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This document contains the global design decisions made by the vulkano library. It can also be a
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This document contains the global design decisions made by the vulkano library. It can also be a
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good start if you want to contribute to some internal parts of vulkano and don't know how it works.
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good start if you want to contribute to some internal parts of vulkano and don't know how it works.
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This document assumes that you're already familiar with Vulkan and does not introduce the various
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This document assumes that you're already familiar with Vulkan and does not introduce the various
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concepts. However it can still be a good read if you are not so familiar.
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concepts. However it can still be a good read if you are not so familiar.
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If you notice any mistake, feel free to open a PR. If you want to suggest something, feel free to
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If you notice any mistake, feel free to open a PR. If you want to suggest something, feel free to
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open a PR as well.
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open a PR as well.
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# The three kinds of objects
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# The three kinds of objects
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Vulkano provides wrappers around all objects of the Vulkan API. However these objects are split in
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Vulkano provides wrappers around all objects of the Vulkan API. However these objects are split in
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three categories, depending on their access pattern:
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three categories, depending on their access pattern:
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- Objects that are not created often and in very small numbers.
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- Objects that are not created often and in very small numbers.
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- Objects that are typically created at initialization and which are often accessed without mutation
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- Objects that are typically created at initialization and which are often accessed without mutation
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by performance-critical code.
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by performance-critical code.
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- Objects that are created, destroyed or modified during performance-critical code, and that
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- Objects that are created, destroyed or modified during performance-critical code, and that
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usually require a synchronization strategy to avoid race conditions.
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usually require a synchronization strategy to avoid race conditions.
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The first category are objects that are not created often and created in very small numbers:
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The first category are objects that are not created often and created in very small numbers:
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Instances, Devices, Surfaces, Swapchains. In a typical application each of these objects is only
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Instances, Devices, Surfaces, Swapchains. In a typical application each of these objects is only
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created once and destroyed when the application exits. Vulkano's API provides a struct that
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created once and destroyed when the application exits. Vulkano's API provides a struct that
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corresponds to each of these objects, and this struct is typically wrapped in an `Arc`.
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corresponds to each of these objects, and this struct is typically wrapped in an `Arc`.
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Their `new` method in fact returns an `Arc<T>` instead of just a `T` in order to encourage users to
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Their `new` method in fact returns an `Arc<T>` instead of just a `T` in order to encourage users to
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use `Arc`s. You use these objects by cloning them around like you would use objects in a
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use `Arc`s. You use these objects by cloning them around like you would use objects in a
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garbage-collected language such as Java.
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garbage-collected language such as Java.
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The second category are objects like the GraphicsPipeline, ComputePipeline, PipelineLayout,
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The second category are objects like the GraphicsPipeline, ComputePipeline, PipelineLayout,
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RenderPass and Framebuffer. They are usually created at initialization and don't perform any
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RenderPass and Framebuffer. They are usually created at initialization and don't perform any
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operations themselves, but they describe to the Vulkan implementation operations that we are going
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operations themselves, but they describe to the Vulkan implementation operations that we are going
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to perform and are thus frequently accessed in order to determine whether the operation that the
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to perform and are thus frequently accessed in order to determine whether the operation that the
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vulkano user requested is compliant to what was described. Just like the first category, each of
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vulkano user requested is compliant to what was described. Just like the first category, each of
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these objects has a struct that corresponds to them, but in order to make these checks as fast as
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these objects has a struct that corresponds to them, but in order to make these checks as fast as
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possible these structs have a template parameter that describes in a strongly-typed fashion the
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possible these structs have a template parameter that describes in a strongly-typed fashion the
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operation on the CPU side. This makes it possible to move many checks to compile-time instead of
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operation on the CPU side. This makes it possible to move many checks to compile-time instead of
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runtime. More information in another section of this document.
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runtime. More information in another section of this document.
