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//! Multi-consumer generalization of triple buffering //! //! This crate is an extension to the triple buffering mechanism proposed as //! part of triple_buffer, which allows it to work with multiple consumers at //! the expense of some additional CPU and memory overhead. //! //! Like a triple buffer, the SPMC buffer presented here can be used when a //! single producer thread is frequently updating a shared data block, which is //! to be subsequently read by a set of consumer threads. Reading is always //! wait-free, writing can also be if sufficient storage is allocated. //! //! # Examples //! //! ``` //! // Create an SPMC buffer of any Clone type //! use spmc_buffer::SPMCBuffer; //! let buf = SPMCBuffer::new(2, 1.0); //! //! // Split it into an input and output interface //! let (mut buf_input, mut buf_output) = buf.split(); //! //! // Create as many extra output interfaces as needed //! let mut buf_output2 = buf_output.clone(); //! //! // The producer can move a value into the buffer at any time //! buf_input.write(4.2); //! //! // A consumer can access the latest value from the producer at any time //! let mut latest_value_ref = buf_output.read(); //! assert_eq!(*latest_value_ref, 4.2); //! let latest_value_ref2 = buf_output2.read(); //! assert_eq!(*latest_value_ref2, 4.2); //! ``` #![deny(missing_docs)] extern crate testbench; use std::cell::UnsafeCell; use std::ops::BitAnd; use std::sync::atomic::{AtomicUsize, Ordering}; use std::sync::Arc; /// A single-producer multiple-consumer buffer, useful for thread-safe data /// sharing in scenarios where triple buffering won't cut it. /// /// Triple buffering is an extremely efficient synchronization protocol when /// a producer thread wants to constantly update a value that is visible by a /// single consumer thread. However, it is not safe to use in the presence of /// multiple consumers, because a consumer thread can no longer assume that it /// is the only thread having access to the read buffer and discard said read /// buffer at will. /// /// Reference counting techniques can be used to build a variant of triple /// buffering which works for multiple consumers, remains provably wait-free /// if one uses two buffers per consumer, and degrades gracefully when a smaller /// amount of buffers is used as long as consumers frequently fetch updates from /// the producer. I call the resulting synchronization primitive an SPMC buffer. /// #[derive(Debug)] pub struct SPMCBuffer<T: Clone + PartialEq + Send> { /// Input object used by the producer to send updates input: SPMCBufferInput<T>, /// Clonable output object, used by consumers to read the current value output: SPMCBufferOutput<T>, } // impl<T: Clone + PartialEq + Send> SPMCBuffer<T> { /// Initialize an SPMC buffer with a certain amount of read buffers (which /// roughly determines how many readers can be accessing the structure at /// a slow pace before the writer starts to block). pub fn new(read_buffers: usize, initial: T) -> Self { // Check that the amount of read buffers fits implementation limits assert!(read_buffers <= MAX_READ_BUFFERS); // Compute the actual buffer count let num_buffers = 2 + read_buffers; // Create the shared state. Buffer 0 is initially considered the latest, // and has one reader accessing it (corresponding to a refcount of 1). let shared_state = Arc::new(SPMCBufferSharedState { buffers: vec![Buffer { data: UnsafeCell::new(initial), done_readers: AtomicRefCount::new(0), }; num_buffers], latest_info: AtomicSharedIndex::new(1), }); // ...then construct the input and output structs let mut result = SPMCBuffer { input: SPMCBufferInput { shared: shared_state.clone(), reader_counts: vec![0; num_buffers], }, output: SPMCBufferOutput { shared: shared_state, read_idx: 0, }, }; // Mark the latest buffer with an "infinite" reference count, to forbid // selecting it as a write buffer (it's reader-visible!) result.input.reader_counts[0] = INFINITE_REFCOUNT; // Return the resulting valid SPMC buffer result } /// Extract input and output of the SPMC buffer pub fn split(self) -> (SPMCBufferInput<T>, SPMCBufferOutput<T>) { (self.input, self.output) } } // // The Clone and PartialEq traits are used internally for testing. // impl<T: Clone + PartialEq + Send> Clone for SPMCBuffer<T> { fn clone(&self) -> Self { // Clone the shared state. This is safe because at this layer of the // interface, one needs an Input/Output &mut to mutate the shared state. let shared_state = Arc::new(unsafe { (*self.input.shared).clone() }); // ...then the input and output structs SPMCBuffer { input: SPMCBufferInput { shared: shared_state.clone(), reader_counts: self.input.reader_counts.clone(), }, output: SPMCBufferOutput { shared: shared_state, read_idx: self.output.read_idx, }, } } } // impl<T: Clone + PartialEq + Send> PartialEq for SPMCBuffer<T> { fn eq(&self, other: &Self) -> bool { // Compare the shared states. This is safe because at this layer of the // interface, one needs an Input/Output &mut to mutate the shared state. let shared_states_equal = unsafe { (*self.input.shared).eq(&*other.input.shared) }; // Compare the rest of the triple buffer states shared_states_equal && (self.input.reader_counts == other.input.reader_counts) && (self.output.read_idx == other.output.read_idx) } } /// Producer interface to SPMC buffers /// /// The producer can use this struct to submit updates to the SPMC buffer /// whenever he likes. These updates may or may not be nonblocking depending /// on the buffer size and the readout pattern. /// #[derive(Debug)] pub struct SPMCBufferInput<T: Clone + PartialEq + Send> { /// Reference-counted shared state shared: Arc<SPMCBufferSharedState<T>>, /// Amount of readers who potentially have access to each (unreachable) /// buffer. The latest buffer, which is still reachable, is marked with an /// "infinite" reference count, to warn that we don't know the true value. reader_counts: Vec<RefCount>, } // impl<T: Clone + PartialEq + Send> SPMCBufferInput<T> { /// Write a new value into the SPMC buffer pub fn write(&mut self, value: T) { // Access the shared state let ref shared_state = *self.shared; // Go into a spin-loop, waiting for an "old" buffer with no live reader. // This loop will finish in a finite amount of iterations if each thread // is allocated two private buffers, because readers can hold at most // two buffers simultaneously. With less buffers, we may need to wait. let mut write_pos: Option<usize> = None; while write_pos == None { // We want to iterate over both buffers and associated refcounts let mut buf_rc_iter = shared_state.buffers.iter().zip(self.reader_counts.iter()); // We want to find a buffer which is unreachable, and whose previous // readers have all moved on to more recent data. We identify // unreachable buffers by having previously tagged the latest buffer // with an infinite reference count. write_pos = buf_rc_iter.position(|tuple| { let (buffer, refcount) = tuple; *refcount == buffer.done_readers .load(Ordering::Relaxed) }); } let write_idx = write_pos.unwrap(); // The buffer that we just obtained has been freed by old readers and is // unreachable by new readers, so we can safely allocate it as a write // buffer and put our new data into it let ref write_buffer = shared_state.buffers[write_idx]; let write_ptr = write_buffer.data.get(); unsafe { *write_ptr = value; } // No one has read this version of the buffer yet, so we reset all // reference-counting information to zero. write_buffer.done_readers.store(0, Ordering::Relaxed); // Publish our write buffer as the new latest buffer, and retrieve // the old buffer's shared index let former_latest_info = shared_state.latest_info.swap( write_idx * SHARED_INDEX_MULTIPLIER, Ordering::Release // Publish updated buffer state to the readers ); // In debug mode, make sure that overflow did not occur debug_assert!(former_latest_info.bitand(SHARED_OVERFLOW_BIT) == 0); // Decode the information contained in the former shared index let former_idx = former_latest_info.bitand(SHARED_INDEX_MASK) / SHARED_INDEX_MULTIPLIER; let former_readcount = former_latest_info.bitand(SHARED_READCOUNT_MASK); // Write down the former buffer's refcount, and set the latest buffer's // refcount to infinity so that we don't accidentally write to it self.reader_counts[former_idx] = former_readcount; self.reader_counts[write_idx] = INFINITE_REFCOUNT; } } /// Consumer interface to SPMC buffers /// /// A consumer of data can use this struct to access the latest published update /// from the producer whenever he