&gt; Do you actually touch it and write to it?<p>calloc itself calls memset in user space.  That&#x27;s what separates it from malloc.  Is it so hard to believe a modern computer can memset 20 GB&#x2F;s?<p>If I run &quot;dd if=&#x2F;dev&#x2F;zero of=&#x2F;dev&#x2F;null bs=1M count=10000&quot;, I get 21.0 GB&#x2F;s.  Reading from &#x2F;dev&#x2F;zero is basically performing a memset to dd&#x27;s buffer in kernel space.  Writing to &#x2F;dev&#x2F;null is a no-op, so it&#x27;s a very similar test.