diff options
Diffstat (limited to 'boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp')
-rw-r--r-- | boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp | 63 |
1 files changed, 44 insertions, 19 deletions
diff --git a/boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp b/boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp index 61ab47691e..0f0b191e67 100644 --- a/boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp +++ b/boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp @@ -26,21 +26,28 @@ namespace detail { class spinlock_ttas_adaptive_futex { private: - std::atomic< std::int32_t > value_{ 0 }; - std::atomic< std::int32_t > tests_{ 0 }; + template< typename FBSplk > + friend class spinlock_rtm; + + std::atomic< std::int32_t > value_{ 0 }; + std::atomic< std::int32_t > retries_{ 0 }; public: - spinlock_ttas_adaptive_futex() noexcept = default; + spinlock_ttas_adaptive_futex() = default; spinlock_ttas_adaptive_futex( spinlock_ttas_adaptive_futex const&) = delete; spinlock_ttas_adaptive_futex & operator=( spinlock_ttas_adaptive_futex const&) = delete; void lock() noexcept { - std::int32_t collisions = 0, tests = 0, expected = 0; - const std::int32_t prev_tests = tests_.load( std::memory_order_relaxed); - const std::int32_t max_tests = (std::min)( static_cast< std::int32_t >( BOOST_FIBERS_SPIN_MAX_TESTS), 2 * prev_tests + 10); + static thread_local std::minstd_rand generator{ std::random_device{}() }; + std::int32_t collisions = 0, retries = 0, expected = 0; + const std::int32_t prev_retries = retries_.load( std::memory_order_relaxed); + const std::int32_t max_relax_retries = (std::min)( + static_cast< std::int32_t >( BOOST_FIBERS_SPIN_BEFORE_SLEEP0), 2 * prev_retries + 10); + const std::int32_t max_sleep_retries = (std::min)( + static_cast< std::int32_t >( BOOST_FIBERS_SPIN_BEFORE_YIELD), 2 * prev_retries + 10); // after max. spins or collisions suspend via futex - while ( max_tests > tests && BOOST_FIBERS_SPIN_MAX_COLLISIONS > collisions) { + while ( retries++ < BOOST_FIBERS_RETRY_THRESHOLD) { // 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 @@ -52,26 +59,39 @@ public: // if 'value_' was released by other fiber // cached 'value_' is invalidated -> cache miss if ( 0 != ( expected = value_.load( std::memory_order_relaxed) ) ) { - ++tests; #if !defined(BOOST_FIBERS_SPIN_SINGLE_CORE) - // 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(); + if ( max_relax_retries > retries) { + // 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 if ( max_sleep_retries > retries) { + // 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() allows this_thread to give up the remaining part of its time slice, + // but only to another thread on the same processor + // instead of constant checking, a thread only checks if no other useful work is pending + std::this_thread::yield(); + } #else // std::this_thread::yield() allows this_thread to give up the remaining part of its time slice, // but only to another thread on the same processor // instead of constant checking, a thread only checks if no other useful work is pending std::this_thread::yield(); #endif - } else if ( ! value_.compare_exchange_strong( expected, 1, std::memory_order_acquire, std::memory_order_release) ) { + } else if ( ! value_.compare_exchange_strong( expected, 1, 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::int32_t > distribution{ 0, static_cast< std::int32_t >( 1) << collisions }; + std::uniform_int_distribution< std::int32_t > distribution{ + 0, static_cast< std::int32_t >( 1) << (std::min)(collisions, static_cast< std::int32_t >( BOOST_FIBERS_CONTENTION_WINDOW_THRESHOLD)) }; const std::int32_t z = distribution( generator); ++collisions; for ( std::int32_t i = 0; i < z; ++i) { @@ -81,7 +101,7 @@ public: } } else { // success, lock acquired - tests_.store( prev_tests + (tests - prev_tests) / 8, std::memory_order_relaxed); + retries_.store( prev_retries + (retries - prev_retries) / 8, std::memory_order_relaxed); return; } } @@ -95,7 +115,12 @@ public: expected = value_.exchange( 2, std::memory_order_acquire); } // success, lock acquired - tests_.store( prev_tests + (tests - prev_tests) / 8, std::memory_order_relaxed); + retries_.store( prev_retries + (retries - prev_retries) / 8, std::memory_order_relaxed); + } + + bool try_lock() noexcept { + std::int32_t expected = 0; + return value_.compare_exchange_strong( expected, 1, std::memory_order_acquire); } void unlock() noexcept { |