likes. Readout is nonblocking: a collision /// between the producer and a consumer will result cache contention induced /// slowdown, but deadlocks and scheduling-induced slowdowns cannot happen. /// #[derive(Debug)] pub struct SPMCBufferOutput<T: Clone + PartialEq + Send> { /// Reference-counted shared state shared: Arc<SPMCBufferSharedState<T>>, /// Index of the buffer which the consumer is currently reading from read_idx: BufferIndex, } // impl<T: Clone + PartialEq + Send> SPMCBufferOutput<T> { /// Access the latest value from the SPMC buffer pub fn read(&mut self) -> &T { // Access the shared state let ref shared_state = *self.shared; // Check if the producer has submitted an update let latest_info = shared_state.latest_info.load(Ordering::Relaxed); let update_available = latest_info.bitand(SHARED_INDEX_MASK) != (self.read_idx * SHARED_INDEX_MULTIPLIER); // If so, drop our current read buffer and go with the latest buffer if update_available { // Acquire access to the latest buffer, incrementing its // refcount to tell the producer that we have access to it let latest_info = shared_state.latest_info.fetch_add( 1, Ordering::Acquire // Fetch the associated buffer state ); // Drop our current read buffer. Because we already used an acquire // fence above, we can safely use relaxed atomic order here: no CPU // or compiler will reorder this operation before the fence. unsafe { self.discard_read_buffer(Ordering::Relaxed); } // In debug mode, make sure that overflow did not occur debug_assert!((latest_info + 1).bitand(SHARED_OVERFLOW_BIT) == 0); // Extract the index of our new read buffer self.read_idx = latest_info.bitand(SHARED_INDEX_MASK) / SHARED_INDEX_MULTIPLIER; } // Access data from the current (read-only) read buffer let read_ptr = shared_state.buffers[self.read_idx].data.get(); unsafe { &*read_ptr } } /// Drop the current read buffer. This is unsafe because it allows the /// writer to write into it, which means that the read buffer must never be /// accessed again after this operation completes. Be extremely careful with /// memory ordering: this operation must NEVER be reordered before a read! unsafe fn discard_read_buffer(&self, order: Ordering) { self.shared.buffers[self.read_idx].done_readers.fetch_add(1, order); } } // impl<T: Clone + PartialEq + Send> Clone for SPMCBufferOutput<T> { // Create a new output interface associated with a given SPMC buffer fn clone(&self) -> Self { // Clone the current shared state let shared_state = self.shared.clone(); // Acquire access to the latest buffer, incrementing its refcount let latest_info = shared_state.latest_info.fetch_add( 1, Ordering::Acquire // Fetch the associated buffer state ); // Extract the index of this new read buffer let new_read_idx = latest_info.bitand(SHARED_INDEX_MASK) / SHARED_INDEX_MULTIPLIER; // Build a new output interface from this information SPMCBufferOutput { shared: shared_state, read_idx: new_read_idx, } } } // impl<T: Clone + PartialEq + Send> Drop for SPMCBufferOutput<T> { // Discard our read buffer on thread exit fn drop(&mut self) { // We must use release ordering here in order to prevent preceding // buffer reads from being reordered after the buffer is discarded unsafe { self.discard_read_buffer(Ordering::Release); } } } /// Shared state for SPMC buffers /// /// This struct provides both a set of shared buffers for single-producer /// multiple-consumer broadcast communication and a way to know which of these /// buffers contains the most up to date data with reader reference counting. /// /// The number of buffers N is a design tradeoff: the larger it is, the more /// robust the primitive is against contention, at the cost of increased memory /// usage. An SPMC buffer is wait free for readers, and almost wait-free for /// writers, if N = Nreaders + 2, where Nreaders is the amount of consumers. But /// it can work correctly in a degraded regime which is wait-free for readers /// and potentially blocking for writers as long as N >= 2. /// /// Note that I said "almost" wait-free. True writer wait-freedom can only be /// proven in any circumstances by adding extra memory barriers to the /// consumer's algorithm, which can have a high cost on relaxed-memory archs /// like ARM and POWER. I do not consider that to be worth it when one can