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<p class='location'><a href='../../index.html'>crossbeam</a>::<wbr><a href='../index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'epoch', ty: 'mod', relpath: '../'};</script><script defer src="../sidebar-items.js"></script>
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<section id='main' class="content mod">
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<h1 class='fqn'><span class='in-band'>Module <a href='../../index.html'>crossbeam</a>::<wbr><a href='../index.html'>mem</a>::<wbr><a class='mod' href=''>epoch</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</a>
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</span><a id='src-9' class='srclink' href='../../../src/crossbeam/mem/epoch/mod.rs.html#1-263' title='goto source code'>[src]</a></span></h1>
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<div class='docblock'><p>Epoch-based memory management</p>
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<p>This module provides fast, easy to use memory management for lock free data
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structures. It's inspired by <a href="https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf">Keir Fraser's <em>epoch-based
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reclamation</em></a>.</p>
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<p>The basic problem this is solving is the fact that when one thread has
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removed a node from a data structure, other threads may still have pointers
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to that node (in the form of snapshots that will be validated through things
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like compare-and-swap), so the memory cannot be immediately freed. Put differently:</p>
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<ol>
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<li><p>There are two sources of reachability at play -- the data structure, and
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the snapshots in threads accessing it. Before we delete a node, we need to know
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that it cannot be reached in either of these ways.</p></li>
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<li><p>Once a node has been unliked from the data structure, no <em>new</em> snapshots
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reaching it will be created.</p></li>
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</ol>
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<p>Using the epoch scheme is fairly straightforward, and does not require
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understanding any of the implementation details:</p>
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<ul>
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<li><p>When operating on a shared data structure, a thread must "pin the current
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epoch", which is done by calling <code>pin()</code>. This function returns a <code>Guard</code>
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which unpins the epoch when destroyed.</p></li>
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<li><p>When the thread subsequently reads from a lock-free data structure, the
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pointers it extracts act like references with lifetime tied to the
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<code>Guard</code>. This allows threads to safely read from snapshotted data, being
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guaranteed that the data will remain allocated until they exit the epoch.</p></li>
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</ul>
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<p>To put the <code>Guard</code> to use, Crossbeam provides a set of three pointer types meant to work together:</p>
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<ul>
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<li><p><code>Owned<T></code>, akin to <code>Box<T></code>, which points to uniquely-owned data that has
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not yet been published in a concurrent data structure.</p></li>
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<li><p><code>Shared<'a, T></code>, akin to <code>&'a T</code>, which points to shared data that may or may
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not be reachable from a data structure, but it guaranteed not to be freed
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during lifetime <code>'a</code>.</p></li>
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<li><p><code>Atomic<T></code>, akin to <code>std::sync::atomic::AtomicPtr</code>, which provides atomic
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updates to a pointer using the <code>Owned</code> and <code>Shared</code> types, and connects them
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to a <code>Guard</code>.</p></li>
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</ul>
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<p>Each of these types provides further documentation on usage.</p>
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<h1 id='example' class='section-header'><a href='#example'>Example</a></h1>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>sync</span>::<span class='ident'>atomic</span>::<span class='ident'>Ordering</span>::{<span class='ident'>Acquire</span>, <span class='ident'>Release</span>, <span class='ident'>Relaxed</span>};
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>ptr</span>;
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<span class='kw'>use</span> <span class='ident'>crossbeam</span>::<span class='ident'>mem</span>::<span class='ident'>epoch</span>::{<span class='self'>self</span>, <span class='ident'>Atomic</span>, <span class='ident'>Owned</span>};
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<span class='kw'>struct</span> <span class='ident'>TreiberStack</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='ident'>head</span>: <span class='ident'>Atomic</span><span class='op'><</span><span class='ident'>Node</span><span class='op'><</span><span class='ident'>T</span><span class='op'>>></span>,
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}
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<span class='kw'>struct</span> <span class='ident'>Node</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='ident'>data</span>: <span class='ident'>T</span>,
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<span class='ident'>next</span>: <span class='ident'>Atomic</span><span class='op'><</span><span class='ident'>Node</span><span class='op'><</span><span class='ident'>T</span><span class='op'>>></span>,
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}
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<span class='kw'>impl</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> <span class='ident'>TreiberStack</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='kw'>fn</span> <span class='ident'>new</span>() <span class='op'>-></span> <span class='ident'>TreiberStack</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='ident'>TreiberStack</span> {
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<span class='ident'>head</span>: <span class='ident'>Atomic</span>::<span class='ident'>null</span>()
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}
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}
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<span class='kw'>fn</span> <span class='ident'>push</span>(<span class='kw-2'>&</span><span class='self'>self</span>, <span class='ident'>t</span>: <span class='ident'>T</span>) {
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<span class='comment'>// allocate the node via Owned</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>n</span> <span class='op'>=</span> <span class='ident'>Owned</span>::<span class='ident'>new</span>(<span class='ident'>Node</span> {
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<span class='ident'>data</span>: <span class='ident'>t</span>,
