1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385
//! A lock-free, eventually consistent, concurrent multi-value map. //! //! This map implementation allows reads and writes to execute entirely in parallel, with no //! implicit synchronization overhead. Reads never take locks on their critical path, and neither //! do writes assuming there is a single writer (multi-writer is possible using a `Mutex`), which //! significantly improves performance under contention. See the [`left-right` crate](left_right) //! for details on the underlying concurrency primitive. //! //! The trade-off exposed by this type is one of eventual consistency: writes are not visible to //! readers except following explicit synchronization. Specifically, readers only see the //! operations that preceeded the last call to `WriteHandle::refresh` by a writer. This lets //! writers decide how stale they are willing to let reads get. They can refresh the map after //! every write to emulate a regular concurrent `HashMap`, or they can refresh only occasionally to //! reduce the synchronization overhead at the cost of stale reads. //! //! For read-heavy workloads, the scheme used by this module is particularly useful. Writers can //! afford to refresh after every write, which provides up-to-date reads, and readers remain fast //! as they do not need to ever take locks. //! //! The map is multi-value, meaning that every key maps to a *collection* of values. This //! introduces some memory cost by adding a layer of indirection through a `Vec` for each value, //! but enables more advanced use. This choice was made as it would not be possible to emulate such //! functionality on top of the semantics of this map (think about it -- what would the operational //! log contain?). //! //! To faciliate more advanced use-cases, each of the two maps also carry some customizeable //! meta-information. The writers may update this at will, and when a refresh happens, the current //! meta will also be made visible to readers. This could be useful, for example, to indicate what //! time the refresh happened. //! //! # Examples //! //! Single-reader, single-writer //! //! ``` //! // new will use the default HashMap hasher, and a meta of () //! // note that we get separate read and write handles //! // the read handle can be cloned to have more readers //! let (mut book_reviews_w, book_reviews_r) = evmap::new(); //! //! // review some books. //! book_reviews_w.insert("Adventures of Huckleberry Finn", "My favorite book."); //! book_reviews_w.insert("Grimms' Fairy Tales", "Masterpiece."); //! book_reviews_w.insert("Pride and Prejudice", "Very enjoyable."); //! book_reviews_w.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot."); //! //! // at this point, reads from book_reviews_r will not see any of the reviews! //! assert_eq!(book_reviews_r.len(), 0); //! // we need to refresh first to make the writes visible //! book_reviews_w.publish(); //! assert_eq!(book_reviews_r.len(), 4); //! // reads will now return Some() because the map has been initialized //! assert_eq!(book_reviews_r.get("Grimms' Fairy Tales").map(|rs| rs.len()), Some(1)); //! //! // remember, this is a multi-value map, so we can have many reviews //! book_reviews_w.insert("Grimms' Fairy Tales", "Eh, the title seemed weird."); //! book_reviews_w.insert("Pride and Prejudice", "Too many words."); //! //! // but again, new writes are not yet visible //! assert_eq!(book_reviews_r.get("Grimms' Fairy Tales").map(|rs| rs.len()), Some(1)); //! //! // we need to refresh first //! book_reviews_w.publish(); //! assert_eq!(book_reviews_r.get("Grimms' Fairy Tales").map(|rs| rs.len()), Some(2)); //! //! // oops, this review has a lot of spelling mistakes, let's delete it. //! // remove_entry deletes *all* reviews (though in this case, just one) //! book_reviews_w.remove_entry("The Adventures of Sherlock Holmes"); //! // but again, it's not visible to readers until we refresh //! assert_eq!(book_reviews_r.get("The Adventures of Sherlock Holmes").map(|rs| rs.len()), Some(1)); //! book_reviews_w.publish(); //! assert_eq!(book_reviews_r.get("The Adventures of Sherlock Holmes").map(|rs| rs.len()), None); //! //! // look up the values associated with some keys. //! let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"]; //! for book in &to_find { //! if let Some(reviews) = book_reviews_r.get(book) { //! for review in &*reviews { //! println!