often /// just use more buffers if writer contention starts to be problematic. /// #[derive(Debug)] struct SPMCBufferSharedState<T: Clone + PartialEq + Send> { /// Data storage buffers buffers: Vec<Buffer<T>>, /// Combination of reader count and latest buffer index (see below) latest_info: AtomicSharedIndex, } // impl<T: Clone + PartialEq + Send> SPMCBufferSharedState<T> { /// Cloning the shared state is unsafe because you must ensure that no one /// is concurrently accessing it, since &self is enough for writing. unsafe fn clone(&self) -> Self { SPMCBufferSharedState { buffers: self.buffers.clone(), latest_info: AtomicSharedIndex::new( self.latest_info.load(Ordering::Relaxed) ), } } /// Equality is unsafe for the same reason as cloning: you must ensure that /// no one is concurrently accessing the triple buffer to avoid data races. unsafe fn eq(&self, other: &Self) -> bool { // Determine whether the contents of all buffers are equal let buffers_equal = self.buffers .iter() .zip(other.buffers.iter()) .all(|tuple| -> bool { let (buf1, buf2) = tuple; let dr1 = buf1.done_readers.load(Ordering::Relaxed); let dr2 = buf2.done_readers.load(Ordering::Relaxed); (*buf1.data.get() == *buf2.data.get()) && (dr1 == dr2) }); // Use that to deduce if the entire shared state is equivalent buffers_equal && (self.latest_info.load(Ordering::Relaxed) == other.latest_info.load(Ordering::Relaxed)) } } // unsafe impl<T: Clone + PartialEq + Send> Sync for SPMCBufferSharedState<T> {} // // #[derive(Debug)] struct Buffer<T: Clone + PartialEq + Send> { /// Actual data must be in an UnsafeCell so that Rust knows it's mutable data: UnsafeCell<T>, /// Amount of readers who are done with this buffer and switched to another done_readers: AtomicRefCount, } // impl<T: Clone + PartialEq + Send> Clone for Buffer<T> { /// WARNING: Buffers are NOT safe to clone, because a writer might be /// concurrently writing to them. The only reason why I'm not /// marking this function as unsafe is Rust would then not accept /// it as a Clone implementation, which would make Vec manipulation /// a lot more painful. fn clone(&self) -> Self { Buffer { data: UnsafeCell::new(unsafe { (*self.data.get()).clone() }), done_readers: AtomicRefCount::new(self.done_readers .load(Ordering::Relaxed)), } } } /// Atomic "shared index", combining "latest buffer" and "reader count" info /// in a single large integer through silly bit tricks. /// /// At the start of the readout process, a reader must atomically announce /// itself as in the process of reading the current buffer (so that said buffer /// does not get reclaimed) and determine which buffer is the current buffer. /// /// Here is why these operations cannot be separated: /// /// - Assume that the reader announces that it is reading, then determines which /// buffer is the current buffer. In this case, the reader can only make the /// generic announcement that it is reading "some" buffer, because it does not /// know yet which buffer it'll be reading. This means that other threads do /// not know which buffers are busy, and no buffer can be liberated until the /// reader clarifies its intent or goes away. This way of operating is thus /// effectively equivalent to a reader-directed update lock. /// - Assume that the reader determines which buffer is the current buffer, then /// announces itself as being in the process of reading this specific buffer. /// Inbetween these two actions, the current buffer may have changed, so the /// reader may increment the wrong refcount. Furthermore, the buffer that is /// now targeted by the reader may have already be tagged as safe for reuse or /// deletion by the writer, so if the reader proceeds with reading it, it may /// accidentally end up in a data race with the writer. This follows the /// classical rule of thumb that one should always reserve resources before /// accessing them, however lightly. /// /// To combine latest buffer index readout and reader count increment, we need /// to pack both of these quantities into a single shared integer variable that /// we can manipulate through a atomic operations. For refcounting, fetch_add /// sounds like a good choice, so we want an atomic integer type whose low-order /// bits act as a refcount and whose high-order bit act as a buffer index. /// Here's an example for a 16-bit unsigned integer, allowing up to 64 buffers /// and 511 concurrent readers on each buffer: /// /// bit (high-order first): 15 .. 