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Updated documentation chapter about lost allocations.

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Adam Sawicki 2018-03-12 13:23:06 +01:00
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@ -67,17 +67,17 @@ $(function() {
</div><!--header-->
<div class="contents">
<div class="textblock"><p>If your game oversubscribes video memory, if may work OK in previous-generation graphics APIs (DirectX 9, 10, 11, OpenGL) because resources are automatically paged to system RAM. In Vulkan you can't do it because when you run out of memory, an allocation just fails. If you have more data (e.g. textures) that can fit into VRAM and you don't need it all at once, you may want to upload them to GPU on demand and "push out" ones that are not used for a long time to make room for the new ones, effectively using VRAM (or a cartain memory pool) as a form of cache. Vulkan Memory Allocator can help you with that by supporting a concept of "lost allocations".</p>
<p>To create an allocation that can become lost, include <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a5f436af6c8fe8540573a6d22627a6fd2">VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT</a> flag in <a class="el" href="struct_vma_allocation_create_info.html#add09658ac14fe290ace25470ddd6d41b" title="Use VmaAllocationCreateFlagBits enum. ">VmaAllocationCreateInfo::flags</a>. Before using a buffer or image bound to such allocation in every new frame, you need to query it if it's not lost. To check it: call <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> and see if <a class="el" href="struct_vma_allocation_info.html#ae0bfb7dfdf79a76ffefc9a94677a2f67" title="Handle to Vulkan memory object. ">VmaAllocationInfo::deviceMemory</a> is not <code>VK_NULL_HANDLE</code>. If the allocation is lost, you should not use it or buffer/image bound to it. You mustn't forget to destroy this allocation and this buffer/image.</p>
<p>To create an allocation that can become lost, include <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a5f436af6c8fe8540573a6d22627a6fd2">VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT</a> flag in <a class="el" href="struct_vma_allocation_create_info.html#add09658ac14fe290ace25470ddd6d41b" title="Use VmaAllocationCreateFlagBits enum. ">VmaAllocationCreateInfo::flags</a>. Before using a buffer or image bound to such allocation in every new frame, you need to query it if it's not lost. To check it, call <a class="el" href="vk__mem__alloc_8h.html#a43d8ba9673c846f049089a5029d5c73a" title="Returns VK_TRUE if allocation is not lost and atomically marks it as used in current frame...">vmaTouchAllocation()</a>. If the allocation is lost, you should not use it or buffer/image bound to it. You mustn't forget to destroy this allocation and this buffer/image. <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> can also be used for checking status of the allocation. Allocation is lost when returned <a class="el" href="struct_vma_allocation_info.html#ae0bfb7dfdf79a76ffefc9a94677a2f67" title="Handle to Vulkan memory object. ">VmaAllocationInfo::deviceMemory</a> == <code>VK_NULL_HANDLE</code>.</p>
<p>To create an allocation that can make some other allocations lost to make room for it, use <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a68686d0ce9beb0d4d1b9f2b8b1389a7e">VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT</a> flag. You will usually use both flags <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a68686d0ce9beb0d4d1b9f2b8b1389a7e">VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT</a> and <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a5f436af6c8fe8540573a6d22627a6fd2">VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT</a> at the same time.</p>
<p>Warning! Current implementation uses quite naive, brute force algorithm, which can make allocation calls that use <a class="el" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a68686d0ce9beb0d4d1b9f2b8b1389a7e">VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT</a> flag quite slow. A new, more optimal algorithm and data structure to speed this up is planned for the future.</p>
<p><b>Q: When interleaving creation of new allocations with usage of existing ones, how do you make sure that an allocation won't become lost while it's used in the current frame?</b></p>
<p>It is ensured because <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> not only returns allocation parameters and checks whether it's not lost, but when it's not, it also atomically marks it as used in the current frame, which makes it impossible to become lost in that frame. It uses lockless algorithm, so it works fast and doesn't involve locking any internal mutex.</p>
<p>It is ensured because <a class="el" href="vk__mem__alloc_8h.html#a43d8ba9673c846f049089a5029d5c73a" title="Returns VK_TRUE if allocation is not lost and atomically marks it as used in current frame...">vmaTouchAllocation()</a> / <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> not only returns allocation status/parameters and checks whether it's not lost, but when it's not, it also atomically marks it as used in the current frame, which makes it impossible to become lost in that frame. It uses lockless algorithm, so it works fast and doesn't involve locking any internal mutex.</p>
<p><b>Q: What if my allocation may still be in use by the GPU when it's rendering a previous frame while I already submit new frame on the CPU?</b></p>
