<a class="site-logo" href="https://github.com/KhronosGroup/MoltenVK" title="MoltenVK">
	<img src="images/MoltenVK-Logo-Banner.png" alt="MoltenVK" style="width:256px;height:auto">
</a>


MoltenVK Runtime User Guide
===========================

Copyright (c) 2015-2021 [The Brenwill Workshop Ltd.](http://www.brenwill.com)

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Table of Contents
-----------------

- [About This Document](#about_this)
- [About **MoltenVK**](#about_moltenvk)
- [Installing **MoltenVK** in Your *Vulkan* Application](#install)
	- [Install *MoltenVK* as a Universal `XCFramework`](#install_xcfwk)
	- [Install *MoltenVK* as a Dynamic Library](#install_dylib)
	- [Build and Runtime Requirements](#requirements)
- [Interacting with the **MoltenVK** Runtime](#interaction)
	- [MoltenVK `VK_MVK_moltenvk` Extension](#moltenvk_extension)
	- [Configuring MoltenVK](#moltenvk_config)
- [*Metal Shading Language* Shaders](#shaders)
	- [Troubleshooting Shader Conversion](#spv_vs_msl)
- [Performance Considerations](#performance)
	- [Shader Loading Time](#shader_load_time)
	- [Swapchains](#swapchains)
	- [Xcode Configuration](#xcode_config)
	- [Metal System Trace Tool](#trace_tool)
- [Known **MoltenVK** Limitations](#limitations)


<a name="about_this"></a>
About This Document
-------------------

This document describes how to integrate the **MoltenVK** runtime distribution package into a game or
application, once **MoltenVK** has been built into a framework or library for *macOS*, *iOS*, or *tvOS*.

To learn how to use the **MoltenVK** open-source repository to build a **MoltenVK** runtime 
distribution package, see the main [`README.md`](../README.md) document in the `MoltenVK` repository.


<a name="about_moltenvk"></a>
About **MoltenVK**
------------------

**MoltenVK** is a layered implementation of [*Vulkan 1.1*](https://www.khronos.org/vulkan) 
graphics and compute functionality, that is built on Apple's [*Metal*](https://developer.apple.com/metal) 
graphics and compute framework on *macOS*, *iOS*, and *tvOS*. **MoltenVK** allows you to use *Vulkan* 
graphics and compute functionality to develop modern, cross-platform, high-performance graphical games 
and applications, and to run them across many platforms, including *macOS*, *iOS*, *tvOS*, *Simulators*,
and *Mac Catalyst* on *macOS 11.0+*.

*Metal* uses a different shading language, the *Metal Shading Language (MSL)*, than 
*Vulkan*, which uses *SPIR-V*. **MoltenVK** automatically converts your *SPIR-V* shaders 
to their *MSL* equivalents.

To provide *Vulkan* capability to the *macOS*, *iOS*, and *tvOS* platforms, **MoltenVK** uses 
*Apple's* publicly available API's, including *Metal*. **MoltenVK** does **_not_** use any 
private or undocumented API calls or features, so your app will be compatible with all 
standard distribution channels, including *Apple's App Store*.


<a name="install"></a>
Installing **MoltenVK** in Your *Vulkan* Application
----------------------------------------------------

Installation of **MoltenVK** in your application is straightforward and easy!

Depending on your build and deployment needs, you can link **MoltenVK** to your application either 
as a universal `XCFramework` or as a *dynamic library* (`.dylib`). Distributing an app containing 
a dynamic library via the *iOS App Store* or *tvOS App Store* can require specialized bundling. 
If you are unsure about which linking and deployment option you need, or on *iOS* or *tvOS*, 
unless you have specific needs for dynamic libraries, follow the steps for linking **MoltenVK** 
as an `XCFramework`, as it is the simpler option, and encompasses the largest set of supported platforms.


<a name="install_xcfwk"></a>
### Install *MoltenVK* as a Universal `XCFramework`

To link **MoltenVK** to your application as an `XCFramework`, follow these steps:

1. Open your application in *Xcode* and select your application's target in the 
   *Project Navigator* panel.

2. Open the *Build Settings* tab.

	1. In the **Header Search Paths** (aka `HEADER_SEARCH_PATHS`) setting, 
           add an entry that points to the `MoltenVK/include` folder.

	2. If using `IOSurfaces` on *iOS*, open the **iOS Deployment Target** (aka `IPHONEOS_DEPLOYMENT_TARGET`) 
	   setting, and ensure it is set to a value of `iOS 11.0` or greater,  or if using `IOSurfaces` on *tvOS*, 
	   open the **tvOS Deployment Target** (aka `TVOS_DEPLOYMENT_TARGET`) setting, and ensure it is set to a
	   value of `tvOS 11.0` or greater.