10 9 8 .. 0 /// +--------+--+-------+ /// Contents: |BUFFERID|OF|READCNT| /// +--------+--+-------+ /// /// In this scheme, BUFFERID is the index of the "latest buffer", which contains /// the newest data from the writer, and READCNT is the amount of readers who /// have acquired access to this data. In principle, the later counter could /// overflow in the presence of 512+ concurrent readers, all accessing the same /// buffer without a single update happening in meantime. This scenario is /// highly implausible on current hardware architectures (even many-core ones), /// but we nevertheless account for it by adding an overflow "OF" bit, which is /// checked in debug builds. A thread which detects such overflow should panic. /// /// TODO: Switch to U16 / AtomicU16 once the later is stable /// type BufferIndex = usize; // type RefCount = usize; const INFINITE_REFCOUNT: RefCount = 0xffff; type AtomicRefCount = AtomicUsize; // type SharedIndex = usize; type AtomicSharedIndex = AtomicUsize; const SHARED_READCOUNT_MASK: SharedIndex = 0b0000_0001_1111_1111; const SHARED_OVERFLOW_BIT: SharedIndex = 0b0000_0010_0000_0000; const SHARED_INDEX_MASK: SharedIndex = 0b1111_1100_0000_0000; const SHARED_INDEX_MULTIPLIER: SharedIndex = 0b0000_0100_0000_0000; // const MAX_BUFFERS: usize = SHARED_INDEX_MASK / SHARED_INDEX_MULTIPLIER + 1; const MAX_READ_BUFFERS: usize = MAX_BUFFERS - 2; /// Unit tests #[cfg(test)] mod tests { use std::ops::BitAnd; use std::sync::{Arc, Condvar, Mutex}; use std::sync::atomic::Ordering; use std::thread; use std::time::Duration; use testbench; /// Check that SPMC buffers are properly initialized as long as the /// requested amount of concurrent readers stays in implementation limits. #[test] fn initial_state() { // Test for 0 readers (writer-blocking double-buffering limit) test_initialization(0); // Test for 1 concurrent reader (quadruple buffering) test_initialization(1); // Test for maximal amount of concurrent readers test_initialization(::MAX_READ_BUFFERS); } /// Check that SPMC buffer initialization panics if too many readers are /// requested with respect to implementation limits. #[test] #[should_panic] fn too_many_readers() { test_initialization(::MAX_READ_BUFFERS + 1); } /// Check that writing to an SPMC buffer works, but can be blocking #[test] fn write_write_sequence() { // Let's create a double buffer let mut buf = ::SPMCBuffer::new(0, 1.0); // Backup the initial buffer state let old_buf = buf.clone(); // Perform a write buf.input.write(4.2); // Analyze the new buffer state { // Starting from the old buffer state... let mut expected_buf = old_buf.clone(); let ref expected_shared = expected_buf.input.shared; // We expect the buffer which is NOT accessed by the current reader // to have received the new value from the writer. let old_read_idx = old_buf.output.read_idx; let write_idx = 1 - old_read_idx; let write_ptr = expected_shared.buffers[write_idx].data.get(); unsafe { *write_ptr = 4.2; } // We expect the latest buffer information to now point towards // this write buffer let new_latest_info = write_idx * ::SHARED_INDEX_MULTIPLIER; expected_shared.latest_info.store(new_latest_info, Ordering::Relaxed); // We expect the writer to have marked this write index as // unreachable, since it is now reader-visible, and to have fetched // the reference count of the former read buffer expected_buf.input.reader_counts[write_idx] = ::INFINITE_REFCOUNT; expected_buf.input.reader_counts[old_read_idx] = 1; // Nothing else should have changed assert_eq!(buf, expected_buf); } // At this point, all buffers are busy: the reader holds one buffer, and // the other is publicly visible. So trying to commit another write // should lead the writer into a waiting loop, from which it can only // exit if the reader drops its current buffer. Let's check that. { // Prepare some synchronization structures to follow writer progress let sync = Arc::new((Mutex::new(0), Condvar::new())); let writer_sync = sync.clone(); // Send a thread on a suicide mission to write into the buffer let (mut buf_input, mut buf_output) = buf.split(); let writer = thread::spawn(move || { *writer_sync.0.lock().unwrap() = 1; buf_input.write(2.4); *writer_sync.0.lock().unwrap() = 2; writer_sync.1.notify_all(); }); // Wait a bit to make sure that the writer cannot proceed let shared_lock = sync.0.lock().unwrap(); let wait_result = sync.1.wait_timeout(shared_lock, Duration::from_millis(100)); let (shared_lock, timeout_result) = wait_result.unwrap(); assert!