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The third category are objects like CommandBuffers, CommandPools, DescriptorSets, DescriptorPools,
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The third category are objects like CommandBuffers, CommandPools, DescriptorSets, DescriptorPools,
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Buffers, Images, and memory pools (although not technically a Vulkan object). The way they are
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Buffers, Images, and memory pools (although not technically a Vulkan object). The way they are
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implemented has a huge impact on the performance of the application. Contrary to the first two
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implemented has a huge impact on the performance of the application. Contrary to the first two
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categories, each of these objects is represented in vulkano by an unsafe trait (and not by a
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categories, each of these objects is represented in vulkano by an unsafe trait (and not by a
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struct) that can be freely implemented by the user if they wish. Vulkano provides unsafe structs
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struct) that can be freely implemented by the user if they wish. Vulkano provides unsafe structs
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such as `UnsafeBuffer`, `UnsafeImage`, etc. which have zero overhead and do not perform any safety
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such as `UnsafeBuffer`, `UnsafeImage`, etc. which have zero overhead and do not perform any safety
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checks, and are the tools used by the safe implementations of the traits. Vulkano also provides
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checks, and are the tools used by the safe implementations of the traits. Vulkano also provides
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some safe implementations for convenience such as `CpuAccessibleBuffer` or `AttachmentImage`.
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some safe implementations for convenience such as `CpuAccessibleBuffer` or `AttachmentImage`.
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# Runtime vs compile-time checks
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# Runtime vs compile-time checks
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The second category of objects described above are objects that describe to the Vulkan
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The second category of objects described above are objects that describe to the Vulkan
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implementation an operation that we are going to perform later. For example a `ComputePipeline`
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implementation an operation that we are going to perform later. For example a `ComputePipeline`
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object describes to the Vulkan implementation a compute operation and contains the shader's code
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object describes to the Vulkan implementation a compute operation and contains the shader's code
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and the list of resources that we are going to bind and that are going to be accessed by the shader.
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and the list of resources that we are going to bind and that are going to be accessed by the shader.
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Since vulkano is a safe library, it needs to check whether the operation the user requests (eg.
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Since vulkano is a safe library, it needs to check whether the operation the user requests (eg.
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executing a compute operation) matches the corresponding `ComputePipeline` (for example, check
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executing a compute operation) matches the corresponding `ComputePipeline` (for example, check
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that the list of resources passed by the user matches what the compute pipeline expects).
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that the list of resources passed by the user matches what the compute pipeline expects).
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These checks can be expensive. For example when it comes to buffers, vulkano needs to check whether
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These checks can be expensive. For example when it comes to buffers, vulkano needs to check whether
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the layout of the buffers passed by the user is the same as what is expected, by looping through all
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the layout of the buffers passed by the user is the same as what is expected, by looping through all
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the members and following several indirections. If you multiply this by several dozens or hundreds
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the members and following several indirections. If you multiply this by several dozens or hundreds
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of operations, it can become very expensive.
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of operations, it can become very expensive.
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In order to reduce the stress caused by these checks, structs such as `ComputePipeline` have a
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In order to reduce the stress caused by these checks, structs such as `ComputePipeline` have a
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template parameter which describes the operation. Whenever vulkano performs a check, it queries
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template parameter which describes the operation. Whenever vulkano performs a check, it queries
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the templated object through a trait, and each safety check has its own trait. This means
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the templated object through a trait, and each safety check has its own trait. This means
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that we can build strongly-typed objects at compile-time that describe a very precise operation and
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that we can build strongly-typed objects at compile-time that describe a very precise operation and
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whose method implementations are trivial. For example, we can create a `MyComputeOpDesc` type which
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whose method implementations are trivial. For example, we can create a `MyComputeOpDesc` type which
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implements the `ResourcesListMatch<MyResourcesList>` trait (which was made up for the sake of the
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implements the `ResourcesListMatch<MyResourcesList>` trait (which was made up for the sake of the
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example), and the user will only be able to pass a `MyResourcesList` object for the list of
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example), and the user will only be able to pass a `MyResourcesList` object for the list of
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resources. This moves the check to compile-time and totally eliminates any runtime check. The
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resources. This moves the check to compile-time and totally eliminates any runtime check. The
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compute pipeline is then expressed as `ComputePipeline<MyComputeOpDesc>`.
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compute pipeline is then expressed as `ComputePipeline<MyComputeOpDesc>`.
