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    <p>It's almost a year and I figured it out.</p> <p><strong>clean memory</strong> </p> <p>clean memory are memories that can be recreated, on iOS it is memory of:</p> <ul> <li>system framework</li> <li>binary executable of your app</li> <li>memory mapped files</li> </ul> <p>Also notice this situation: when your app link to a framework, the clean memory will increase by the size of the framework binary. But most of time, only part of binary is really loaded in physical memory.</p> <p><strong>dirty memory</strong></p> <p>All memory that is not clean memory is dirty memory, dirty memory can't be recreated by system. </p> <p>When there is a memory pressure, system will unload some clean memory, when the memory is needed again, system will recreate them. </p> <p>But for dirty memory, system can't unload them, and iOS has no swap mechanism, so dirty memory will always be kept in physical memory, till it reach a certain limit, then your App will be terminated and all memory for it is recycled by system.</p> <p><strong>virtual memory</strong></p> <pre><code>virtual memory = clean memory + dirty memory. </code></pre> <p>That means virtual memory is all the memory your App want.</p> <p><strong>resident memory</strong></p> <pre><code>resident memory = dirty memory + clean memory that loaded in physical memory </code></pre> <p>resident memory is the memory really loaded in your physical memory, it mean all the dirty memory and parts of your clean memory.</p> <p><strong>conclusion</strong></p> <p>At any time, this is always true:</p> <pre><code>virtual memory == (clean memory + dirty memory) &gt; resident memory &gt; dirty memory </code></pre> <p>If you are worrying about the physical memory your App is taking(which is the key reason your App is terminated due to low memory), you should mainly focus on resident memory.</p>
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