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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="A lock-free concurrent slab."><title>sharded_slab - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="sharded_slab" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">&#9776;</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../sharded_slab/index.html">sharded_slab</a><span class="version">0.1.7</span></h2></div><div class="sidebar-elems"><ul class="block">
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../sharded_slab/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press S to search, ? for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">sharded_slab</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/sharded_slab/lib.rs.html#1-1106">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A lock-free concurrent slab.</p>
<p>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.</p>
<p>This crate implements a lock-free concurrent slab, indexed by <code>usize</code>s.</p>
<h3 id="usage"><a href="#usage">Usage</a></h3>
<p>First, add this to your <code>Cargo.toml</code>:</p>
<div class="example-wrap"><pre class="language-toml"><code>sharded-slab = &quot;0.1.1&quot;
</code></pre></div>
<p>This crate provides two types, <a href="https://crates.io/crates/loom"><code>Slab</code></a> and <a href="struct.Pool.html" title="struct sharded_slab::Pool"><code>Pool</code></a>, which provide
slightly different APIs for using a sharded slab.</p>
<p><a href="https://crates.io/crates/loom"><code>Slab</code></a> implements a slab for <em>storing</em> small types, sharing them between
threads, and accessing them by index. New entries are allocated by
<a href="struct.Slab.html#method.insert" title="method sharded_slab::Slab::insert">inserting</a> data, moving it in by value. Similarly, entries may be
deallocated by <a href="struct.Slab.html#method.take" title="method sharded_slab::Slab::take">taking</a> from the slab, moving the value out. This API is
similar to a <code>Vec&lt;Option&lt;T&gt;&gt;</code>, but allowing lock-free concurrent insertion
and removal.</p>
<p>In contrast, the <a href="struct.Pool.html" title="struct sharded_slab::Pool"><code>Pool</code></a> type provides an <a href="https://en.wikipedia.org/wiki/Object_pool_pattern">object pool</a> style API for
<em>reusing storage</em>. Rather than constructing values and moving them into the
pool, as with <a href="https://crates.io/crates/loom"><code>Slab</code></a>, <a href="struct.Pool.html#method.create" title="method sharded_slab::Pool::create">allocating an entry</a> from the pool takes a
closure thats provided with a mutable reference to initialize the entry in
place. When entries are deallocated, they are <a href="trait.Clear.html" title="trait sharded_slab::Clear">cleared</a> in place. Types
which own a heap allocation can be cleared by dropping any <em>data</em> they
store, but retaining any previously-allocated capacity. This means that a
<a href="struct.Pool.html" title="struct sharded_slab::Pool"><code>Pool</code></a> may be used to reuse a set of existing heap allocations, reducing
allocator load.</p>
<h2 id="examples"><a href="#examples">Examples</a></h2>
<p>Inserting an item into the slab, returning an index:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>slab = Slab::new();
<span class="kw">let </span>key = slab.insert(<span class="string">"hello world"</span>).unwrap();
<span class="macro">assert_eq!</span>(slab.get(key).unwrap(), <span class="string">"hello world"</span>);</code></pre></div>
<p>To share a slab across threads, it may be wrapped in an <code>Arc</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">let </span>slab = Arc::new(Slab::new());
<span class="kw">let </span>slab2 = slab.clone();
<span class="kw">let </span>thread2 = std::thread::spawn(<span class="kw">move </span>|| {
<span class="kw">let </span>key = slab2.insert(<span class="string">"hello from thread two"</span>).unwrap();
<span class="macro">assert_eq!</span>(slab2.get(key).unwrap(), <span class="string">"hello from thread two"</span>);
key
});
<span class="kw">let </span>key1 = slab.insert(<span class="string">"hello from thread one"</span>).unwrap();
<span class="macro">assert_eq!</span>(slab.get(key1).unwrap(), <span class="string">"hello from thread one"</span>);
<span class="comment">// Wait for thread 2 to complete.
</span><span class="kw">let </span>key2 = thread2.join().unwrap();
<span class="comment">// The item inserted by thread 2 remains in the slab.
</span><span class="macro">assert_eq!</span>(slab.get(key2).unwrap(), <span class="string">"hello from thread two"</span>);</code></pre></div>
<p>If items in the slab must be mutated, a <code>Mutex</code> or <code>RwLock</code> may be used for
each item, providing granular locking of items rather than of the slab:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::{Arc, Mutex};
<span class="kw">let </span>slab = Arc::new(Slab::new());
<span class="kw">let </span>key = slab.insert(Mutex::new(String::from(<span class="string">"hello world"</span>))).unwrap();
<span class="kw">let </span>slab2 = slab.clone();
<span class="kw">let </span>thread2 = std::thread::spawn(<span class="kw">move </span>|| {
<span class="kw">let </span>hello = slab2.get(key).expect(<span class="string">"item missing"</span>);
<span class="kw">let </span><span class="kw-2">mut </span>hello = hello.lock().expect(<span class="string">"mutex poisoned"</span>);
<span class="kw-2">*</span>hello = String::from(<span class="string">"hello everyone!"</span>);
});
thread2.join().unwrap();
<span class="kw">let </span>hello = slab.get(key).expect(<span class="string">"item missing"</span>);
<span class="kw">let </span><span class="kw-2">mut </span>hello = hello.lock().expect(<span class="string">"mutex poisoned"</span>);
<span class="macro">assert_eq!</span>(hello.as_str(), <span class="string">"hello everyone!"</span>);</code></pre></div>
<h2 id="configuration"><a href="#configuration">Configuration</a></h2>
<p>For performance reasons, several values used by the slab are calculated as
constants. In order to allow users to tune the slabs parameters, we provide
a <a href="trait.Config.html"><code>Config</code></a> trait which defines these parameters as associated <code>consts</code>.