(timeout_result.timed_out()); assert_eq!(*shared_lock, 1); // Make the reader check out the new buffer state, freeing the // buffer that it was previously holding let _ = buf_output.read(); // Check that the writer can now proceed let wait_result = sync.1.wait_timeout(shared_lock, Duration::from_millis(100)); let (shared_lock, timeout_result) = wait_result.unwrap(); assert!(!timeout_result.timed_out()); assert_eq!(*shared_lock, 2); // Wait for the writer to finish writer.join().unwrap(); } } /// Check that reading from an SPMC buffer works #[test] fn write_read_read_sequence() { // Let's create an SPMC buffer and write into it let mut buf = ::SPMCBuffer::new(0, false); buf.input.write(true); // Test readout from a dirty (freshly written) buffer { // Back up the initial buffer state let old_buf = buf.clone(); let ref old_shared = old_buf.input.shared; // Read from the buffer let result = *buf.output.read(); // Output value should be correct assert_eq!(result, true); // Starting from the old buffer state... let mut expected_buf = old_buf.clone(); let ref expected_shared = expected_buf.input.shared; // We expect the reader to have discarded its former read buffer let old_read_idx = old_buf.output.read_idx; expected_shared.buffers[old_read_idx] .done_readers .store(1, Ordering::Relaxed); // We expect the reader to be now accessing the new latest buffer let latest_idx = old_shared.latest_info .load(Ordering::Relaxed) .bitand(::SHARED_INDEX_MASK) / ::SHARED_INDEX_MULTIPLIER; expected_buf.output.read_idx = latest_idx; // We expect the latest buffer's reference count to have increased expected_shared.latest_info.fetch_add(1, Ordering::Relaxed); // Nothing else should have changed assert_eq!(buf, expected_buf); } // Test readout from a clean (unchanged) buffer { // Back up the initial buffer state let old_buf = buf.clone(); // Read from the buffer let result = *buf.output.read(); // Output value should be correct assert_eq!(result, true); // Buffer state should be unchanged assert_eq!(buf, old_buf); } } /// Check that writing after a dirty read works #[test] fn dirty_read_write_sequence() { // Let's create an SPMC buffer, write into it, and perform a dirty read let mut buf = ::SPMCBuffer::new(0, [1, 2, 3]); buf.input.write([4, 5, 6]); let _ = buf.output.read(); // Back up the current buffer state let old_buf = buf.clone(); // Write to the buffer again buf.input.write([7, 8, 9]); // Analyze the new buffer state { // Starting from the old buffer state... let mut expected_buf = old_buf.clone(); let ref expected_shared = expected_buf.input.shared; // We expect the buffer which is NOT accessed by the current reader // to have received the new value from the writer. let old_read_idx = old_buf.output.read_idx; let write_idx = 1 - old_read_idx; let ref write_buffer = expected_shared.buffers[write_idx]; let write_ptr = write_buffer.data.get(); unsafe { *write_ptr = [7, 8, 9]; } // We expect the buffer's reference counts to have been cleared write_buffer.done_readers.store(0, Ordering::Relaxed); // We expect the latest buffer information to now point towards // this write buffer let new_latest_info = write_idx * ::SHARED_INDEX_MULTIPLIER; expected_shared.latest_info.store(new_latest_info, Ordering::Relaxed); // We expect the writer to have marked this write index as // unreachable, since it is now reader-visible, and to have fetched // the reference count of the former read buffer expected_buf.input.reader_counts[write_idx] = ::INFINITE_REFCOUNT; expected_buf.input.reader_counts[old_read_idx] = 1; // Nothing else should have changed assert_eq!