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//! A lock-free concurrent slab. //! //! Slabs provide pre-allocated storage for many instances of a single data //! type. When a large number of values of a single type are required, //! this can be more efficient than allocating each item individually. Since the //! allocated items are the same size, memory fragmentation is reduced, and //! creating and removing new items can be very cheap. //! //! This crate implements a lock-free concurrent slab, indexed by `usize`s. //! //! # Examples //! //! Inserting an item into the slab, returning an index: //! ```rust //! # use sharded_slab::Slab; //! let slab = Slab::new(); //! //! let key = slab.insert("hello world").unwrap(); //! assert_eq!(slab.get(key).unwrap(), "hello world"); //! ``` //! //! To share a slab across threads, it may be wrapped in an `Arc`: //! ```rust //! # use sharded_slab::Slab; //! use std::sync::Arc; //! let slab = Arc::new(Slab::new()); //! //! let slab2 = slab.clone(); //! let thread2 = std::thread::spawn(move || { //! let key = slab2.insert("hello from thread two").unwrap(); //! assert_eq!(slab2.get(key).unwrap(), "hello from thread two"); //! key //! }); //! //! let key1 = slab.insert("hello from thread one").unwrap(); //! assert_eq!(slab.get(key1).unwrap(), "hello from thread one"); //! //! // Wait for thread 2 to complete. //! let key2 = thread2.join().unwrap(); //! //! // The item inserted by thread 2 remains in the slab. //! assert_eq!(slab.get(key2).unwrap(), "hello from thread two"); //!``` //! //! If items in the slab must be mutated, a `Mutex` or `RwLock` may be used for //! each item, providing granular locking of items rather than of the slab: //! //! ```rust //! # use sharded_slab::Slab; //! use std::sync::{Arc, Mutex}; //! let slab = Arc::new(Slab::new()); //! //! let key = slab.insert(Mutex::new(String::from("hello world"))).unwrap(); //! //! let slab2 = slab.clone(); //! let thread2 = std::thread::spawn(move || { //! let hello = slab2.get(key).expect("item missing"); //! let mut hello = hello.lock().expect("mutex poisoned"); //! *hello = String::from("hello everyone!"); //! }); //! //! thread2.join().unwrap(); //! //! let hello = slab.get(key).expect("item missing"); //! let mut hello = hello.lock().expect("mutex poisoned"); //! assert_eq!(hello.as_str(), "hello everyone!"); //! ``` //! //! # Configuration //! //! For performance reasons, several values used by the slab are calculated as //! constants. In order to allow users to tune the slab's parameters, we provide //! a [`Config`] trait which defines these parameters as associated `consts`. //! The `Slab` type is generic over a `C: Config` parameter. //! //! [`Config`]: trait.Config.html //! //! # Comparison with Similar Crates //! //! - [`slab`]: Carl Lerche's `slab` crate provides a slab implementation with a //! similar API, implemented by storing all data in a single vector. //! //! Unlike `sharded_slab`, inserting and removing elements from the slab //! requires mutable access. This means that if the slab is accessed //! concurrently by multiple threads, it is necessary for it to be protected //! by a `Mutex` or `RwLock`. Items may not be inserted or removed (or //! accessed, if a `Mutex` is used) concurrently, even when they are //! unrelated. In many cases, the lock can become a significant bottleneck. On //! the other hand, this crate allows separate indices in the slab to be //! accessed, inserted, and removed concurrently without requiring a global //! lock. Therefore, when the slab is shared across multiple threads, this //! crate offers significantly better performance than `slab`. //! //! However, the lock free slab introduces some additional constant-factor //! overhead. This means that in use-cases where a slab is _not_ shared by //! multiple threads and locking is not required, this crate will likely offer //! slightly worse performance. //! //! In summary: `sharded-slab` offers significantly improved performance in //! concurrent use-cases, while `slab` should be preferred in single-threaded //! use-cases. //! //! [`slab`]: https://crates.io/crates/loom //! //! # Safety and Correctness //! //! Most implementations of lock-free data structures in Rust require some //! amount of unsafe code, and this crate is not an exception. In order to catch //! potential bugs in this unsafe code, we make use of [`loom`], a //! permutation-testing tool for concurrent Rust programs. All `unsafe` blocks //! this crate occur in accesses to `loom` `CausalCell`s. This means that when //! those accesses occur in this crate's tests, `loom` will assert that they are //! valid under the C11 memory model across multiple permutations of concurrent //! executions of those tests. //! //! In order to guard against the [ABA problem][aba], this crate makes use of //! _generational indices_. Each slot in the slab tracks a generation counter //! which is incremented every time a value is inserted into that slot, and the //! indices returned by [`Slab::insert`] include the generation of the slot when //! the value was inserted, packed into the high-order bits of the index. This //! ensures that if a value is inserted, removed, and a new value is inserted //! into the same slot in the slab, the key returned by the first call to //! `insert` will not map to the new value. //! //! Since a fixed number of bits are set aside to use for storing the generation //! counter, the counter will wrap around after being incremented a number of //! times. To avoid situations where a returned index lives long enough to see the //! generation counter wrap around to the same value, it is good to be fairly //! generous when configuring the allocation of index bits. //! //! [`loom`]: https://crates.io/crates/loom //! [aba]: https://en.wikipedia.org/wiki/ABA_problem //! [`Slab::insert`]: struct.Slab.html#method.insert //! //! # Performance //! //! These graphs were produced by [benchmarks] of the sharded slab implementation, //! using the [`criterion`] crate. //! //! The first shows the results of a benchmark where an increasing number of //! items are inserted and then removed into a slab concurrently by five //! threads. It compares the performance of the sharded slab implementation //! with a `RwLock<slab::Slab>`: //! //! <img width="1124" alt="Screen Shot 2019-10-01 at 5 09 49 PM" src="https://user-images.githubusercontent.com/2796466/66078398-cd6c9f80-e516-11e9-9923-0ed6292e8498.png"> //! //! The second graph shows the results of a benchmark where an increasing //! number of items are inserted and then removed by a _single_ thread. It //! compares the performance of the sharded slab implementation with an //! `RwLock<slab::Slab>` and a `mut slab::Slab`. //! //! <img width="925" alt="Screen Shot 2019-10-01 at 5 13 45 PM" src="https://user-images.githubusercontent.com/2796466/66078469-f0974f00-e516-11e9-95b5-f65f0aa7e494.png"> //! //! These benchmarks demonstrate that, while the sharded approach introduces //! a small constant-factor overhead, it offers significantly better //! performance across concurrent accesses. //! //! [benchmarks]: https://github.com/hawkw/sharded-slab/blob/master/benches/bench.rs //! [`criterion`]: https://crates.io/crates/criterion //! //! # Implementation Notes //! //! See [this page](implementation/index.html) for details on this crate's design //! and implementation. //! #![doc(html_root_url = "https://docs.rs/sharded-slab/0.0.3")] #[cfg(test)] macro_rules! thread_local { ($($tts:tt)+) => { loom::thread_local!{ $($tts)+ } } } #[cfg(not(test))] macro_rules! thread_local { ($($tts:tt)+) => { std::thread_local!{ $($tts)+ } } } macro_rules! test_println { ($($arg:tt)*) => { if cfg!(test) { println!("{:?} {}", crate::Tid::<crate::DefaultConfig>::current(), format_args!($($arg)*)) } } } pub mod implementation; mod page; pub(crate) mod sync; mod tid; pub(crate) use tid::Tid; pub(crate) mod cfg; mod iter; use cfg::CfgPrivate; pub use cfg::{Config, DefaultConfig}; use std::{fmt, marker::PhantomData}; /// A sharded slab. /// /// See the [crate-level documentation](index.html) for details on using this type. pub struct Slab<T, C: cfg::Config = DefaultConfig> { shards: Box<[Shard<T, C>]>, _cfg: PhantomData<C>, } /// A guard that allows access to an object in a slab. /// /// While the guard exists, it indicates to the slab that the item the guard /// references is currently being accessed. If the item is removed from the slab /// while a guard exists, the removal will be deferred until all guards are dropped. pub struct Guard<'a, T, C: cfg::Config = DefaultConfig> { inner: page::slot::Guard<'a, T, C>, shard: &'a Shard<T, C>, key: usize, } // ┌─────────────┐ ┌────────┐ // │ page 1 │ │ │ // ├─────────────┤ ┌───▶│ next──┼─┐ // │ page 2 │ │ ├────────┤ │ // │ │ │ │XXXXXXXX│ │ // │ local_free──┼─┘ ├────────┤ │ // │ global_free─┼─┐ │ │◀┘ // ├─────────────┤ └───▶│ next──┼─┐ // │ page 3 │ ├────────┤ │ // └─────────────┘ │XXXXXXXX│ │ // ... ├────────┤ │ // ┌─────────────┐ │XXXXXXXX│ │ // │ page n │ ├────────┤ │ // └─────────────┘ │ │◀┘ // │ next──┼───▶ // ├────────┤ // │XXXXXXXX│ // └────────┘ // ... struct Shard<T, C: cfg::Config> { /// The shard's parent thread ID. tid: usize, /// The local free list for each page. /// /// These are only ever accessed from this shard's thread, so they are /// stored separately from the shared state for the page that can be /// accessed concurrently, to minimize false sharing. local: Box<[page::Local]>, /// The shared state for each page in this shard. /// /// This consists of the page's metadata (size, previous size), remote free /// list, and a pointer to the actual array backing that page. shared: Box<[page::Shared<T, C>]>, } impl<T> Slab<T> { /// Returns a new slab with the default configuration parameters. pub fn new() -> Self { Self::new_with_config() } /// Returns a new slab with the provided configuration parameters. pub fn new_with_config<C: cfg::Config>() -> Slab<T, C> { C::validate(); let shards = (0..C::MAX_SHARDS).map(Shard::new).collect(); Slab { shards, _cfg: PhantomData, } } } impl<T, C: cfg::Config> Slab<T, C> { /// The number of bits in each index which are used by the slab. /// /// If other data is packed into the `usize` indices returned by /// [`Slab::insert`], user code is free to use any bits higher than the /// `USED_BITS`-th bit freely. /// /// This is determined by the [`Config`] type that configures the slab's /// parameters. By default, all bits are used; this can be changed by /// overriding the [`Config::RESERVED_BITS`][res] constant. /// /// [`Config`]: trait.Config.html /// [res]: trait.Config.html#associatedconstant.RESERVED_BITS /// [`Slab::insert`]: struct.Slab.html#method.insert pub const USED_BITS: usize = C::USED_BITS; /// Inserts a value into the slab, returning a key that can be used to /// access it. /// /// If this function returns `None`, then the shard for the current thread /// is full and no items can be added until some are removed, or the maximum /// number of shards has been reached. /// /// # Examples /// ```rust /// # use sharded_slab::Slab; /// let slab = Slab::new(); /// /// let key = slab.insert("hello world").unwrap(); /// assert_eq!(slab.get(key).unwrap(), "hello world"); /// ``` pub fn insert(&self, value: T) -> Option<usize> { let tid = Tid::<C>::current(); test_println!("insert {:?}", tid); self.shards[tid.as_usize()] .insert(value) .map(|idx| tid.pack(idx)) } /// Remove the value associated with the given key from the slab, returning /// `true` if a value was removed. /// /// Unlike [`take`], this method does _not_ block the current thread until /// the value can be removed. Instead, if another thread is currently /// accessing that value, this marks it to be removed by that thread when it /// finishes accessing the value. /// /// # Examples /// /// ```rust /// let slab = sharded_slab::Slab::new(); /// let key = slab.insert("hello world").unwrap(); /// /// // Remove the item from the slab. /// assert!(slab.remove(key)); /// /// // Now, the slot is empty. /// assert!(!slab.contains(key)); /// ``` /// /// ```rust /// use std::sync::Arc; /// /// let slab = Arc::new(sharded_slab::Slab::new()); /// let key = slab.insert("hello world").unwrap(); /// /// let slab2 = slab.clone(); /// let thread2 = std::thread::spawn(move || { /// // Depending on when this thread begins executing, the item may /// // or may not have already been removed... /// if let Some(item) = slab2.get(key) { /// assert_eq!(item, "hello world"); /// } /// }); /// /// // The item will be removed by thread2 when it finishes accessing it. /// assert!(slab.remove(key)); /// /// thread2.join().unwrap(); /// assert!(!slab.contains(key)); /// ``` /// [`take`]: #method.take pub fn remove(&self, idx: usize) -> bool { let tid = C::unpack_tid(idx); test_println!("rm_deferred {:?}", tid); self.shards .get(tid.as_usize()) .map(|shard| shard.remove(idx)) .unwrap_or(false) } /// Removes the value associated with the given key from the slab, returning /// it. /// /// If the slab does not contain a value for that key, `None` is returned /// instead. /// /// **Note**: If the value associated with the given key is currently being /// accessed by another thread, this method will block the current thread /// until the item is no longer accessed. If this is not desired, use /// [`remove`] instead. /// /// # Examples /// /// ```rust /// let slab = sharded_slab::Slab::new(); /// let key = slab.insert("hello world").unwrap(); /// /// // Remove the item from the slab, returning it. /// assert_eq!