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Updated documentation of memory mapping with known bug in MoltenVK, based on #47. Thanks @DiegoAce !

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Adam Sawicki 2018-11-16 13:43:34 +01:00
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docs/html/memory_mapping.html
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@ -82,6 +82,7 @@ Persistently mapped memory</h1>
<div class="fragment"><div class="line">VkBufferCreateInfo bufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };</div><div class="line">bufCreateInfo.size = <span class="keyword">sizeof</span>(ConstantBuffer);</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"><a class="code" href="struct_vma_allocation.html">VmaAllocation</a> 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, &amp;bufCreateInfo, &amp;allocCreateInfo, &amp;buf, &amp;alloc, &amp;allocInfo);</div><div class="line"></div><div class="line"><span class="comment">// Buffer is already 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>, &amp;constantBufferData, <span class="keyword">sizeof</span>(constantBufferData));</div></div><!-- fragment --><p>There are some exceptions though, when you should consider mapping memory only for a short period of time:</p> <div class="fragment"><div class="line">VkBufferCreateInfo bufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };</div><div class="line">bufCreateInfo.size = <span class="keyword">sizeof</span>(ConstantBuffer);</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"><a class="code" href="struct_vma_allocation.html">VmaAllocation</a> 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, &amp;bufCreateInfo, &amp;allocCreateInfo, &amp;buf, &amp;alloc, &amp;allocInfo);</div><div class="line"></div><div class="line"><span class="comment">// Buffer is already 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>, &amp;constantBufferData, <span class="keyword">sizeof</span>(constantBufferData));</div></div><!-- fragment --><p>There are some exceptions though, when you should consider mapping memory only for a short period of time:</p>
<ul> <ul>
<li>When operating system is Windows 7 or 8.x (Windows 10 is not affected because it uses WDDM2), device is discrete AMD GPU, and memory type is the special 256 MiB pool of <code>DEVICE_LOCAL + HOST_VISIBLE</code> memory (selected when you use <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca9066b52c5a7079bb74a69aaf8b92ff67">VMA_MEMORY_USAGE_CPU_TO_GPU</a>), then whenever a memory block allocated from this memory type stays 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.</li> <li>When operating system is Windows 7 or 8.x (Windows 10 is not affected because it uses WDDM2), device is discrete AMD GPU, and memory type is the special 256 MiB pool of <code>DEVICE_LOCAL + HOST_VISIBLE</code> memory (selected when you use <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca9066b52c5a7079bb74a69aaf8b92ff67">VMA_MEMORY_USAGE_CPU_TO_GPU</a>), then whenever a memory block allocated from this memory type stays 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.</li>
<li>On Mac/MoltenVK there is a known bug - <a href="https://github.com/KhronosGroup/MoltenVK/issues/175">Issue #175</a> which requires unmapping before GPU can see updated texture.</li>
<li>Keeping many large memory blocks mapped may impact performance or stability of some debugging tools.</li> <li>Keeping many large memory blocks mapped may impact performance or stability of some debugging tools.</li>
</ul> </ul>
<h1><a class="anchor" id="memory_mapping_cache_control"></a> <h1><a class="anchor" id="memory_mapping_cache_control"></a>

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src/vk_mem_alloc.h
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@ -392,6 +392,8 @@ There are some exceptions though, when you should consider mapping memory only f
block is migrated by WDDM to system RAM, which degrades performance. It doesn't 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 matter if that particular memory block is actually used by the command buffer
being submitted. being submitted.
- On Mac/MoltenVK there is a known bug - [Issue #175](https://github.com/KhronosGroup/MoltenVK/issues/175)
which requires unmapping before GPU can see updated texture.
- Keeping many large memory blocks mapped may impact performance or stability of some debugging tools. - Keeping many large memory blocks mapped may impact performance or stability of some debugging tools.
\section memory_mapping_cache_control Cache control \section memory_mapping_cache_control Cache control