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<h1 class="title">Nim's Memory Management</h1>
<table class="docinfo" frame="void" rules="none"><col class="docinfo-name" /><col class="docinfo-content" /><tbody valign="top"><tr><th class="docinfo-name">Author:</th><td>Andreas Rumpf</td></tr>
<tr><th class="docinfo-name">Version:</th><td>2.3.1</td></tr>
</tbody></table><blockquote class="markdown-quote"><p>&quot;The road to hell is paved with good intentions.&quot;</p></blockquote>
<h1 id="multiminusparadigm-memory-management-strategies">Multi-paradigm Memory Management Strategies</h1><p>Nim offers multiple different memory management strategies. To choose the memory management strategy use the <tt class="docutils literal"><span class="pre option">--mm:</span></tt> switch.</p>
<div class="admonition admonition-info"><span class="admonition-info-text"><b>Hint:</b></span>
<strong>The recommended switch for newly written Nim code is `--mm:orc`.</strong></div>
<h2 id="arcslashorc">ARC/ORC</h2><p>ORC is the default memory management strategy. It is a memory management mode primarily based on reference counting. Reference cycles are handled by a cycle collection mechanism based on &quot;trial deletion&quot;. Since algorithms based on &quot;tracing&quot; are not used, the runtime behavior is oblivious to the involved heap and stack sizes.</p>
<p>The reference counting operations (= &quot;RC ops&quot;) do not use atomic instructions and do not have to -- instead entire subgraphs are <em>moved</em> between threads. The Nim compiler also aggressively optimizes away RC ops and exploits <a class="reference external" href="destructors.html#move-semantics">move semantics</a>.</p>
<p>Nim performs a fair share of optimizations for ARC/ORC; you can inspect what it did to your time critical function via <tt class="docutils literal"><span class="pre option">--expandArc:functionName</span></tt>. Likewise, you can inspect the whole module via <tt class="docutils literal"><span class="pre option">--expandArc:fileName</span></tt>.</p>
<p><tt class="docutils literal"><span class="pre option">--mm:arc</span></tt> uses the same mechanism as <tt class="docutils literal"><span class="pre option">--mm:orc</span></tt>, but it leaves out the cycle collector. Both ARC and ORC offer deterministic performance for <span id="hard-realtime_1">hard realtime</span> systems, but ARC can be easier to reason about for people coming from Ada/C++/C -- roughly speaking the memory for a variable is freed when it goes &quot;out of scope&quot;.</p>
<p>We generally advise you to use the <tt class="docutils literal"><span class="pre option">acyclic</span></tt> annotation in order to optimize away the cycle collector's overhead but <tt class="docutils literal"><span class="pre option">--mm:orc</span></tt> also produces more machine code than <tt class="docutils literal"><span class="pre option">--mm:arc</span></tt>, so if you're on a target where code size matters and you know that your code does not produce cycles, you can use <tt class="docutils literal"><span class="pre option">--mm:arc</span></tt>. Notice that the default <span id="async_1">async</span> implementation produces cycles and leaks memory with <tt class="docutils literal"><span class="pre option">--mm:arc</span></tt>, in other words, for <tt class="docutils literal"><span class="pre option">async</span></tt> you need to use <tt class="docutils literal"><span class="pre option">--mm:orc</span></tt>.</p>
<h2 id="other-mm-modes">Other MM modes</h2><div class="admonition admonition-info"><span class="admonition-info-text"><b>Note:</b></span>
The <tt class="docutils literal"><span class="pre option">refc</span></tt> GC is incremental, thread-local and not &quot;stop-the-world&quot;.</div>
<div class="option-list"><div class="option-list-item odd"><div class="option-list-label"><tt><span class="option">--mm:refc</span></tt></div><div class="option-list-description">It's a deferred reference counting based garbage collector with a simple Mark&amp;Sweep backup GC in order to collect cycles. Heaps are thread-local. <a class="reference external" href="refc.html">This document</a> contains further information.</div></div>
<div class="option-list-item"><div class="option-list-label"><tt><span class="option">--mm:markAndSweep</span></tt></div><div class="option-list-description">Simple Mark-And-Sweep based garbage collector. Heaps are thread-local.</div></div>
<div class="option-list-item odd"><div class="option-list-label"><tt><span class="option">--mm:boehm</span></tt></div><div class="option-list-description">Boehm based garbage collector, it offers a shared heap.</div></div>
<div class="option-list-item"><div class="option-list-label"><tt><span class="option">--mm:go</span></tt></div><div class="option-list-description">Go's garbage collector, useful for interoperability with Go. Offers a shared heap.</div></div>
<div class="option-list-item odd"><div class="option-list-label"><tt><span class="option">--mm:none</span></tt></div><div class="option-list-description">No memory management strategy nor a garbage collector. Allocated memory is simply never freed. You should use <tt class="docutils literal"><span class="pre option">--mm:arc</span></tt> instead.</div></div>
</div><p>Here is a comparison of the different memory management modes:</p>
<table border="1" class="docutils"><tr><th>Memory Management</th><th>Heap</th><th>Reference Cycles</th><th>Stop-The-World</th><th>Atomic</th><th>Valgrind compatible</th><th>Command line switch</th></tr>
<tr><td>ORC</td><td>Shared</td><td>Cycle Collector</td><td>No</td><td>No</td><td>Yes</td><td><tt class="docutils literal"><span class="pre option">--mm:orc</span></tt></td></tr>
<tr><td>ARC</td><td>Shared</td><td>Leak</td><td>No</td><td>No</td><td>Yes</td><td><tt class="docutils literal"><span class="pre option">--mm:arc</span></tt></td></tr>
<tr><td>Atomic ARC</td><td>Shared</td><td>Leak</td><td>No</td><td>Yes</td><td>Yes</td><td><tt class="docutils literal"><span class="pre option">--mm:atomicArc</span></tt></td></tr>
<tr><td>RefC</td><td>Local</td><td>Cycle Collector</td><td>No</td><td>No</td><td>No</td><td><tt class="docutils literal"><span class="pre option">--mm:refc</span></tt></td></tr>
<tr><td>Mark &amp; Sweep</td><td>Local</td><td>Cycle Collector</td><td>No</td><td>No</td><td>No</td><td><tt class="docutils literal"><span class="pre option">--mm:markAndSweep</span></tt></td></tr>
<tr><td>Boehm</td><td>Shared</td><td>Cycle Collector</td><td>Yes</td><td>No</td><td>No</td><td><tt class="docutils literal"><span class="pre option">--mm:boehm</span></tt></td></tr>
<tr><td>Go</td><td>Shared</td><td>Cycle Collector</td><td>Yes</td><td>No</td><td>No</td><td><tt class="docutils literal"><span class="pre option">--mm:go</span></tt></td></tr>
<tr><td>None</td><td>Manual</td><td>Manual</td><td>Manual</td><td>Manual</td><td>Manual</td><td><tt class="docutils literal"><span class="pre option">--mm:none</span></tt></td></tr>
</table><p>JavaScript's garbage collector is used for the <a class="reference external" href=" backends.html#backends-the-javascript-target">JavaScript and NodeJS</a> compilation targets. The <a class="reference external" href="nims.html">NimScript</a> target uses the memory management strategy built into the Nim compiler. </p>
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