(slab.take(key), Some("hello world")); /// /// // Now, the slot is empty. /// assert!(!slab.contains(key)); /// ``` /// /// ```rust /// use std::sync::Arc; /// /// let slab = Arc::new(sharded_slab::Slab::new()); /// let key = slab.insert("hello world").unwrap(); /// /// let slab2 = slab.clone(); /// let thread2 = std::thread::spawn(move || { /// // Depending on when this thread begins executing, the item may /// // or may not have already been removed... /// if let Some(item) = slab2.get(key) { /// assert_eq!(item, "hello world"); /// } /// }); /// /// // The item will only be removed when the other thread finishes /// // accessing it. /// assert_eq!(slab.take(key), Some("hello world")); /// /// thread2.join().unwrap(); /// assert!(!slab.contains(key)); /// ``` /// [`remove`]: #method.remove pub fn take(&self, idx: usize) -> Option<T> { let tid = C::unpack_tid(idx); test_println!("rm {:?}", tid); let shard = &self.shards[tid.as_usize()]; if tid.is_current() { shard.remove_local(idx) } else { shard.remove_remote(idx) } } /// Return a reference to the value associated with the given key. /// /// If the slab does not contain a value for the given key, `None` is /// returned instead. /// /// # Examples /// /// ``` /// let slab = sharded_slab::Slab::new(); /// let key = slab.insert("hello world").unwrap(); /// /// assert_eq!(slab.get(key).unwrap(), "hello world"); /// assert!(slab.get(12345).is_none()); /// ``` pub fn get(&self, key: usize) -> Option<Guard<'_, T, C>> { let tid = C::unpack_tid(key); test_println!("get {:?}; current={:?}", tid, Tid::<C>::current()); self.shards.get(tid.as_usize())?.get(key) } /// Returns `true` if the slab contains a value for the given key. /// /// # Examples /// /// ``` /// let slab = sharded_slab::Slab::new(); /// /// let key = slab.insert("hello world").unwrap(); /// assert!(slab.contains(key)); /// /// slab.take(key).unwrap(); /// assert!(!slab.contains(key)); /// ``` pub fn contains(&self, key: usize) -> bool { self.get(key).is_some() } /// Returns an iterator over all the items in the slab. pub fn unique_iter(&mut self) -> iter::UniqueIter<'_, T, C> { let mut shards = self.shards.iter_mut(); let shard = shards.next().expect("must be at least 1 shard"); let mut pages = shard.iter(); let slots = pages.next().and_then(page::Shared::iter); iter::UniqueIter { shards, slots, pages, } } } impl<T> Default for Slab<T> { fn default() -> Self { Self::new() } } impl<T: fmt::Debug, C: cfg::Config> fmt::Debug for Slab<T, C> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.debug_struct("Slab") .field("shards", &self.shards) .field("Config", &C::debug()) .finish() } } unsafe impl<T: Send, C: cfg::Config> Send for Slab<T, C> {} unsafe impl<T: Sync, C: cfg::Config> Sync for Slab<T, C> {} // === impl Shard === impl<T, C: cfg::Config> Shard<T, C> { fn new(tid: usize) -> Self { let mut total_sz = 0; let shared = (0..C::MAX_PAGES) .map(|page_num| { let sz = C::page_size(page_num); let prev_sz = total_sz; total_sz += sz; page::Shared::new(sz, prev_sz) }) .collect(); let local = (0..C::MAX_PAGES).map(|_| page::Local::new()).collect(); Self { tid, local, shared } } #[inline(always)] fn page_indices(idx: usize) -> (page::Addr<C>, usize) { let addr = C::unpack_addr(idx); (addr, addr.index()) } fn insert(&self, value: T) -> Option<usize> { let mut value = Some(value); // Can we fit the value into an existing page? for (page_idx, page) in self.shared.iter().enumerate() { let local = self.local(page_idx); test_println!("-> page {}; {:?}; {:?}", page_idx, local, page); if let Some(poff) = page.insert(local, &mut value) { return Some(poff); } } None } #[inline(always)] fn get(&self, idx: usize) -> Option<Guard<'_, T, C>> { debug_assert_eq!(Tid::<C>::from_packed(idx).as_usize(), self.tid); let (addr, page_index) = Self::page_indices(idx); test_println!("-> {:?}", addr); if page_index > self.shared.len() { return None; } let inner = self.shared[page_index].get(addr, idx)?; Some(Guard { inner, shard: self, key: idx, }) } fn remove(&self, idx: usize) -> bool { debug_assert_eq!(Tid::<C>::from_packed(idx).as_usize(), self.tid); let (addr, page_index) = Self::page_indices(idx); if page_index > self.shared.len() { return false; } self.shared[page_index].remove(addr, C::unpack_gen(idx)) } /// Remove an item on the shard's local thread. fn remove_local(&self, idx: usize) -> Option<T> { debug_assert_eq!