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<div class="title">Memory mapping </div> </div>
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<div class="textblock"><h1><a class="anchor" id="persistently_mapped_memory"></a>
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Persistently mapped memory</h1>
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<p>If you need to map memory on host, it may happen that two allocations are assigned to the same <code>VkDeviceMemory</code> block, so if you map them both at the same time, it will cause error because mapping single memory block multiple times is illegal in Vulkan.</p>
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<p>TODO update this...</p>
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<p>It is safer, more convenient and more efficient to use special feature designed for that: persistently mapped memory. Allocations made with <code>VMA_ALLOCATION_CREATE_MAPPED_BIT</code> flag set in <a class="el" href="struct_vma_allocation_create_info.html#add09658ac14fe290ace25470ddd6d41b" title="Use VmaAllocationCreateFlagBits enum. ">VmaAllocationCreateInfo::flags</a> are returned from device memory blocks that stay mapped all the time, so you can just access CPU pointer to it. <a class="el" href="struct_vma_allocation_info.html#a5eeffbe2d2f30f53370ff14aefbadbe2" title="Pointer to the beginning of this allocation as mapped data. ">VmaAllocationInfo::pMappedData</a> pointer is already offseted to the beginning of particular allocation. Example:</p>
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<div class="fragment"><div class="line">VkBufferCreateInfo bufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };</div><div class="line">bufCreateInfo.size = 1024;</div><div class="line">bufCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_SRC_BIT;</div><div class="line"></div><div class="line"><a class="code" href="struct_vma_allocation_create_info.html">VmaAllocationCreateInfo</a> allocCreateInfo = {};</div><div class="line">allocCreateInfo.<a class="code" href="struct_vma_allocation_create_info.html#accb8b06b1f677d858cb9af20705fa910">usage</a> = <a class="code" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca40bdf4cddeffeb12f43d45ca1286e0a5">VMA_MEMORY_USAGE_CPU_ONLY</a>;</div><div class="line">allocCreateInfo.<a class="code" href="struct_vma_allocation_create_info.html#add09658ac14fe290ace25470ddd6d41b">flags</a> = <a class="code" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a11da372cc3a82931c5e5d6146cd9dd1f">VMA_ALLOCATION_CREATE_MAPPED_BIT</a>;</div><div class="line"></div><div class="line">VkBuffer buf;</div><div class="line">VmaAllocation alloc;</div><div class="line"><a class="code" href="struct_vma_allocation_info.html">VmaAllocationInfo</a> allocInfo;</div><div class="line"><a class="code" href="vk__mem__alloc_8h.html#ac72ee55598617e8eecca384e746bab51">vmaCreateBuffer</a>(allocator, &bufCreateInfo, &allocCreateInfo, &buf, &alloc, &allocInfo);</div><div class="line"></div><div class="line"><span class="comment">// Buffer is immediately mapped. You can access its memory.</span></div><div class="line">memcpy(allocInfo.<a class="code" href="struct_vma_allocation_info.html#a5eeffbe2d2f30f53370ff14aefbadbe2">pMappedData</a>, myData, 1024);</div></div><!-- fragment --><p>Memory in Vulkan doesn't need to be unmapped before using it e.g. for transfers, but if you are not sure whether it's <code>HOST_COHERENT</code> (here is surely is because it's created with <code>VMA_MEMORY_USAGE_CPU_ONLY</code>), you should check it. If it's not, you should call <code>vkInvalidateMappedMemoryRanges()</code> before reading and <code>vkFlushMappedMemoryRanges()</code> after writing to mapped memory on CPU. Example:</p>
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<div class="fragment"><div class="line">VkMemoryPropertyFlags memFlags;</div><div class="line"><a class="code" href="vk__mem__alloc_8h.html#a8701444752eb5de4464adb5a2b514bca">vmaGetMemoryTypeProperties</a>(allocator, allocInfo.<a class="code" href="struct_vma_allocation_info.html#a7f6b0aa58c135e488e6b40a388dad9d5">memoryType</a>, &memFlags);</div><div class="line"><span class="keywordflow">if</span>((memFlags & VK_MEMORY_PROPERTY_HOST_COHERENT_BIT) == 0)</div><div class="line">{</div><div class="line"> VkMappedMemoryRange memRange = { VK_STRUCTURE_TYPE_MAPPED_MEMORY_RANGE };</div><div class="line"> memRange.memory = allocInfo.<a class="code" href="struct_vma_allocation_info.html#ae0bfb7dfdf79a76ffefc9a94677a2f67">deviceMemory</a>;</div><div class="line"> memRange.offset = allocInfo.<a class="code" href="struct_vma_allocation_info.html#a4a3c732388dbdc7a23f9365b00825268">offset</a>;</div><div class="line"> memRange.size = allocInfo.<a class="code" href="struct_vma_allocation_info.html#aac76d113a6a5ccbb09fea00fb25fd18f">size</a>;</div><div class="line"> vkFlushMappedMemoryRanges(device, 1, &memRange);</div><div class="line">}</div></div><!-- fragment --><h1><a class="anchor" id="amd_perf_note"></a>
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Note on performance</h1>
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<p>There is a situation that you should be careful about. It happens only if all of following conditions are met:</p>
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<ol type="1">
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<li>You use AMD GPU.</li>
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<li>You use the memory type that is both <code>DEVICE_LOCAL</code> and <code>HOST_VISIBLE</code> (used when you specify <code>VMA_MEMORY_USAGE_CPU_TO_GPU</code>).</li>
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<li>Operating system is Windows 7 or 8.x (Windows 10 is not affected because it uses WDDM2).</li>
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</ol>
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<p>Then whenever a <code>VkDeviceMemory</code> block allocated from this memory type is mapped for the time of any call to <code>vkQueueSubmit()</code> or <code>vkQueuePresentKHR()</code>, this block is migrated by WDDM to system RAM, which degrades performance. It doesn't matter if that particular memory block is actually used by the command buffer being submitted.</p>
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<p>To avoid this problem, either make sure to unmap all allocations made from this memory type before your Submit and Present, or use <code>VMA_MEMORY_USAGE_GPU_ONLY</code> and transfer from a staging buffer in <code>VMA_MEMORY_USAGE_CPU_ONLY</code>, which can safely stay mapped all the time. </p>
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