3. Open the *Build Phases* tab and open the *Link Binary With Libraries* list.
   
	1. Drag `MoltenVK/MoltenVK.xcframework` to the *Link Binary With Libraries* list.

	2. If your application does **_not_** use use `C++`, click the **+** button, 
	   and add `libc++.tbd` by selecting it from the list of system frameworks. 
	   This is needed because **MoltenVK** uses `C++` system libraries internally.
      
	3. If you do **_not_** have the **Link Frameworks Automatically** (aka `CLANG_MODULES_AUTOLINK`) and 
       **Enable Modules (C and Objective-C)** (aka `CLANG_ENABLE_MODULES`) settings enabled, click the 
       **+** button, and add the following items by selecting them from the list of system frameworks:
	   - `libc++.tbd` *(if not already done in Step 2)*
	   - `Metal.framework`
	   - `Foundation.framework`.
	   - `QuartzCore.framework`
	   - `IOKit.framework` (*macOS*)
	   - `UIKit.framework` (*iOS* or *tvOS*)
	   - `IOSurface.framework` (*macOS*, or *iOS* if `IPHONEOS_DEPLOYMENT_TARGET` is at least `iOS 11.0`, 
	      or *tvOS* if `TVOS_DEPLOYMENT_TARGET` is at least `tvOS 11.0`)


<a name="install_dylib"></a>
### Install *MoltenVK* as a Dynamic Library

To link **MoltenVK** to your application as a dynamic library (`.dylib`), follow these steps:

1. Open your application in *Xcode* and select your application's target in the 
   *Project Navigator* panel.


2. Open the *Build Settings* tab.

    1. In the **Header Search Paths** (aka `HEADER_SEARCH_PATHS`) setting, 
       add an entry that points to the `MoltenVK/include` folder.
       
    2. In the **Library Search Paths** (aka `LIBRARY_SEARCH_PATHS`) setting, 
       add an entry that points to **_one_** of the following folders:
          - `MoltenVK/dylib/macOS` *(macOS)*
          - `MoltenVK/dylib/iOS` *(iOS)*
          - `MoltenVK/dylib/tvOS` *(tvOS)*
          
    3. In the **Runpath Search Paths** (aka `LD_RUNPATH_SEARCH_PATHS`) setting, 
       add an entry that matches where the dynamic library will be located in your runtime
       environment. If the dynamic library is to be embedded within your application, 
       you would typically set this to  **_one_** of these values:

       - `@executable_path/../Frameworks` *(macOS)*
       - `@executable_path/Frameworks` *(iOS or tvOS)*
       
       The `libMoltenVK.dylib` library is internally configured to be located at 
       `@rpath/libMoltenVK.dylib`.

	3. If using `IOSurfaces` on *iOS*, open the **iOS Deployment Target** (aka `IPHONEOS_DEPLOYMENT_TARGET`) 
	   setting, and ensure it is set to a value of `iOS 11.0` or greater,  or if using `IOSurfaces` on *tvOS*, 
	   open the **tvOS Deployment Target** (aka `TVOS_DEPLOYMENT_TARGET`) setting, and ensure it is set to a
	   value of `tvOS 11.0` or greater.

3. Open the *Build Phases* tab and open the *Link Binary With Libraries* list.
   
	1. Drag **_one_** of the following files to the *Link Binary With Libraries* list:
      - `MoltenVK/dylib/macOS/libMoltenVK.dylib` *(macOS)* 
      - `MoltenVK/dylib/iOS/libMoltenVK.dylib` *(iOS)* 
      - `MoltenVK/dylib/tvOS/libMoltenVK.dylib` *(tvOS)* 

	2. If your application does **_not_** use use `C++`, click the **+** button, 
	   and add `libc++.tbd` by selecting it from the list of system frameworks. 
	   This is needed because **MoltenVK** uses `C++` system libraries internally.
      
	3. If you do **_not_** have the **Link Frameworks Automatically** (aka `CLANG_MODULES_AUTOLINK`) and 
       **Enable Modules (C and Objective-C)** (aka `CLANG_ENABLE_MODULES`) settings enabled, click the 
       **+** button, and add the following items by selecting them from the list of system frameworks:
	   - `libc++.tbd` *(if not already done in Step 2)*
	   - `Metal.framework`
	   - `Foundation.framework`.
	   - `QuartzCore.framework`
	   - `IOKit.framework` (*macOS*)
	   - `UIKit.framework` (*iOS* or *tvOS*)
	   - `IOSurface.framework` (*macOS*, or *iOS* if `IPHONEOS_DEPLOYMENT_TARGET` is at least `iOS 11.0`, 
	      or *tvOS* if `TVOS_DEPLOYMENT_TARGET` is at least `tvOS 11.0`)

4. Arrange to install the `libMoltenVK.dylib` file in your application environment:

   - To copy the `libMoltenVK.dylib` file into your application or component library:
   
	   1. On the *Build Phases* tab, add a new *Copy Files* build phase.
	    
	   2. Set the *Destination* into which you want to place  the `libMoltenVK.dylib` file.
	      Typically this will be *Frameworks* (and it should match the **Runpath Search Paths** 
	      (aka `LD_RUNPATH_SEARCH_PATHS`) build setting you added above).
	    