("{}: {}", book, review); //! } //! } else { //! println!("{} is unreviewed.", book); //! } //! } //! //! // iterate over everything. //! for (book, reviews) in &book_reviews_r.enter().unwrap() { //! for review in reviews { //! println!("{}: \"{}\"", book, review); //! } //! } //! ``` //! //! Reads from multiple threads are possible by cloning the `ReadHandle`. //! //! ``` //! use std::thread; //! let (mut book_reviews_w, book_reviews_r) = evmap::new(); //! //! // start some readers //! let readers: Vec<_> = (0..4).map(|_| { //! let r = book_reviews_r.clone(); //! thread::spawn(move || { //! loop { //! let l = r.len(); //! if l == 0 { //! thread::yield_now(); //! } else { //! // the reader will either see all the reviews, //! // or none of them, since refresh() is atomic. //! assert_eq!(l, 4); //! break; //! } //! } //! }) //! }).collect(); //! //! // do some writes //! book_reviews_w.insert("Adventures of Huckleberry Finn", "My favorite book."); //! book_reviews_w.insert("Grimms' Fairy Tales", "Masterpiece."); //! book_reviews_w.insert("Pride and Prejudice", "Very enjoyable."); //! book_reviews_w.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot."); //! // expose the writes //! book_reviews_w.publish(); //! //! // you can read through the write handle //! assert_eq!(book_reviews_w.len(), 4); //! //! // the original read handle still works too //! assert_eq!(book_reviews_r.len(), 4); //! //! // all the threads should eventually see .len() == 4 //! for r in readers.into_iter() { //! assert!(r.join().is_ok()); //! } //! ``` //! //! If multiple writers are needed, the `WriteHandle` must be protected by a `Mutex`. //! //! ``` //! use std::thread; //! use std::sync::{Arc, Mutex}; //! let (mut book_reviews_w, book_reviews_r) = evmap::new(); //! //! // start some writers. //! // since evmap does not support concurrent writes, we need //! // to protect the write handle by a mutex. //! let w = Arc::new(Mutex::new(book_reviews_w)); //! let writers: Vec<_> = (0..4).map(|i| { //! let w = w.clone(); //! thread::spawn(move || { //! let mut w = w.lock().unwrap(); //! w.insert(i, true); //! w.publish(); //! }) //! }).collect(); //! //! // eventually we should see all the writes //! while book_reviews_r.len() < 4 { thread::yield_now(); }; //! //! // all the threads should eventually finish writing //! for w in writers.into_iter() { //! assert!(w.join().is_ok()); //! } //! ``` //! //! [`ReadHandle`] is not `Sync` as sharing a single instance amongst threads would introduce a //! significant performance bottleneck. A fresh `ReadHandle` needs to be created for each thread //! either by cloning a [`ReadHandle`] or from a [`handles::ReadHandleFactory`]. For further //! information, see [`left_right::ReadHandle`]. //! //! # Implementation //! //! Under the hood, the map is implemented using two regular `HashMap`s and some magic. Take a look //! at [`left-right`](left_right) for a much more in-depth discussion. Since the implementation //! uses regular `HashMap`s under the hood, table resizing is fully supported. It does, however, //! also mean that the memory usage of this implementation is approximately twice of that of a //! regular `HashMap`, and more if writers rarely refresh after writing. //! //! # Value storage //! //! The values for each key in the map are stored in [`refs::Values`]. Conceptually, each `Values` //! is a _bag_ or _multiset_; it can store multiple copies of the same value. `evmap` applies some //! cleverness in an attempt to reduce unnecessary allocations and keep the cost of operations on //! even large value-bags small. For small bags, `Values` uses the `smallvec` crate. This avoids //! allocation entirely for single-element bags, and uses a `Vec` if the bag is relatively small. //! For large bags, `Values` uses the `hashbag` crate, which enables `evmap` to efficiently look up //! and remove specific elements in the value bag. For bags larger than one element, but smaller //! than the threshold for moving to `hashbag`, we use `smallvec` to avoid unnecessary hashing. //! Operations such as `Fit` and `Replace` will automatically switch back to the inline storage if //! possible. This is ideal for maps that mostly use one element per key, as it can improvate //! memory