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