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However this design has a drawback, which is that is can be difficult to explicitly express such a
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However this design has a drawback, which is that is can be difficult to explicitly express such a
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type. A compute pipeline in the example above could be expressed as
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type. A compute pipeline in the example above could be expressed as
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`ComputePipeline<MyComputeOpDesc>`, but in practice these types (like `MyComputeOpDesc`) would be
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`ComputePipeline<MyComputeOpDesc>`, but in practice these types (like `MyComputeOpDesc`) would be
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built by builders and can become extremely long and annoying to put in a struct (just like for
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built by builders and can become extremely long and annoying to put in a struct (just like for
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example the type of `(10..).filter(|n| n*2).skip(3).take(5)` can be very long and annoying to put
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example the type of `(10..).filter(|n| n*2).skip(3).take(5)` can be very long and annoying to put
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in a struct). This is especially problematic as it concerns objects that are usually created at
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in a struct). This is especially problematic as it concerns objects that are usually created at
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initialization and stay alive for a long time, in other words the kind of objects that you would
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initialization and stay alive for a long time, in other words the kind of objects that you would
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put in a struct.
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put in a struct.
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In order to solve this naming problem, all the traits that are used to describe operations must be
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In order to solve this naming problem, all the traits that are used to describe operations must be
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boxable so that we can turn `ComputePipeline<Very<Long<And<Complicated, Type>>>>` into
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boxable so that we can turn `ComputePipeline<Very<Long<And<Complicated, Type>>>>` into
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`ComputePipeline<Box<ComputePipelineDesc>>`. This means that we can't use associated types and
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`ComputePipeline<Box<ComputePipelineDesc>>`. This means that we can't use associated types and
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templates for any of the trait methods. Ideologically it is a bit annoying to have to restrict
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templates for any of the trait methods. Ideologically it is a bit annoying to have to restrict
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ourselves in what we can do just because the user needs to be able to write out the precise type,
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ourselves in what we can do just because the user needs to be able to write out the precise type,
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but it's the only pragmatic solution for now.
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but it's the only pragmatic solution for now.
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# Submissions
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# Submissions
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Any object that can be submitted to a GPU queue (for example a command buffer) implements
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Any object that can be submitted to a GPU queue (for example a command buffer) implements
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the `Submit` trait.
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the `Submit` trait.
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The `Submit` trait provides a function named `build` which returns a `Submission<Self>` object
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The `Submit` trait provides a function named `build` which returns a `Submission<Self>` object
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(where `Self` is the type that implements the `Submit` trait). The `Submission` object must be kept
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(where `Self` is the type that implements the `Submit` trait). The `Submission` object must be kept
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alive by the user for as long as the GPU hasn't finished executing the submission. Trying to
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alive by the user for as long as the GPU hasn't finished executing the submission. Trying to
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destroy a `Submission` will block until it is the case. Since the `Submission` holds the object
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destroy a `Submission` will block until it is the case. Since the `Submission` holds the object
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that was submitted, this object is also kept alive for as long as the GPU hasn't finished executing
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that was submitted, this object is also kept alive for as long as the GPU hasn't finished executing
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it.
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it.
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For the moment submitting an object always creates a fence, which is how the `Submission` knows
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For the moment submitting an object always creates a fence, which is how the `Submission` knows
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whether the GPU has finished executing it. Eventually this will need to be modified for the sake of
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whether the GPU has finished executing it. Eventually this will need to be modified for the sake of
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performance.
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performance.
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In order to make the `Submit` trait safer to implement, the method that actually needs to be
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In order to make the `Submit` trait safer to implement, the method that actually needs to be
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implemented is not `build` but `append_submission`. This method uses a API/lifetime trick to
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implemented is not `build` but `append_submission`. This method uses a API/lifetime trick to
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guarantee that the GPU only executes command buffers that outlive the struct that implements
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guarantee that the GPU only executes command buffers that outlive the struct that implements
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`Submit`.
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`Submit`.
