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<div class="section" id="memory-in-skiboot">
<h1>Memory in skiboot<a class="headerlink" href="#memory-in-skiboot" title="Permalink to this headline">¶</a></h1>
<p>There are regions of memory we statically allocate for firmware as well as
a HEAP region for boot and runtime allocations.</p>
<p>A design principle of skiboot is to attempt not to allocate memory at runtime,
or at least keep it to a minimum, and not do so in any critical code path
for the system to remain running.</p>
<p>At no point during runtime should a skiboot memory allocation failure cause
the system to stop functioning.</p>
<div class="section" id="heap">
<h2>HEAP<a class="headerlink" href="#heap" title="Permalink to this headline">¶</a></h2>
<p>Dynamic memory allocations go in a single heap. This is identified as
Region ibm,firmware-heap and appears as a reserved section in the device tree.</p>
<p>Originally, it was 12582912 bytes in size (declared in mem_map.h).
Now, it is 13631488 bytes after being bumped as part of the GCOV work.</p>
<p>We increased heap size as on larger systems, we were getting close to using
all the heap once skiboot became 2MB with GCOV.</p>
<p>Heap usage is printed before running the payload.</p>
<p>For example, as of writing, on a dual socket Tuleta:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">45215870591</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">SkiBoot</span> <span class="n">skiboot</span><span class="o">-</span><span class="mf">5.0</span><span class="o">.</span><span class="mi">1</span><span class="o">-</span><span class="mi">94</span><span class="o">-</span><span class="n">gb759ce2</span> <span class="n">starting</span><span class="o">...</span>
<span class="p">[</span><span class="mi">3680939340</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">CUPD</span><span class="p">:</span> <span class="n">T</span> <span class="n">side</span> <span class="n">MI</span> <span class="n">Keyword</span> <span class="o">=</span> <span class="n">SV830_027</span>
<span class="p">[</span><span class="mi">3680942658</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">CUPD</span><span class="p">:</span> <span class="n">T</span> <span class="n">side</span> <span class="n">ML</span> <span class="n">Keyword</span> <span class="o">=</span> <span class="n">FW830</span><span class="o">.</span><span class="mi">00</span>
<span class="p">[</span><span class="mi">15404383291</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">Region</span> <span class="n">ibm</span><span class="p">,</span><span class="n">firmware</span><span class="o">-</span><span class="n">heap</span> <span class="n">free</span><span class="p">:</span> <span class="mi">5378072</span>
</pre></div>
</div>
<p>and on a palmetto:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">24748502575</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">SkiBoot</span> <span class="n">skiboot</span><span class="o">-</span><span class="mf">5.0</span><span class="o">.</span><span class="mi">1</span><span class="o">-</span><span class="mi">94</span><span class="o">-</span><span class="n">gb759ce2</span> <span class="n">starting</span><span class="o">...</span>
<span class="p">[</span><span class="mi">9870429550</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> <span class="n">Region</span> <span class="n">ibm</span><span class="p">,</span><span class="n">firmware</span><span class="o">-</span><span class="n">heap</span> <span class="n">free</span><span class="p">:</span> <span class="mi">10814856</span>
</pre></div>
</div>
<p>Our memory allocator is simple, a use pattern of:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">A</span> <span class="o">=</span> <span class="n">malloc</span><span class="p">();</span>
<span class="n">B</span> <span class="o">=</span> <span class="n">malloc</span><span class="p">();</span>
<span class="n">free</span><span class="p">(</span><span class="n">A</span><span class="p">);</span>
</pre></div>
</div>
<p>is likely to generate fragmentation, so it should generally be avoided
where possible.</p>
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