MPSTemporaryImage(3) MetalPerformanceShaders.framework MPSTemporaryImage(3)
NAME
MPSTemporaryImage
SYNOPSIS
#import <MPSImage.h>
Inherits MPSImage.
Instance Methods
(nonnull instancetype) - initWithTexture:featureChannels:
(nonnull instancetype) - initWithDevice:imageDescriptor:
Class Methods
(nonnull id< MPSImageAllocator >) + defaultAllocator
(nonnull instancetype) + temporaryImageWithCommandBuffer:imageDescriptor:
(nonnull instancetype) + temporaryImageWithCommandBuffer:textureDescriptor:
(nonnull instancetype) + temporaryImageWithCommandBuffer:textureDescriptor:featureChannels:
(void) + prefetchStorageWithCommandBuffer:imageDescriptorList:
Properties
NSUInteger readCount
Detailed Description
MPSImage MPSTemporaryImages are for MPSImages with short lifetimes.
What is temporary memory? It is memory, plain and simple. Analogy: If we use an app as an analogy for a command buffer, then 'Regular
memory' (such as what backs a MPSImage or the typical MTLTexture) would be memory that you allocate at launch and never free. Temporary
memory would be memory that you free when you are done with it so it can be used for something else as needed later in your app. You
/could/ write your app to allocate everything you will ever need up front, but this is very inefficient and quite frankly a pain to plan
out in advance. You don't do it for your app, so why would you do it for your command buffers?
Welcome to the 1970's! We have added a heap.
Unsurprisingly, MPSTemporaryImages can provide for profound reduction in the the amount of memory used by your application. Like malloc,
MPS maintains a heap of memory usable in a command buffer. Over the lifetime of a command buffer, the same piece of memory may be reused
many times. This means that each time the same meory is reused, it aliases with previous uses. If we aren't careful, we might find that
needed data is overwritten by successive allocations. However, this is no different than accessing freed memory only to discover it doesn't
contain what you thought it did anymore, so you should be able to keep out of trouble by following a few simple rules, like with malloc.
To this end, we added some restrictions to help you out and get a bit more performance. Some comments are appended in parentheses below to
extend the analogy of command buffer = program:
o The textures are MTLStorageModePrivate. You can not, for example, use [MTLTexture getBytes...] or [MTLTexture replaceRegion...] with
them. MPSTemporaryImages are strictly read and written by the GPU. (There is protected memory to prevent other processes from overwriting
your heap.)
o The temporary image may be used only on a single MTLCommandBuffer. This limits the chronology to a single linear time stream. (The heap
is specific to just one command buffer. There are no mutexes to coordinate timing of simultaneous access by multiple GPUs. Nor are we
likely to like them if there were. So, we disallow it.)
o The readCount property must be managed correctly. Please see the description of the readCount property for full details. (The readCount
is a reference count for the block of memory that holds your data. The usual undefined behaviors apply to reading data that has been
released. We assert when we can to prevent that from happening accidentally, just as a program might segfault. The readCount counts
procedural users of the object -- MPSKernel.encode... calls that read the MPSTemporaryImage. As each reads from it, the readCount is
automatically decremented. The texture data will be freed in typical usage at the right time as the readCount reaches zero, typically
with little user involvement other than to set the readCount up front. We did examine using the main MPSTemporaryImage reference count
for this instead so that ARC would do work for you automatically. Alas, ARC destroys things at end of scope rather than promptly,
sometimes resulting in greatly increased memory usage. These allocations are large! So, we use this method instead.)
Since MPSTemporaryImages can only be used with a single MTLCommandBuffer, and can not be used off the GPU, they generally should not be
kept around past the completion of the MTLCommandBuffer. The lifetime of MPSTemporaryImages is expected to be typically extremely short,
perhaps only a few lines of code. Like malloc, it is intended to be fairly cheap to make MPSTemporaryImages and throw them away. Please do
so.
To keep the lifetime of the underlying texture allocation as short as possible, the underlying texture is not allocated until the first
time the MPSTemporaryImage is used by a MPSCNNKernel or the .texture property is read. The readCount property serves to limit the lifetime
on the other end.
You may use the MPSTemporaryImage.texture with MPSUnaryImageKernel -encode... methods, iff featureChannels <= 4 and the MTLTexture conforms
to requirements of that MPSKernel. There is no locking mechanism provided to prevent a MTLTexture returned from the .texture property from
becoming invalid when the readCount reaches 0.
MPSTemporaryImages can otherwise be used wherever MPSImages are used.