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SAFETY ISSUE HERE HOWEVER: the user can use mem::forget on the Submission and then drop the
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SAFETY ISSUE HERE HOWEVER: the user can use mem::forget on the Submission and then drop the
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objects referenced by it. There are two solutions to this: either store a bunch of Arc<Fence> in
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objects referenced by it. There are two solutions to this: either store a bunch of Arc<Fence> in
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every single object referenced by submissions (eg. pipeline objects), or force the user to use
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every single object referenced by submissions (eg. pipeline objects), or force the user to use
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either Arcs or give ownership of the object. The latter is preferred but not yet implemented.
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either Arcs or give ownership of the object. The latter is preferred but not yet implemented.
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# Pools
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# Pools
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There are three kinds of pools in vulkano: memory pools, descriptor pools, and command pools. Only
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There are three kinds of pools in vulkano: memory pools, descriptor pools, and command pools. Only
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the last two are technically Vulkan concepts, but using a memory pool is also a very common
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the last two are technically Vulkan concepts, but using a memory pool is also a very common
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pattern that you are strongly encouraged to embrace when you write a Vulkan application.
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pattern that you are strongly encouraged to embrace when you write a Vulkan application.
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These three kinds of pools are each represented in vulkano by a trait. When you use the Vulkan API,
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These three kinds of pools are each represented in vulkano by a trait. When you use the Vulkan API,
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you are expected to create multiple command pools and multiple descriptor pools for maximum
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you are expected to create multiple command pools and multiple descriptor pools for maximum
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performance. In vulkano however, it is the implementation of the pool trait that is responsible
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performance. In vulkano however, it is the implementation of the pool trait that is responsible
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for managing multiple actual pool objects. In other words a pool in vulkano is just a trait that
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for managing multiple actual pool objects. In other words a pool in vulkano is just a trait that
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provides a method to allocate or free some resource, and the advanced functionality of Vulkan
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provides a method to allocate or free some resource, and the advanced functionality of Vulkan
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pools (like resetting a command buffer, resetting a pool, or managing the descriptor pool's
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pools (like resetting a command buffer, resetting a pool, or managing the descriptor pool's
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capacity) is handled internally by the implementation of the trait. For example freeing a
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capacity) is handled internally by the implementation of the trait. For example freeing a
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command buffer can be implemented by resetting it and reusing it, instead of actually freeing it.
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command buffer can be implemented by resetting it and reusing it, instead of actually freeing it.
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One of the goals of vulkano is to be easy to use by default. Therefore vulkano provides a default
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One of the goals of vulkano is to be easy to use by default. Therefore vulkano provides a default
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implementation for each of these pools, and the `new` constructors of types that need a pool (ie.
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implementation for each of these pools, and the `new` constructors of types that need a pool (ie.
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buffers, images, descriptor sets, and command buffers) will use the default implementation. It is
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buffers, images, descriptor sets, and command buffers) will use the default implementation. It is
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possible for the user to use an alternative implementation of a pool by using an alternative
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possible for the user to use an alternative implementation of a pool by using an alternative
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constructor, but the default implementations should be good for most usages. This is similar to
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constructor, but the default implementations should be good for most usages. This is similar to
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memory allocators in languages such as C++ and Rust, in the sense that some users want to be able
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memory allocators in languages such as C++ and Rust, in the sense that some users want to be able
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to use a custom allocator but most of the time it's not worth bothering with that.
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to use a custom allocator but most of the time it's not worth bothering with that.
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# Command buffers
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# Command buffers
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Command buffer objects belong to the last category of objects that were described above. They are
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Command buffer objects belong to the last category of objects that were described above. They are
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represented by an unsafe trait and can be implemented manually by the user if they wish.
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represented by an unsafe trait and can be implemented manually by the user if they wish.
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However this poses a practical problem, which is that creating a command buffer in a safe way
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However this poses a practical problem, which is that creating a command buffer in a safe way
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is really complicated. There are tons of commands to implement, and each command has a ton of
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is really complicated. There are tons of commands to implement, and each command has a ton of
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safety requirements. If a user wants to create a custom command buffer type, it is just not an
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safety requirements. If a user wants to create a custom command buffer type, it is just not an
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option to ask them to reimplement these safety checks themselves.
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option to ask them to reimplement these safety checks themselves.
