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// if and only if a thread of equal or greater priority is ready to run
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static constexpr std::chrono::microseconds us0{ 0 };
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std::this_thread::sleep_for( us0);
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} else {
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// std::this_thread::yield() allows this_thread to give up the remaining part of its time slice,
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// but only to another thread on the same processor
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// instead of constant checking, a thread only checks if no other useful work is pending
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std::this_thread::yield();
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}
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#else
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std::this_thread::yield();
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#endif
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}
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// test-and-set shared variable 'status_'
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// everytime 'status_' is signaled over the bus, even if the test failes
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if ( spinlock_status::locked == state_.exchange( spinlock_status::locked, std::memory_order_acquire) ) {
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// spinlock now contended
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// utilize 'Binary Exponential Backoff' algorithm
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// linear_congruential_engine is a random number engine based on Linear congruential generator (LCG)
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static thread_local std::minstd_rand generator;
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const std::size_t z =
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std::uniform_int_distribution< std::size_t >{ 0, static_cast< std::size_t >( 1) << collisions }( generator);
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++collisions;
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for ( std::size_t i = 0; i < z; ++i) {
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cpu_relax();
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}
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} else {
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// success, thread has acquired the lock
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break;
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}
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}
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}
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void unlock() noexcept {
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state_.store( spinlock_status::unlocked, std::memory_order_release);
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}
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};
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}}}
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#endif // BOOST_FIBERS_SPINLOCK_TTAS_H
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