(buf, expected_buf); } } // Check that spawning a new reader and using it works #[test] fn spawn_new_reader() { // Let's create a double buffer let buf = ::SPMCBuffer::new(0, (64, 4.6)); // Backup the initial buffer state let old_buf = buf.clone(); // Clone the output let new_output = buf.output.clone(); // Analyze the new buffer state { // Starting from the old buffer state... let expected_buf = old_buf.clone(); let ref expected_shared = expected_buf.input.shared; // We expect the latest buffer's reference count to have increased let old_latest = expected_shared.latest_info .fetch_add(1, Ordering::Relaxed); // We expect the new reader to be pointing towards it let latest_idx = old_latest.bitand(::SHARED_INDEX_MASK) / ::SHARED_INDEX_MULTIPLIER; assert_eq!(new_output.read_idx, latest_idx); // Nothing else should have changed assert_eq!(buf, expected_buf); } } /// Check that uncontended concurrent reads and writes work /// /// **WARNING:** This test unfortunately needs to have timing-dependent /// behaviour to do its job. If it fails for you, try the following: /// /// - Close running applications in the background /// - Re-run the tests with only one OS thread (--test-threads=1) /// - Increase the writer sleep period /// #[test] #[ignore] fn uncontended_concurrent_access() { // Try it in the double-buffering regime test_rate_limited_writes(false); // Try it in the wait-free regime test_rate_limited_writes(true); } /// Check that contended reads and writes work /// /// **WARNING:** Caveats of uncontended concurrent tests also apply here. /// #[test] #[ignore] fn contended_concurrent_access() { // Try it in the double-buffering regime test_max_rate_writes(false); // Try it in the wait-free regime test_max_rate_writes(true); } /// Try initializing a buffer for some maximal wait-free readout concurrency fn test_initialization(read_buffers: usize) { // Create a buffer with the requested wait-free read concurrency let buf = ::SPMCBuffer::new(read_buffers, 42); // Access the shared state let ref buf_shared = *buf.input.shared; // Check that we have an appropriate amount of buffers let num_buffers = buf_shared.buffers.len(); assert_eq!(num_buffers, 2 + read_buffers); // Decode and check the latest buffer metadata: we should have one // reader, no refcount overflow, and a valid latest buffer index let latest_info = buf_shared.latest_info.load(Ordering::Relaxed); let reader_count = latest_info.bitand(::SHARED_READCOUNT_MASK); assert_eq!(reader_count, 1); let overflow = latest_info.bitand(::SHARED_OVERFLOW_BIT) != 0; assert!(!overflow); let latest_idx = latest_info.bitand(::SHARED_INDEX_MASK) / ::SHARED_INDEX_MULTIPLIER; assert!(latest_idx < num_buffers); // The reader must initially use the latest buffer as a read buffer assert_eq!(buf.output.read_idx, latest_idx); // The read buffer must be properly initialized let ref buffers = buf_shared.buffers; let read_ptr = buffers[latest_idx].data.get(); assert_eq!(unsafe { *read_ptr }, 42); // The outgoing reader count of each buffer must be 0 initially. for buffer in buffers { assert_eq!(buffer.done_readers.load(Ordering::Relaxed), 0); } // Every buffer except for the read buffer should be considered free // in the writer's internal reference counting records. The read buffer // should use a special infinite refcount to completely forbid writing. let indexes_and_refcounts = buf.input .reader_counts .iter() .enumerate(); for tuple in indexes_and_refcounts { let (index, refcount) = tuple; if index != latest_idx { assert_eq!(*refcount, 0); } else { assert_eq!(*refcount, ::INFINITE_REFCOUNT); } } } // Test concurrent access with a rate-limited writer, either in the double // buffering or in the wait_free regime fn test_rate_limited_writes(wait_free_regime: bool) { // We will stress the infrastructure by performing this many writes // as two readers continuously read the latest value const TEST_WRITE_COUNT: u64 = 5000; // Run the concurrent test run_concurrent_test( wait_free_regime, 0u64, |mut buf_input: ::SPMCBufferInput<u64>| { // The writer continuously increments the buffered value, with // some rate limiting to ensure the reader can see the updates for value in 1..(TEST_WRITE_COUNT + 1) { buf_input.write(value); thread::yield_now(); thread::sleep(Duration::from_millis(1)); } }, |mut buf_output: ::SPMCBufferOutput<u64>| { // The readers continuously check the buffered value, and should // see every update without any incoherent value in the middle let mut last_value = 0u64; while last_value != TEST_WRITE_COUNT { let new_value = *buf_output.read(); assert!