locality with less indirection. #![warn( missing_docs, rust_2018_idioms, missing_debug_implementations, broken_intra_doc_links )] #![allow(clippy::type_complexity)] // This _should_ detect if we ever accidentally leak aliasing::NoDrop. // But, currently, it does not.. #![deny(unreachable_pub)] use crate::inner::Inner; use crate::read::ReadHandle; use crate::write::WriteHandle; use left_right::aliasing::Aliased; use std::collections::hash_map::RandomState; use std::fmt; use std::hash::{BuildHasher, Hash}; mod inner; mod read; mod values; mod write; /// Handles to the read and write halves of an `evmap`. pub mod handles { pub use crate::write::WriteHandle; // These cannot use ::{..} syntax because of // https://github.com/rust-lang/rust/issues/57411 pub use crate::read::ReadHandle; pub use crate::read::ReadHandleFactory; } /// Helper types that give access to values inside the read half of an `evmap`. pub mod refs { // Same here, ::{..} won't work. pub use super::values::Values; pub use crate::read::MapReadRef; pub use crate::read::ReadGuardIter; // Expose `ReadGuard` since it has useful methods the user will likely care about. #[doc(inline)] pub use left_right::ReadGuard; } // NOTE: It is _critical_ that this module is not public. mod aliasing; /// Options for how to initialize the map. /// /// In particular, the options dictate the hashing function, meta type, and initial capacity of the /// map. pub struct Options<M, S> where S: BuildHasher, { meta: M, hasher: S, capacity: Option<usize>, } impl<M, S> fmt::Debug for Options<M, S> where S: BuildHasher, M: fmt::Debug, { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.debug_struct("Options") .field("meta", &self.meta) .field("capacity", &self.capacity) .finish() } } impl Default for Options<(), RandomState> { fn default() -> Self { Options { meta: (), hasher: RandomState::default(), capacity: None, } } } impl<M, S> Options<M, S> where S: BuildHasher, { /// Set the initial meta value for the map. pub fn with_meta<M2>(self, meta: M2) -> Options<M2, S> { Options { meta, hasher: self.hasher, capacity: self.capacity, } } /// Set the hasher used for the map. pub fn with_hasher<S2>(self, hash_builder: S2) -> Options<M, S2> where S2: BuildHasher, { Options { meta: self.meta, hasher: hash_builder, capacity: self.capacity, } } /// Set the initial capacity for the map. pub fn with_capacity(self, capacity: usize) -> Options<M, S> { Options { meta: self.meta, hasher: self.hasher, capacity: Some(capacity), } } /// Create the map, and construct the read and write handles used to access it. #[allow(clippy::type_complexity)] pub fn construct<K, V>(self) -> (WriteHandle<K, V, M, S>, ReadHandle<K, V, M, S>) where K: Eq + Hash + Clone, S: BuildHasher + Clone, V: Eq + Hash, M: 'static + Clone, { let inner = if let Some(cap) = self.capacity { Inner::with_capacity_and_hasher(self.meta, cap, self.hasher) } else { Inner::with_hasher(self.meta, self.hasher) }; let (mut w, r) = left_right::new_from_empty(inner); w.append(write::Operation::MarkReady); (WriteHandle::new(w), ReadHandle::new(r)) } } /// Create an empty eventually consistent map. /// /// Use the [`Options`](./struct.Options.html) builder for more control over initialization. #[allow(clippy::type_complexity)] pub fn new<K, V>() -> ( WriteHandle<K, V, (), RandomState>, ReadHandle<K, V, (), RandomState>, ) where K: Eq + Hash + Clone, V: Eq + Hash, { Options::default().construct() } /// Create an empty eventually consistent map with meta information. /// /// Use the [`Options`](./struct.Options.html) builder for more control over initialization. #[allow(clippy::type_complexity)] pub fn with_meta<K, V, M>( meta: M, ) -> ( WriteHandle<K, V, M, RandomState>, ReadHandle<K, V, M, RandomState>, ) where K: Eq + Hash + Clone, V: Eq + Hash, M: 'static + Clone, { Options::default().with_meta(meta).construct() } /// Create an empty eventually consistent map with meta information and custom hasher. /// /// Use the [`Options`](./struct.Options.html) builder for more control over initialization. #[allow(clippy::type_complexity)] pub fn with_hasher<K, V, M, S>( meta: M, hasher: S, ) -> (WriteHandle<K, V, M, S>, ReadHandle<K, V, M, S>) where K: Eq + Hash + Clone, V: Eq + Hash, M: 'static + Clone, S: BuildHasher + Clone, { Options::default() .with_hasher(hasher) .with_meta(meta) .construct() }