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The reason why users may want to create their own command buffer types is to implement
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The reason why users may want to create their own command buffer types is to implement
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synchronization themselves. Vulkano's default implementation (which is `AutobarriersCommandBuffer`)
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synchronization themselves. Vulkano's default implementation (which is `AutobarriersCommandBuffer`)
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will automatically place pipeline barriers in order to handle cache flushes and image layout
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will automatically place pipeline barriers in order to handle cache flushes and image layout
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transitions and avoid data races, but this automatic computation can be seen as expensive.
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transitions and avoid data races, but this automatic computation can be seen as expensive.
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In order to make it possible to customize the synchronization story of command buffers, vulkano has
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In order to make it possible to customize the synchronization story of command buffers, vulkano has
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split the command buffer building process in two steps. First the user builds a list of commands
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split the command buffer building process in two steps. First the user builds a list of commands
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through an iterator-like API (and vulkano will check their validity), and then they are turned into
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through an iterator-like API (and vulkano will check their validity), and then they are turned into
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a command buffer through a trait. This means that the user can customize the synchronization
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a command buffer through a trait. This means that the user can customize the synchronization
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strategy (by customizing the second step) while still using the same command-building process
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strategy (by customizing the second step) while still using the same command-building process
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(the first step). Commands are not opinionated towards one strategy or another. The
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(the first step). Commands are not opinionated towards one strategy or another. The
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command-building code is totally isolated from the synchronization strategy and only checks
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command-building code is totally isolated from the synchronization strategy and only checks
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whether the commands themselves are valid.
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whether the commands themselves are valid.
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The fact that all the commands are added at once can be a little surprising for a user coming from
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The fact that all the commands are added at once can be a little surprising for a user coming from
|
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Vulkan. Vulkano's API looks very similar to Vulkan's API, but there is a major difference: in
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Vulkan. Vulkano's API looks very similar to Vulkan's API, but there is a major difference: in
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Vulkan the cost of creating a command buffer is distributed between each function call, but in
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Vulkan the cost of creating a command buffer is distributed between each function call, but in
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vulkano it is done all at once. For example creating a command buffer with 6 commands with Vulkan
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vulkano it is done all at once. For example creating a command buffer with 6 commands with Vulkan
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requires 8 function calls that take say 5µs each, while creating the same command buffer with
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requires 8 function calls that take say 5µs each, while creating the same command buffer with
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vulkano requires 8 function calls, but the first 7 are almost free and the last one takes 40µs.
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vulkano requires 8 function calls, but the first 7 are almost free and the last one takes 40µs.
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After some thinking, it was considered to not be a problem.
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After some thinking, it was considered to not be a problem.
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Creating a list of commands with an iterator-like API has the problem that the type of the list of
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Creating a list of commands with an iterator-like API has the problem that the type of the list of
|
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commands changes every time you add a new command to the list
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commands changes every time you add a new command to the list
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(just like for example `let iterator = iterator.skip(1)` changes the type of `iterator`). This is
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(just like for example `let iterator = iterator.skip(1)` changes the type of `iterator`). This is
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a problem in situations where we don't know at compile-time the number of commands that we are
|
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a problem in situations where we don't know at compile-time the number of commands that we are
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going to add. In order to solve this, it is required that the `CommandsList` trait be boxable,
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going to add. In order to solve this, it is required that the `CommandsList` trait be boxable,
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so that the user can use a `Box<CommandsList>`. This is unfortunately not optimal as you will need
|
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so that the user can use a `Box<CommandsList>`. This is unfortunately not optimal as you will need
|
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a memory allocation for each command that is added to the list. The situation here could still be
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a memory allocation for each command that is added to the list. The situation here could still be
|
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improved.
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improved.
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# The auto-barriers builder
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# The auto-barriers builder
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As explained above, the default implementation of a command buffer provided by vulkano
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As explained above, the default implementation of a command buffer provided by vulkano
|
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automatically places pipeline barriers to avoid issues such as caches not being flushed, commands
|
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automatically places pipeline barriers to avoid issues such as caches not being flushed, commands
|
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being executed simultaneously when they shouldn't, or images having the wrong layout.