	   3. Drag **_one_** of the following files to the *Copy Files* list in this new build phase:
	     - `MoltenVK/dylib/macOS/libMoltenVK.dylib` *(macOS)* 
	     - `MoltenVK/dylib/iOS/libMoltenVK.dylib` *(iOS)* 
	     - `MoltenVK/dylib/tvOS/libMoltenVK.dylib` *(tvOS)* 
   
   - Alternately, you may create your own installation mechanism to install one of the following 
     files into a standard *macOS*, *iOS*, or  *tvOS* system library folder on the user's device:
      - `MoltenVK/dylib/macOS/libMoltenVK.dylib` *(macOS)* 
      - `MoltenVK/dylib/iOS/libMoltenVK.dylib` *(iOS)* 
      - `MoltenVK/dylib/tvOS/libMoltenVK.dylib` *(tvOS)* 
     

<a name="requirements"></a>
### Build and Runtime Requirements

**MoltenVK** references the latest *Apple SDK* frameworks. To access these frameworks when building
your app, and to avoid build errors, be sure to use the latest publicly available version of *Xcode*.

>***Note:*** To support `IOSurfaces` on *iOS* or *tvOS*, any app that uses **MoltenVK** must be 
built with a minimum **iOS Deployment Target** (aka `IPHONEOS_DEPLOYMENT_TARGET `) build setting 
of `iOS 11.0` or greater, or a minimum **tvOS Deployment Target** (aka `TVOS_DEPLOYMENT_TARGET `)
build setting of `tvOS 11.0` or greater.

Once built, your app integrating the **MoltenVK** libraries can be run on *macOS*, *iOS* or *tvOS* 
devices that support *Metal*, or on the *Xcode* *iOS Simulator* or *tvOS Simulator*.

- At runtime, **MoltenVK** requires at least *macOS 10.11*, *iOS 9*, or *tvOS 9* 
  (or *iOS 11* or *tvOS 11* if using `IOSurfaces`).
- Information on *macOS* devices that are compatible with *Metal* can be found in 
  [this article](http://www.idownloadblog.com/2015/06/22/how-to-find-mac-el-capitan-metal-compatible).
- Information on *iOS* devices that are compatible with *Metal* can be found in 
  [this article](https://developer.apple.com/library/content/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/HardwareGPUInformation/HardwareGPUInformation.html).

When a *Metal* app is running from *Xcode*, the default ***Scheme*** settings may reduce performance. 
To improve performance and gain the benefits of *Metal*, perform the following in *Xcode*:
   
1. Open the ***Scheme Editor*** for building your main application. You can do 
   this by selecting ***Edit Scheme...*** from the ***Scheme*** drop-down menu, or select 
   ***Product -> Scheme -> Edit Scheme...*** from the main menu.
2. On the ***Info*** tab, set the ***Build Configuration*** to ***Release***, and disable the 
   ***Debug executable*** check-box.
3. On the ***Options*** tab, disable both the ***Metal API Validation*** and ***GPU Frame Capture***
   options. For optimal performance, you may also consider disabling the other simulation
   and debugging options on this tab. For further information, see the 
   [Xcode Scheme Settings and Performance](https://developer.apple.com/library/ios/documentation/Miscellaneous/Conceptual/MetalProgrammingGuide/Dev-Technique/Dev-Technique.html#//apple_ref/doc/uid/TP40014221-CH8-SW3) 
   section of Apple's *Metal Programming Guide* documentation.


<a name="interaction"></a>
Interacting with the **MoltenVK** Runtime
-----------------------------------------

You programmatically configure and interact with the **MoltenVK** runtime through function 
calls, enumeration values, and capabilities, in exactly the same way you do with other
*Vulkan* implementations. **MoltenVK** contains several header files that define access 
to *Vulkan* and **MoltenVK** function calls.