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<span class='ident'>next</span>: <span class='ident'>Atomic</span>::<span class='ident'>null</span>(),
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});
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<span class='comment'>// become active</span>
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<span class='kw'>let</span> <span class='ident'>guard</span> <span class='op'>=</span> <span class='ident'>epoch</span>::<span class='ident'>pin</span>();
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<span class='kw'>loop</span> {
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<span class='comment'>// snapshot current head</span>
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<span class='kw'>let</span> <span class='ident'>head</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>head</span>.<span class='ident'>load</span>(<span class='ident'>Relaxed</span>, <span class='kw-2'>&</span><span class='ident'>guard</span>);
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<span class='comment'>// update `next` pointer with snapshot</span>
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<span class='ident'>n</span>.<span class='ident'>next</span>.<span class='ident'>store_shared</span>(<span class='ident'>head</span>, <span class='ident'>Relaxed</span>);
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<span class='comment'>// if snapshot is still good, link in the new node</span>
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<span class='kw'>match</span> <span class='self'>self</span>.<span class='ident'>head</span>.<span class='ident'>cas_and_ref</span>(<span class='ident'>head</span>, <span class='ident'>n</span>, <span class='ident'>Release</span>, <span class='kw-2'>&</span><span class='ident'>guard</span>) {
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<span class='prelude-val'>Ok</span>(_) <span class='op'>=></span> <span class='kw'>return</span>,
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<span class='prelude-val'>Err</span>(<span class='ident'>owned</span>) <span class='op'>=></span> <span class='ident'>n</span> <span class='op'>=</span> <span class='ident'>owned</span>,
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}
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}
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}
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<span class='kw'>fn</span> <span class='ident'>pop</span>(<span class='kw-2'>&</span><span class='self'>self</span>) <span class='op'>-></span> <span class='prelude-ty'>Option</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='comment'>// become active</span>
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<span class='kw'>let</span> <span class='ident'>guard</span> <span class='op'>=</span> <span class='ident'>epoch</span>::<span class='ident'>pin</span>();
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<span class='kw'>loop</span> {
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<span class='comment'>// take a snapshot</span>
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<span class='kw'>match</span> <span class='self'>self</span>.<span class='ident'>head</span>.<span class='ident'>load</span>(<span class='ident'>Acquire</span>, <span class='kw-2'>&</span><span class='ident'>guard</span>) {
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<span class='comment'>// the stack is non-empty</span>
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<span class='prelude-val'>Some</span>(<span class='ident'>head</span>) <span class='op'>=></span> {
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<span class='comment'>// read through the snapshot, *safely*!</span>
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<span class='kw'>let</span> <span class='ident'>next</span> <span class='op'>=</span> <span class='ident'>head</span>.<span class='ident'>next</span>.<span class='ident'>load</span>(<span class='ident'>Relaxed</span>, <span class='kw-2'>&</span><span class='ident'>guard</span>);
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<span class='comment'>// if snapshot is still good, update from `head` to `next`</span>
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<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>head</span>.<span class='ident'>cas_shared</span>(<span class='prelude-val'>Some</span>(<span class='ident'>head</span>), <span class='ident'>next</span>, <span class='ident'>Release</span>) {
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<span class='kw'>unsafe</span> {
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<span class='comment'>// mark the node as unlinked</span>
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<span class='ident'>guard</span>.<span class='ident'>unlinked</span>(<span class='ident'>head</span>);
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<span class='comment'>// extract out the data from the now-unlinked node</span>
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<span class='kw'>return</span> <span class='prelude-val'>Some</span>(<span class='ident'>ptr</span>::<span class='ident'>read</span>(<span class='kw-2'>&</span>(<span class='op'>*</span><span class='ident'>head</span>).<span class='ident'>data</span>))
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}
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}
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}
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<span class='comment'>// we observed the stack empty</span>
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<span class='prelude-val'>None</span> <span class='op'>=></span> <span class='kw'>return</span> <span class='prelude-val'>None</span>
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}
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}
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}
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}</pre>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class='struct' href='struct.Atomic.html'
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title='crossbeam::mem::epoch::Atomic'>Atomic</a></td>
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<td class='docblock short'>
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<p>Like <code>std::sync::atomic::AtomicPtr</code>.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class='struct' href='struct.Guard.html'
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title='crossbeam::mem::epoch::Guard'>Guard</a></td>
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<td class='docblock short'>
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<p>An RAII-style guard for pinning the current epoch.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class='struct' href='struct.Owned.html'
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title='crossbeam::mem::epoch::Owned'>Owned</a></td>
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<td class='docblock short'>
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<p>Like <code>Box<T></code>: an owned, heap-allocated data value of type <code>T</code>.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class='struct' href='struct.Shared.html'
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title='crossbeam::mem::epoch::Shared'>Shared</a></td>
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<td class='docblock short'>
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<p>Like <code>&'a T</code>: a shared reference valid for lifetime <code>'a</code>.</p>
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</td>
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</tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class='fn' href='fn.pin.html'
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title='crossbeam::mem::epoch::pin'>pin</a></td>
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<td class='docblock short'>
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<p>Pin the current epoch.</p>
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</td>
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</tr></table></section>
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