((new_value >= last_value) && (new_value - last_value <= 1)); last_value = new_value; } } ); } // Test concurrent access with a writer writing at the maximal rate, either // in the double buffering or in the wait_free regime fn test_max_rate_writes(wait_free_regime: bool) { // We will stress the infrastructure by performing this many writes // as two readers continuously read the latest value const TEST_WRITE_COUNT: u64 = 20_000_000; // Run the concurrent test run_concurrent_test( wait_free_regime, 0u64, |mut buf_input: ::SPMCBufferInput<u64>| { // The writer increments the buffered value as fast as possible for value in 1..(TEST_WRITE_COUNT + 1) { buf_input.write(value); } }, |mut buf_output: ::SPMCBufferOutput<u64>| { // The readers continuously check the buffered value, and should // not spot any garbage value slipping in the middle let mut last_value = 0u64; while last_value != TEST_WRITE_COUNT { let new_value = *buf_output.read(); assert!((new_value >= last_value) && (new_value <= TEST_WRITE_COUNT)); last_value = new_value; } } ); } // Run a concurrent test with one producer and two consumers fn run_concurrent_test<T, P, C>(wait_free_regime: bool, initial: T, producer: P, consumer: C) where T: Clone + PartialEq + Send + 'static, P: FnOnce(::SPMCBufferInput<T>) + Send + 'static, C: Fn(::SPMCBufferOutput<T>) + Send + Sync + 'static { // Create an SPMC buffer with appropriate dimensions and initial content let wf_conc_readers = if wait_free_regime { 2 } else { 0 }; let buffer = ::SPMCBuffer::new(wf_conc_readers, initial); // Split the buffer into one input and two outputs let (buf_input, buf_output1) = buffer.split(); let buf_output2 = buf_output1.clone(); // Setup movable closures for the consumer threads let consumer1 = Arc::new(consumer); let consumer2 = consumer1.clone(); // Run the concurrent test testbench::concurrent_test_3(move || producer(buf_input), move || consumer1(buf_output1), move || consumer2(buf_output2)); } } /// Performance benchmarks /// /// These benchmarks masquerading as tests are a stopgap solution until /// benchmarking lands in Stable Rust. They should be compiled in release mode, /// and run with only one OS thread. In addition, the default behaviour of /// swallowing test output should obviously be suppressed. /// /// TL;DR: cargo test --release -- --ignored --nocapture --test-threads=1 /// /// TODO: Switch to standard Rust benchmarks once they are stable /// #[cfg(test)] mod benchmarks { use testbench; /// Benchmark for clean read performance #[test] #[ignore] fn clean_read() { // Create a buffer let mut buf = ::SPMCBuffer::new(1, 0u32); // Benchmark clean reads testbench::benchmark(3_000_000_000u32, || { let read = *buf.output.read(); assert!(read < u32::max_value()); }); } /// Benchmark for write performance #[test] #[ignore] fn write() { // Create a buffer let mut buf = ::SPMCBuffer::new(1, 0u32); // Benchmark writes let mut iter = 1u32; testbench::benchmark(300_000_000u32, || { buf.input.write(iter); iter += 1; }); } /// Benchmark for write + dirty read performance #[test] #[ignore] fn write_and_dirty_read() { // Create a buffer let mut buf = ::SPMCBuffer::new(1, 0u32); // Benchmark writes + dirty reads let mut iter = 1u32; testbench::benchmark(140_000_000u32, || { buf.input.write(iter); iter += 1; let read = *buf.output.read(); assert!(read < u32::max_value()); }); } /// Benchmark read performance under concurrent write pressure #[test] #[ignore] fn concurrent_read() { // Create a buffer let buf = ::SPMCBuffer::new(1, 0u32); let (mut buf_input, mut buf_output) = buf.split(); // Benchmark reads under concurrent write pressure let mut counter = 0u32; testbench::concurrent_benchmark( 80_000_000u32, move || { let read = *buf_output.read(); assert!(read < u32::max_value()); }, move || { buf_input.write(counter); counter = (counter + 1) % u32::max_value(); } ); } /// Benchmark write performance under concurrent read pressure #[test] #[ignore] fn concurrent_write() { // Create a buffer let buf = ::SPMCBuffer::new(1, 0u32); let (mut buf_input, mut buf_output) = buf.split(); // Benchmark writes under concurrent read pressure let mut iter = 1u32; testbench::concurrent_benchmark( 30_000_000u32, move || { buf_input.write(iter); iter += 1; }, move || { let read = *buf_output.read(); assert!(read < u32::max_value()); } ); } }