The <code>Slab</code> type is generic over a <code>C: Config</code> parameter.</p>
<h2 id="comparison-with-similar-crates"><a href="#comparison-with-similar-crates">Comparison with Similar Crates</a></h2>
<ul>
<li>
<p><a href="https://crates.io/crates/loom"><code>slab</code></a>: Carl Lerches <code>slab</code> crate provides a slab implementation with a
similar API, implemented by storing all data in a single vector.</p>
<p>Unlike <code>sharded_slab</code>, 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 <code>Mutex</code> or <code>RwLock</code>. Items may not be inserted or removed (or
accessed, if a <code>Mutex</code> 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 <code>slab</code>.</p>
<p>However, the lock free slab introduces some additional constant-factor
overhead. This means that in use-cases where a slab is <em>not</em> shared by
multiple threads and locking is not required, this crate will likely offer
slightly worse performance.</p>
<p>In summary: <code>sharded-slab</code> offers significantly improved performance in
concurrent use-cases, while <code>slab</code> should be preferred in single-threaded
use-cases.</p>
</li>
</ul>
<h2 id="safety-and-correctness"><a href="#safety-and-correctness">Safety and Correctness</a></h2>
<p>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 <a href="https://crates.io/crates/loom"><code>loom</code></a>, a
permutation-testing tool for concurrent Rust programs. All <code>unsafe</code> blocks
this crate occur in accesses to <code>loom</code> <code>UnsafeCell</code>s. This means that when
those accesses occur in this crates tests, <code>loom</code> will assert that they are
valid under the C11 memory model across multiple permutations of concurrent
executions of those tests.</p>
<p>In order to guard against the <a href="https://en.wikipedia.org/wiki/ABA_problem">ABA problem</a>, this crate makes use of
<em>generational indices</em>. 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 <a href="struct.Slab.html#method.insert"><code>Slab::insert</code></a> 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
<code>insert</code> will not map to the new value.</p>
<p>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.</p>
<h2 id="performance"><a href="#performance">Performance</a></h2>
<p>These graphs were produced by <a href="https://github.com/hawkw/sharded-slab/blob/master/benches/bench.rs">benchmarks</a> of the sharded slab implementation,
using the <a href="https://crates.io/crates/criterion"><code>criterion</code></a> crate.</p>
<p>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 <code>RwLock&lt;slab::Slab&gt;</code>:</p>
<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">
<p>The second graph shows the results of a benchmark where an increasing
number of items are inserted and then removed by a <em>single</em> thread. It
compares the performance of the sharded slab implementation with an
<code>RwLock&lt;slab::Slab&gt;</code> and a <code>mut slab::Slab</code>.</p>
<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">
<p>These benchmarks demonstrate that, while the sharded approach introduces
a small constant-factor overhead, it offers significantly better
performance across concurrent accesses.</p>
<h2 id="implementation-notes"><a href="#implementation-notes">Implementation Notes</a></h2>
<p>See <a href="implementation/index.html" title="mod sharded_slab::implementation">this page</a> for details on this crates design
and implementation.</p>
</div></details><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="implementation/index.html" title="mod sharded_slab::implementation">implementation</a></div><div class="desc docblock-short">Notes on <code>sharded-slab</code>s implementation and design.</div></li><li><div class="item-name"><a class="mod" href="pool/index.html" title="mod sharded_slab::pool">pool</a></div><div class="desc docblock-short">A lock-free concurrent object pool.</div></li></ul><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.DefaultConfig.html" title="struct sharded_slab::DefaultConfig">DefaultConfig</a></div><div class="desc docblock-short">Default slab configuration values.</div></li><li><div class="item-name"><a class="struct" href="struct.Entry.html" title="struct sharded_slab::Entry">Entry</a></div><div class="desc docblock-short">A handle that allows access to an occupied entry in a <a href="struct.Slab.html" title="struct sharded_slab::Slab"><code>Slab</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.OwnedEntry.html" title="struct sharded_slab::OwnedEntry">OwnedEntry</a></div><div class="desc docblock-short">An owned reference to an occupied entry in a <a href="struct.Slab.html" title="struct sharded_slab::Slab"><code>Slab</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Pool.html" title="struct sharded_slab::Pool">Pool</a></div><div class="desc docblock-short">A lock-free concurrent object pool.</div></li><li><div class="item-name"><a class="struct" href="struct.Slab.html" title="struct sharded_slab::Slab">Slab</a></div><div class="desc docblock-short">A sharded slab.</div></li><li><div class="item-name"><a class="struct" href="struct.UniqueIter.html" title="struct sharded_slab::UniqueIter">UniqueIter</a></div><div class="desc docblock-short">An exclusive fused iterator over the items in a <a href="struct.Slab.html" title="struct sharded_slab::Slab"><code>Slab</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.VacantEntry.html" title="struct sharded_slab::VacantEntry">VacantEntry</a></div><div class="desc docblock-short">A handle to a vacant entry in a <a href="struct.Slab.html" title="struct sharded_slab::Slab"><code>Slab</code></a>.</div></li></ul><h2 id="traits" class="section-header"><a href="#traits">Traits</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Clear.html" title="trait sharded_slab::Clear">Clear</a></div><div class="desc docblock-short">Trait implemented by types which can be cleared in place, retaining any
allocated memory.</div></li><li><div class="item-name"><a class="trait" href="trait.Config.html" title="trait sharded_slab::Config">Config</a></div><div class="desc docblock-short">Configuration parameters which can be overridden to tune the behavior of a slab.</div></li></ul></section></div></main></body></html>