<p>You can make sure that allocations "touched" by <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> will not become lost for a number of additional frames back from the current one by specifying this number as <a class="el" href="struct_vma_allocator_create_info.html#a21ea188dd212b8171cb9ecbed4a2a3a7" title="Maximum number of additional frames that are in use at the same time as current frame. ">VmaAllocatorCreateInfo::frameInUseCount</a> (for default memory pool) and <a class="el" href="struct_vma_pool_create_info.html#a9437e43ffbb644dbbf7fc4e50cfad6aa" title="Maximum number of additional frames that are in use at the same time as current frame. ">VmaPoolCreateInfo::frameInUseCount</a> (for custom pool).</p>
<p>You can make sure that allocations "touched" by <a class="el" href="vk__mem__alloc_8h.html#a43d8ba9673c846f049089a5029d5c73a" title="Returns VK_TRUE if allocation is not lost and atomically marks it as used in current frame...">vmaTouchAllocation()</a> / <a class="el" href="vk__mem__alloc_8h.html#a86dd08aba8633bfa4ad0df2e76481d8b" title="Returns current information about specified allocation and atomically marks it as used in current fra...">vmaGetAllocationInfo()</a> will not become lost for a number of additional frames back from the current one by specifying this number as <a class="el" href="struct_vma_allocator_create_info.html#a21ea188dd212b8171cb9ecbed4a2a3a7" title="Maximum number of additional frames that are in use at the same time as current frame. ">VmaAllocatorCreateInfo::frameInUseCount</a> (for default memory pool) and <a class="el" href="struct_vma_pool_create_info.html#a9437e43ffbb644dbbf7fc4e50cfad6aa" title="Maximum number of additional frames that are in use at the same time as current frame. ">VmaPoolCreateInfo::frameInUseCount</a> (for custom pool).</p>
<p><b>Q: How do you inform the library when new frame starts?</b></p>
<p>You need to call function <a class="el" href="vk__mem__alloc_8h.html#ade56bf8dc9f5a5eaddf5f119ed525236" title="Sets index of the current frame. ">vmaSetCurrentFrameIndex()</a>.</p>
<p>Example code:</p>
<div class="fragment"><div class="line"><span class="keyword">struct </span>MyBuffer</div><div class="line">{</div><div class="line"> VkBuffer m_Buf = <span class="keyword">nullptr</span>;</div><div class="line"> VmaAllocation m_Alloc = <span class="keyword">nullptr</span>;</div><div class="line"></div><div class="line"> <span class="comment">// Called when the buffer is really needed in the current frame.</span></div><div class="line"> <span class="keywordtype">void</span> EnsureBuffer();</div><div class="line">};</div><div class="line"></div><div class="line"><span class="keywordtype">void</span> MyBuffer::EnsureBuffer()</div><div class="line">{</div><div class="line"> <span class="comment">// Buffer has been created.</span></div><div class="line"> <span class="keywordflow">if</span>(m_Buf != VK_NULL_HANDLE)</div><div class="line"> {</div><div class="line"> <span class="comment">// Check if its allocation is not lost + mark it as used in current frame.</span></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#a86dd08aba8633bfa4ad0df2e76481d8b">vmaGetAllocationInfo</a>(allocator, m_Alloc, &amp;allocInfo);</div><div class="line"> <span class="keywordflow">if</span>(allocInfo.<a class="code" href="struct_vma_allocation_info.html#ae0bfb7dfdf79a76ffefc9a94677a2f67">deviceMemory</a> != VK_NULL_HANDLE)</div><div class="line"> {</div><div class="line"> <span class="comment">// It&#39;s all OK - safe to use m_Buf.</span></div><div class="line"> <span class="keywordflow">return</span>;</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// Buffer not yet exists or lost - destroy and recreate it.</span></div><div class="line"></div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#a0d9f4e4ba5bf9aab1f1c746387753d77">vmaDestroyBuffer</a>(allocator, m_Buf, m_Alloc);</div><div class="line"></div><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_UNIFORM_BUFFER_BIT | VK_BUFFER_USAGE_TRANSFER_DST_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#aa5846affa1e9da3800e3e78fae2305ccac6b5dc1432d88647aa4cd456246eadf7">VMA_MEMORY_USAGE_GPU_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#ad9889c10c798b040d59c92f257cae597a5f436af6c8fe8540573a6d22627a6fd2">VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT</a> |</div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a68686d0ce9beb0d4d1b9f2b8b1389a7e">VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT</a>;</div><div class="line"></div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#ac72ee55598617e8eecca384e746bab51">vmaCreateBuffer</a>(allocator, &amp;bufCreateInfo, &amp;allocCreateInfo, &amp;m_Buf, &amp;m_Alloc, <span class="keyword">nullptr</span>);</div><div class="line">}</div></div><!-- fragment --><p>When using lost allocations, you may see some Vulkan validation layer warnings about overlapping regions of memory bound to different kinds of buffers and images. This is still valid as long as you implement proper handling of lost allocations (like in the example above) and don't use them.</p>