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being executed simultaneously when they shouldn't, or images having the wrong layout.
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This is not an easy job, because Vulkan allows lots of weird access patterns that we want to make
|
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This is not an easy job, because Vulkan allows lots of weird access patterns that we want to make
|
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|
|
available in vulkano. You can for example create a buffer object split into multiple sub-buffer
|
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available in vulkano. You can for example create a buffer object split into multiple sub-buffer
|
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objects, or make some images and buffers share the same memory.
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objects, or make some images and buffers share the same memory.
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In order to make it possible to handle everything properly, the `Buffer` and `Image` traits need to
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In order to make it possible to handle everything properly, the `Buffer` and `Image` traits need to
|
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|
help us with the `conflicts` methods. Each buffer and image can be queried to know whether it
|
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|
|
help us with the `conflicts` methods. Each buffer and image can be queried to know whether it
|
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|
potentially uses the same memory as any other buffer or image. When two resources conflict, this
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|
|
potentially uses the same memory as any other buffer or image. When two resources conflict, this
|
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|
|
means that you can't write to one and read from the other one simultaneously or write to both
|
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|
|
means that you can't write to one and read from the other one simultaneously or write to both
|
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simultaneously.
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simultaneously.
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But we don't want to check every single combination of buffer and image every time to check whether
|
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|
But we don't want to check every single combination of buffer and image every time to check whether
|
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|
|
they conflict. So in order to improve performance, buffers and images also need to provide a key
|
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|
they conflict. So in order to improve performance, buffers and images also need to provide a key
|
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|
|
that identifies them. Two resources that can potentially conflict must always return the same key.
|
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|
that identifies them. Two resources that can potentially conflict must always return the same key.
|
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|
|
The regular `conflict` functions are still necessary to handle the situation where buffers or
|
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|
|
The regular `conflict` functions are still necessary to handle the situation where buffers or
|
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|
|
images accidentally return the same key but don't actually conflict.
|
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|
|
images accidentally return the same key but don't actually conflict.
|
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This conflict system is also used to make sure that the attachments of a framebuffer don't conflict
|
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|
|
This conflict system is also used to make sure that the attachments of a framebuffer don't conflict
|
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|
|
with each other or that the resources in a descriptor set don't conflict with each other (both
|
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|
|
with each other or that the resources in a descriptor set don't conflict with each other (both
|
|
|
|
situations are forbidden).
|
|
|
|
situations are forbidden).
|
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|
|
# Image layouts
|
|
|
|
# Image layouts
|
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|
|
Tracking image layouts can be tedious. Vulkano uses a simple solution, which is that images must
|
|
|
|
Tracking image layouts can be tedious. Vulkano uses a simple solution, which is that images must
|
|
|
|
always be in a specific layout at the beginning and the end of a command buffer. If a transition
|
|
|
|
always be in a specific layout at the beginning and the end of a command buffer. If a transition
|
|
|
|
is performed during a command buffer, the image must be transitioned back before the end of the
|
|
|
|
is performed during a command buffer, the image must be transitioned back before the end of the
|
|
|
|
command buffer. The layout in question is queried with a method on the `Image` trait.
|
|
|
|
command buffer. The layout in question is queried with a method on the `Image` trait.
|
|
|
|
|
|
|
|
|
|
|
|
For example an `AttachmentImage` must always be in the `ColorAttachmentOptimal` layout for color
|
|
|
|
For example an `AttachmentImage` must always be in the `ColorAttachmentOptimal` layout for color
|
|
|
|
attachment, and the `DepthStencilAttachmentOptimal` layout for depth-stencil attachments. If any
|
|
|
|
attachment, and the `DepthStencilAttachmentOptimal` layout for depth-stencil attachments. If any
|
|
|
|
command switches the image to another layout, then it will need to be switched back before the end
|
|
|
|
command switches the image to another layout, then it will need to be switched back before the end
|
|
|
|
of the command buffer.
|
|
|
|
of the command buffer.
|
|
|
|
|
|
|
|
|
|
|
|
This system works very nicely in practice, and unnecessary layout transitions almost never happen.