Method Documentation
+ (nonnull id <MPSImageAllocator>) defaultAllocator
Get a well known MPSImageAllocator that makes MPSTemporaryImages
Reimplemented from MPSImage.
- (nonnull instancetype) initWithDevice: (nonnull id< MTLDevice >) device(const MPSImageDescriptor *__nonnull) imageDescriptor
Unavailable. Use itemporaryImageForCommandBuffer:textureDescriptor: instead.
Reimplemented from MPSImage.
- (nonnull instancetype) initWithTexture: (nonnull id< MTLTexture >) texture(NSUInteger) featureChannels
Unavailable. Use temporaryImageForCommandBuffer:textureDescriptor: or -temporaryImageForCommandBuffer:imageDescriptor: instead.
Reimplemented from MPSImage.
+ (void) prefetchStorageWithCommandBuffer: (nonnull id< MTLCommandBuffer >) commandBuffer(NSArray< MPSImageDescriptor * > *__nonnull)
descriptorList
Help MPS decide which allocations to make ahead of time The texture cache that underlies the MPSTemporaryImage can automatically allocate
new storage as needed as you create new temporary images. However, sometimes a more global view of what you plan to make is useful for
maximizing memory reuse to get the most efficient operation. This class method hints to the cache what the list of images will be.
It is never necessary to call this method. It is purely a performance and memory optimization.
Parameters:
commandBuffer The command buffer on which the MPSTemporaryImages will be used
descriptorList A NSArray of MPSImageDescriptors, indicating images that will be created
+ (nonnull instancetype) temporaryImageWithCommandBuffer: (nonnull id< MTLCommandBuffer >) commandBuffer(const MPSImageDescriptor *__nonnull)
imageDescriptor
Initialize a MPSTemporaryImage for use on a MTLCommandBuffer
Parameters:
commandBuffer The MTLCommandBuffer on which the MPSTemporaryImage will be exclusively used
imageDescriptor A valid imageDescriptor describing the MPSImage format to create.
Returns:
A valid MPSTemporaryImage. The object will be released when the command buffer is committed. The underlying texture will become invalid
before this time due to the action of the readCount property.
+ (nonnull instancetype) temporaryImageWithCommandBuffer: (nonnull id< MTLCommandBuffer >) commandBuffer(const MTLTextureDescriptor *__nonnull)
textureDescriptor
Low level interface for creating a MPSTemporaryImage using a MTLTextureDescriptor This function provides access to MTLPixelFormats not
typically covered by -initForCommandBuffer:imageDescriptor: The feature channels will be inferred from the MTLPixelFormat without changing
the width. The following restrictions apply:
MTLTextureType must be MTLTextureType2D or MTLTextureType2DArray
MTLTextureUsage must contain at least one of MTLTextureUsageShaderRead, MTLTextureUsageShaderWrite
MTLStorageMode must be MTLStorageModePrivate
depth must be 1
Parameters:
commandBuffer The command buffer on which the MPSTemporaryImage may be used
textureDescriptor A texture descriptor describing the MPSTemporaryImage texture
Returns:
A valid MPSTemporaryImage. The object will be released when the command buffer is committed. The underlying texture will become invalid
before this time due to the action of the readCount property.
+ (nonnull instancetype) temporaryImageWithCommandBuffer: (nonnull id< MTLCommandBuffer >) commandBuffer(const MTLTextureDescriptor *__nonnull)
textureDescriptor(NSUInteger) featureChannels
Low level interface for creating a MPSTemporaryImage using a MTLTextureDescriptor This function provides access to MTLPixelFormats not
typically covered by -initForCommandBuffer:imageDescriptor: The number of images will be inferred from number of slices in the
descriptor.arrayLength and the number of feature channels.
The following restrictions apply:
MTLTextureType must be MTLTextureType2D or MTLTextureType2DArray
MTLTextureUsage must contain at least one of MTLTextureUsageShaderRead, MTLTextureUsageShaderWrite
MTLStorageMode must be MTLStorageModePrivate
Parameters:
commandBuffer The command buffer on which the MPSTemporaryImage may be used
textureDescriptor A texture descriptor describing the MPSTemporaryImage texture
Returns:
A valid MPSTemporaryImage. The object will be released when the command buffer is committed. The underlying texture will become invalid
before this time due to the action of the readCount property.
Property Documentation
- (NSUInteger) readCount [read], [write], [nonatomic], [assign]
Author
Generated automatically by Doxygen for MetalPerformanceShaders.framework from the source code.
Version MetalPerformanceShaders-100 Thu Feb 8 2018 MPSTemporaryImage(3)