114 lines
6.1 KiB
HTML
114 lines
6.1 KiB
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Allocators</title>
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<link rel="stylesheet" href="../../../boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../../index.html" title="Asio">
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<link rel="up" href="../model.html" title="Asynchronous Model">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="executors.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../model.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="cancellation.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.overview.model.allocators"></a><a class="link" href="allocators.html" title="Allocators">Allocators</a>
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</h4></div></div></div>
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<p>
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Every asynchronous agent has an associated <span class="emphasis"><em>allocator</em></span>.
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An agent's allocator is an interface used by the agent's asynchronous operations
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to obtain <span class="emphasis"><em>per-operation stable memory</em></span> resources (POSMs).
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This name reflects the fact that the memory is per-operation because the
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memory is only retained for the lifetime of that operation, and stable,
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because the memory is guaranteed to be available at that location throughout
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the operation.
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</p>
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<p>
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Asynchronous operations may utilise POSMs in a number of different ways:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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The operation doesn't require any POSMs. For example, the operation
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wraps an existing API that performs its own memory management, or is
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copying the long lived state into existing memory like a circular buffer.
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</li>
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<li class="listitem">
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The operation uses a single, fixed-size POSM for as long as the operation
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is outstanding. For example, the operation stores some state in a linked
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list.
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</li>
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<li class="listitem">
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The operation uses a single, runtime-sized POSM. For example, the operation
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stores a copy of a user-supplied buffer, or a runtime-sized array of
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iovec structures.
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</li>
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<li class="listitem">
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The operation uses multiple POSMs concurrently. For example, a fixed
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size POSM for a linked list plus a runtime-sized POSM for a buffer.
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</li>
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<li class="listitem">
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The operation uses multiple POSMs serially, which may vary in size.
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</li>
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</ul></div>
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<p>
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Associated allocators allow users to treat POSM optimisation as a cross-cutting
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concern to the composition of asynchronous operations. Furthermore, using
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allocators as the interface to obtain POSMs grant substantial flexibility
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to both the implementers and users of asynchronous operations:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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Users can ignore the allocator and accept whatever default strategy
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is employed by the application.
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</li>
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<li class="listitem">
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Implementers can ignore the allocator, especially if the operation
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is not considered performance-sensitive.
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</li>
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<li class="listitem">
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Users can co-locate POSMs for related asynchronous operations, for
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better locality of reference.
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</li>
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<li class="listitem">
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For compositions that involve serial POSMs of different sizes, memory
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usage need only be as great as the currently extant POSM. For example,
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consider a composition that contains a short-lived operation that uses
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large POSMs (connection establishment and handshake) followed by a
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long-lived operation that uses small POSMs (transferring data to and
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from the peer).
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</li>
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</ul></div>
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<p>
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As noted previously, all resources must be released prior to calling the
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completion handler. This enables memory to be recycled for subsequent asynchronous
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operations within an agent. This allows applications with long-lived asynchronous
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agents to have no hot-path memory allocations, even though the user code
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is unaware of associated allocators.
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</p>
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<h6>
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<a name="asio.overview.model.allocators.h0"></a>
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<span><a name="asio.overview.model.allocators.see_also"></a></span><a class="link" href="allocators.html#asio.overview.model.allocators.see_also">See
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Also</a>
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</h6>
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<p>
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<a class="link" href="../core/allocation.html" title="Custom Memory Allocation">Custom Memory Allocation</a>.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="executors.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../model.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="cancellation.html"><img src="../../../next.png" alt="Next"></a>
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