|
|
|
|
This system works very nicely in practice, and unnecessary layout transitions almost never happen.
|
|
|
|
The only situation where unnecessary transitions tend to happen in practice is for swapchain images
|
|
|
|
The only situation where unnecessary transitions tend to happen in practice is for swapchain images
|
|
|
|
that are transitioned from `PresentSrc` to `ColorAttachmentOptimal` before the start of the
|
|
|
|
that are transitioned from `PresentSrc` to `ColorAttachmentOptimal` before the start of the
|
|
|
|
render pass, because the initial layout of the render pass attachment is `ColorAttachmentOptimal`
|
|
|
|
render pass, because the initial layout of the render pass attachment is `ColorAttachmentOptimal`
|
|
|
|
by default for color attachments. Vulkano should make it clear in the documentation of render
|
|
|
|
by default for color attachments. Vulkano should make it clear in the documentation of render
|
|
|
|
passes that the user is encouraged to specify when an attachment is expected to be in the
|
|
|
|
passes that the user is encouraged to specify when an attachment is expected to be in the
|
|
|
|
`PresentSrc` layout.
|
|
|
|
`PresentSrc` layout.
|
|
|
|
|
|
|
|
|
|
|
|
The only problematic area concerns the first usage of an image, where it must be transitioned from
|
|
|
|
The only problematic area concerns the first usage of an image, where it must be transitioned from
|
|
|
|
the `Undefined` or `Preinitialized` layout. This is done by making the user pass a command buffer
|
|
|
|
the `Undefined` or `Preinitialized` layout. This is done by making the user pass a command buffer
|
|
|
|
builder in the constructor of images, and the constructor adds a transition command to it. The
|
|
|
|
builder in the constructor of images, and the constructor adds a transition command to it. The
|
|
|
|
image implementation is responsible for making sure that the transition command has been submitted
|
|
|
|
image implementation is responsible for making sure that the transition command has been submitted
|
|
|
|
before any further command that uses the image.
|
|
|
|
before any further command that uses the image.
|
|
|
|
|
|
|
|
|
|
|
|
# Inter-queue synchronization
|
|
|
|
# Inter-queue synchronization
|
|
|
|
|
|
|
|
|
|
|
|
When users submit two command buffers to two different queues, they expect the two command buffers
|
|
|
|
When users submit two command buffers to two different queues, they expect the two command buffers
|
|
|
|
to execute in parallel. However this is forbidden if doing so could result in a data race,
|
|
|
|
to execute in parallel. However this is forbidden if doing so could result in a data race,
|
|
|
|
like for example if one command buffer writes to an image and the other one reads from that same
|
|
|
|
like for example if one command buffer writes to an image and the other one reads from that same
|
|
|
|
image.
|
|
|
|
image.
|
|
|
|
In this situation, the only possible technical solution is to make the execution of the second
|
|
|
|
In this situation, the only possible technical solution is to make the execution of the second
|
|
|
|
command buffer block until the first command buffer has finished executing.
|
|
|
|
command buffer block until the first command buffer has finished executing.
|
|
|
|
This case is similar to spawning two threads that each access the same resource protected by
|
|
|
|
This case is similar to spawning two threads that each access the same resource protected by
|
|
|
|
a `RwLock` or a `Mutex`. One of the two threads will need to block until the first one is finished.
|
|
|
|
a `RwLock` or a `Mutex`. One of the two threads will need to block until the first one is finished.
|
|
|
|
|
|
|
|
|
|
|
|
This raises the question: should vulkano implicitly block command buffers to avoid data races,
|
|
|
|
This raises the question: should vulkano implicitly block command buffers to avoid data races,
|
|
|
|
or should it force the user to explicitly add wait operations? By comparing a CPU-side
|
|
|
|
or should it force the user to explicitly add wait operations? By comparing a CPU-side
|
|
|
|
multithreaded program and a GPU-side multithreaded program, then the answer is to make it implicit,
|
|
|
|
multithreaded program and a GPU-side multithreaded program, then the answer is to make it implicit,
|
|
|
|
as a CPU will also implicitly block when calling a function that happens to lock a `Mutex` or
|
|
|
|
as a CPU will also implicitly block when calling a function that happens to lock a `Mutex` or
|
|
|
|
a `RwLock`. In CPU code, these locking problems are always "fixed" by properly documenting the
|
|
|
|
a `RwLock`. In CPU code, these locking problems are always "fixed" by properly documenting the
|
|
|
|
behavior of the functions you call. Similarly, vulkano should precisely document its behavior.