(Tid::<C>::from_packed(idx).as_usize(), self.tid); let (addr, page_index) = Self::page_indices(idx); test_println!("-> remove_local {:?}", addr); self.shared .get(page_index)? .remove_local(self.local(page_index), addr, C::unpack_gen(idx)) } /// Remove an item, while on a different thread from the shard's local thread. fn remove_remote(&self, idx: usize) -> Option<T> { debug_assert_eq!(Tid::<C>::from_packed(idx).as_usize(), self.tid); debug_assert!(Tid::<C>::current().as_usize() != self.tid); let (addr, page_index) = Self::page_indices(idx); test_println!("-> remove_remote {:?}; page {:?}", addr, page_index); self.shared .get(page_index)? .remove_remote(addr, C::unpack_gen(idx)) } #[inline(always)] fn local(&self, i: usize) -> &page::Local { #[cfg(debug_assertions)] debug_assert_eq!( Tid::<C>::current().as_usize(), self.tid, "tried to access local data from another thread!" ); &self.local[i] } fn iter<'a>(&'a self) -> std::slice::Iter<'a, page::Shared<T, C>> { self.shared.iter() } } impl<T: fmt::Debug, C: cfg::Config> fmt::Debug for Shard<T, C> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { let mut d = f.debug_struct("Shard"); #[cfg(debug_assertions)] d.field("tid", &self.tid); d.field("shared", &self.shared).finish() } } // === impl Guard === impl<'a, T, C: cfg::Config> std::ops::Deref for Guard<'a, T, C> { type Target = T; fn deref(&self) -> &Self::Target { self.inner.item() } } impl<'a, T, C: cfg::Config> Drop for Guard<'a, T, C> { fn drop(&mut self) { use crate::sync::atomic; if self.inner.release() { atomic::fence(atomic::Ordering::Acquire); if Tid::<C>::current().as_usize() == self.shard.tid { self.shard.remove_local(self.key); } else { self.shard.remove_remote(self.key); } } } } impl<'a, T, C> fmt::Debug for Guard<'a, T, C> where T: fmt::Debug, C: cfg::Config, { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Debug::fmt(self.inner.item(), f) } } impl<'a, T, C> PartialEq<T> for Guard<'a, T, C> where T: PartialEq<T>, C: cfg::Config, { fn eq(&self, other: &T) -> bool { self.inner.item().eq(other) } } // === pack === pub(crate) trait Pack<C: cfg::Config>: Sized { // ====== provided by each implementation ================================= /// The number of bits occupied by this type when packed into a usize. /// /// This must be provided to determine the number of bits into which to pack /// the type. const LEN: usize; /// The type packed on the less significant side of this type. /// /// If this type is packed into the least significant bit of a usize, this /// should be `()`, which occupies no bytes. /// /// This is used to calculate the shift amount for packing this value. type Prev: Pack<C>; // ====== calculated automatically ======================================== /// A number consisting of `Self::LEN` 1 bits, starting at the least /// significant bit. /// /// This is the higest value this type can represent. This number is shifted /// left by `Self::SHIFT` bits to calculate this type's `MASK`. /// /// This is computed automatically based on `Self::LEN`. const BITS: usize = { let shift = 1 << (Self::LEN - 1); shift | (shift - 1) }; /// The number of bits to shift a number to pack it into a usize with other /// values. /// /// This is caculated automatically based on the `LEN` and `SHIFT` constants /// of the previous value. const SHIFT: usize = Self::Prev::SHIFT + Self::Prev::LEN; /// The mask to extract only this type from a packed `usize`. /// /// This is calculated by shifting `Self::BITS` left by `Self::SHIFT`. const MASK: usize = Self::BITS << Self::SHIFT; fn as_usize(&self) -> usize; fn from_usize(val: usize) -> Self; #[inline(always)] fn pack(&self, to: usize) -> usize { let value = self.as_usize(); debug_assert!(value <= Self::BITS); (to & !Self::MASK) | (value << Self::SHIFT) } #[inline(always)] fn from_packed(from: usize) -> Self { let value = (from & Self::MASK) >> Self::SHIFT; debug_assert!(value <= Self::BITS); Self::from_usize(value) } } impl<C: cfg::Config> Pack<C> for () { const BITS: usize = 0; const LEN: usize = 0; const SHIFT: usize = 0; const MASK: usize = 0; type Prev = (); fn as_usize(&self) -> usize { unreachable!() } fn from_usize(_val: usize) -> Self { unreachable!() } fn pack(&self, _to: usize) -> usize { unreachable!() } fn from_packed(_from: usize) -> Self { unreachable!() } } #[cfg(test)] pub(crate) use self::tests::util as test_util; #[cfg(test)] mod tests;