<div class="fragment"><div class="line"><span class="keyword">struct </span>MyBuffer</div><div class="line">{</div><div class="line"> VkBuffer m_Buf = <span class="keyword">nullptr</span>;</div><div class="line"> VmaAllocation m_Alloc = <span class="keyword">nullptr</span>;</div><div class="line"></div><div class="line"> <span class="comment">// Called when the buffer is really needed in the current frame.</span></div><div class="line"> <span class="keywordtype">void</span> EnsureBuffer();</div><div class="line">};</div><div class="line"></div><div class="line"><span class="keywordtype">void</span> MyBuffer::EnsureBuffer()</div><div class="line">{</div><div class="line"> <span class="comment">// Buffer has been created.</span></div><div class="line"> <span class="keywordflow">if</span>(m_Buf != VK_NULL_HANDLE)</div><div class="line"> {</div><div class="line"> <span class="comment">// Check if its allocation is not lost + mark it as used in current frame.</span></div><div class="line"> <span class="keywordflow">if</span>(<a class="code" href="vk__mem__alloc_8h.html#a43d8ba9673c846f049089a5029d5c73a">vmaTouchAllocation</a>(allocator, m_Alloc))</div><div class="line"> {</div><div class="line"> <span class="comment">// It&#39;s all OK - safe to use m_Buf.</span></div><div class="line"> <span class="keywordflow">return</span>;</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// Buffer not yet exists or lost - destroy and recreate it.</span></div><div class="line"></div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#a0d9f4e4ba5bf9aab1f1c746387753d77">vmaDestroyBuffer</a>(allocator, m_Buf, m_Alloc);</div><div class="line"></div><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_UNIFORM_BUFFER_BIT | VK_BUFFER_USAGE_TRANSFER_DST_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#aa5846affa1e9da3800e3e78fae2305ccac6b5dc1432d88647aa4cd456246eadf7">VMA_MEMORY_USAGE_GPU_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#ad9889c10c798b040d59c92f257cae597a5f436af6c8fe8540573a6d22627a6fd2">VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT</a> |</div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#ad9889c10c798b040d59c92f257cae597a68686d0ce9beb0d4d1b9f2b8b1389a7e">VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT</a>;</div><div class="line"></div><div class="line"> <a class="code" href="vk__mem__alloc_8h.html#ac72ee55598617e8eecca384e746bab51">vmaCreateBuffer</a>(allocator, &amp;bufCreateInfo, &amp;allocCreateInfo, &amp;m_Buf, &amp;m_Alloc, <span class="keyword">nullptr</span>);</div><div class="line">}</div></div><!-- fragment --><p>When using lost allocations, you may see some Vulkan validation layer warnings about overlapping regions of memory bound to different kinds of buffers and images. This is still valid as long as you implement proper handling of lost allocations (like in the example above) and don't use them.</p>
<p>The library uses following algorithm for allocation, in order:</p>
<ol type="1">
<li>Try to find free range of memory in existing blocks.</li>

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@ -564,11 +564,12 @@ cache. Vulkan Memory Allocator can help you with that by supporting a concept of
To create an allocation that can become lost, include #VMA_ALLOCATION_CREATE_CAN_BECOME_LOST_BIT
flag in VmaAllocationCreateInfo::flags. Before using a buffer or image bound to
such allocation in every new frame, you need to query it if it's not lost. To
check it: call vmaGetAllocationInfo() and see if VmaAllocationInfo::deviceMemory
is not `VK_NULL_HANDLE`. If the allocation is lost, you should not use it or
buffer/image bound to it. You mustn't forget to destroy this allocation and this
buffer/image.
such allocation in every new frame, you need to query it if it's not lost.
To check it, call vmaTouchAllocation().
If the allocation is lost, you should not use it or buffer/image bound to it.
You mustn't forget to destroy this allocation and this buffer/image.
vmaGetAllocationInfo() can also be used for checking status of the allocation.
Allocation is lost when returned VmaAllocationInfo::deviceMemory == `VK_NULL_HANDLE`.
To create an allocation that can make some other allocations lost to make room
for it, use #VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT flag. You will
@ -584,8 +585,8 @@ up is planned for the future.
how do you make sure that an allocation won't become lost while it's used in the
current frame?</b>
It is ensured because vmaGetAllocationInfo() not only returns allocation
parameters and checks whether it's not lost, but when it's not, it also
It is ensured because vmaTouchAllocation() / vmaGetAllocationInfo() not only returns allocation
status/parameters and checks whether it's not lost, but when it's not, it also
atomically marks it as used in the current frame, which makes it impossible to
become lost in that frame. It uses lockless algorithm, so it works fast and
doesn't involve locking any internal mutex.
@ -593,7 +594,7 @@ doesn't involve locking any internal mutex.
<b>Q: What if my allocation may still be in use by the GPU when it's rendering a
previous frame while I already submit new frame on the CPU?</b>
You can make sure that allocations "touched" by vmaGetAllocationInfo() will not
You can make sure that allocations "touched" by vmaTouchAllocation() / vmaGetAllocationInfo() will not
become lost for a number of additional frames back from the current one by
specifying this number as VmaAllocatorCreateInfo::frameInUseCount (for default
memory pool) and VmaPoolCreateInfo::frameInUseCount (for custom pool).
@ -620,9 +621,7 @@ void MyBuffer::EnsureBuffer()
if(m_Buf != VK_NULL_HANDLE)
{
// Check if its allocation is not lost + mark it as used in current frame.
VmaAllocationInfo allocInfo;
vmaGetAllocationInfo(allocator, m_Alloc, &allocInfo);
if(allocInfo.deviceMemory != VK_NULL_HANDLE)
if(vmaTouchAllocation(allocator, m_Alloc))
{
// It's all OK - safe to use m_Buf.
return;