|
|
|
|
behavior of the functions you call. Similarly, vulkano should precisely document its behavior.
|
|
|
|
|
|
|
|
|
|
|
|
More generally users are encouraged to avoid sharing resources between multiple queues unless these
|
|
|
|
More generally users are encouraged to avoid sharing resources between multiple queues unless these
|
|
|
|
resources are read-only, and in practice in a video game it is indeed rarely needed to share
|
|
|
|
resources are read-only, and in practice in a video game it is indeed rarely needed to share
|
|
|
|
resources between multiple queues. Just like for CPU-side multithreading, users are encouraged to
|
|
|
|
resources between multiple queues. Just like for CPU-side multithreading, users are encouraged to
|
|
|
|
have a graph of the ways queues interact with each other.
|
|
|
|
have a graph of the ways queues interact with each other.
|
|
|
|
|
|
|
|
|
|
|
|
However another problem arises. In order to make a command buffer wait for another, you need to
|
|
|
|
However another problem arises. In order to make a command buffer wait for another, you need to
|
|
|
|
make the queue of the first command buffer submit a semaphore after execution, and the queue of
|
|
|
|
make the queue of the first command buffer submit a semaphore after execution, and the queue of
|
|
|
|
the second command buffer wait on that same semaphore before execution. Semaphores can only be used
|
|
|
|
the second command buffer wait on that same semaphore before execution. Semaphores can only be used
|
|
|
|
once. This means that when you submit a command buffer to a queue, you must already know if any
|
|
|
|
once. This means that when you submit a command buffer to a queue, you must already know if any
|
|
|
|
other command buffers are going to wait on the one you are submitting, and if so how many. This is not
|
|
|
|
other command buffers are going to wait on the one you are submitting, and if so how many. This is not
|
|
|
|
something that vulkano can automatically determine. The fact that there is therefore no optimal
|
|
|
|
something that vulkano can automatically determine. The fact that there is therefore no optimal
|
|
|
|
algorithm for implicit synchronization would be a good point in favor of explicit synchronization.
|
|
|
|
algorithm for implicit synchronization would be a good point in favor of explicit synchronization.
|
|
|
|
|
|
|
|
|
|
|
|
The decision was taken to encourage users to explicitly handle synchronization between multiple
|
|
|
|
The decision was taken to encourage users to explicitly handle synchronization between multiple
|
|
|
|
queues, but if they forget to do so then vulkano will automatically fall back to a dumb
|
|
|
|
queues, but if they forget to do so then vulkano will automatically fall back to a dumb
|
|
|
|
worst-case-scenario but safe behavior. Whenever this dumb behavior is triggered, a debug message
|
|
|
|
worst-case-scenario but safe behavior. Whenever this dumb behavior is triggered, a debug message
|
|
|
|
is outputted by vulkano with the `vkDebugReportMessageEXT` function. This message can easily be
|
|
|
|
is outputted by vulkano with the `vkDebugReportMessageEXT` function. This message can easily be
|
|
|
|
caught by the user by registering a callback, or with a debugger.
|
|
|
|
caught by the user by registering a callback, or with a debugger.
|
|
|
|
|
|
|
|
|
|
|
|
It is yet to be determined what exactly the user needs to handle. The user will at least need to
|
|
|
|
It is yet to be determined what exactly the user needs to handle. The user will at least need to
|
|
|
|
specify an optional list of semaphores to signal at each submission, but maybe not the list of
|
|
|
|
specify an optional list of semaphores to signal at each submission, but maybe not the list of
|
|
|
|
semaphores to wait upon if these can be determined automatically. This has yet to be seen.
|
|
|
|
semaphores to wait upon if these can be determined automatically. This has yet to be seen.
|
|
|
|