In your application code, you access *Vulkan* features through the API defined in the standard 
`vulkan.h` header file. This file is included in the **MoltenVK** framework, and can be included 
in your source code files as follows:

	#include <vulkan/vulkan.h>

In addition to core *Vulkan* functionality, **MoltenVK**  also supports the following *Vulkan* extensions:

- `VK_KHR_16bit_storage`
- `VK_KHR_8bit_storage`
- `VK_KHR_bind_memory2`
- `VK_KHR_create_renderpass2`
- `VK_KHR_dedicated_allocation`
- `VK_KHR_depth_stencil_resolve`
- `VK_KHR_descriptor_update_template`
- `VK_KHR_device_group`
- `VK_KHR_device_group_creation`
- `VK_KHR_driver_properties`
- `VK_KHR_get_memory_requirements2`
- `VK_KHR_get_physical_device_properties2`
- `VK_KHR_get_surface_capabilities2`
- `VK_KHR_imageless_framebuffer`
- `VK_KHR_image_format_list`
- `VK_KHR_maintenance1`
- `VK_KHR_maintenance2`
- `VK_KHR_maintenance3`
- `VK_KHR_multiview`
- `VK_KHR_portability_subset`
- `VK_KHR_push_descriptor`
- `VK_KHR_relaxed_block_layout`
- `VK_KHR_sampler_mirror_clamp_to_edge` *(requires a Mac GPU or Apple family 7 GPU)*
- `VK_KHR_sampler_ycbcr_conversion`
- `VK_KHR_shader_draw_parameters`
- `VK_KHR_shader_float16_int8`
- `VK_KHR_shader_subgroup_extended_types` *(requires Metal 2.1 on Mac or Metal 2.2 and Apple family 4 on iOS)*
- `VK_KHR_storage_buffer_storage_class`
- `VK_KHR_surface`
- `VK_KHR_swapchain`
- `VK_KHR_swapchain_mutable_format`
- `VK_KHR_timeline_semaphore`
- `VK_KHR_uniform_buffer_standard_layout`
- `VK_KHR_variable_pointers`
- `VK_EXT_debug_marker`
- `VK_EXT_debug_report`
- `VK_EXT_debug_utils`
- `VK_EXT_descriptor_indexing` *(initial release limited to Metal Tier 1: 96/128 textures, 16 samplers)*
- `VK_EXT_fragment_shader_interlock` *(requires Metal 2.0 and Raster Order Groups)*
- `VK_EXT_host_query_reset`
- `VK_EXT_image_robustness`
- `VK_EXT_inline_uniform_block`
- `VK_EXT_memory_budget` *(requires Metal 2.0)*
- `VK_EXT_metal_surface`
- `VK_EXT_post_depth_coverage` *(iOS and macOS, requires family 4 (A11) or better Apple GPU)*
- `VK_EXT_private_data `
- `VK_EXT_robustness2`
- `VK_EXT_scalar_block_layout`
- `VK_EXT_shader_stencil_export` *(requires Mac GPU family 2 or iOS GPU family 5)*
- `VK_EXT_shader_viewport_index_layer`
- `VK_EXT_subgroup_size_control` *(requires Metal 2.1 on Mac or Metal 2.2 and Apple family 4 on iOS)*
- `VK_EXT_swapchain_colorspace`
- `VK_EXT_vertex_attribute_divisor`
- `VK_EXT_texel_buffer_alignment` *(requires Metal 2.0)*
- `VK_EXT_texture_compression_astc_hdr` *(iOS and macOS, requires family 6 (A13) or better Apple GPU)*
- `VK_MVK_ios_surface` *(iOS) (Obsolete. Use `VK_EXT_metal_surface` instead.)*
- `VK_MVK_macos_surface` *(macOS) (Obsolete. Use `VK_EXT_metal_surface` instead.)*
- `VK_MVK_moltenvk`
- `VK_AMD_gpu_shader_half_float`
- `VK_AMD_negative_viewport_height`
- `VK_AMD_shader_image_load_store_lod` *(requires Apple GPU)*
- `VK_AMD_shader_trinary_minmax` *(requires Metal 2.1)*
- `VK_IMG_format_pvrtc` *(requires Apple GPU)*
- `VK_INTEL_shader_integer_functions2`
- `VK_NV_glsl_shader`

In order to visibly display your content on *macOS*, *iOS*, or *tvOS*, you must enable the
`VK_EXT_metal_surface` extension, and use the function defined in that extension to create a 
*Vulkan* rendering surface. You can enable the `VK_EXT_metal_surface` extension by defining the `VK_USE_PLATFORM_METAL_EXT` guard macro in your compiler build settings. See the description of 
the `mvk_vulkan.h` file below for  a convenient way to enable this extension automatically.


<a name="moltenvk_extension"></a>
### MoltenVK `VK_MVK_moltenvk` Extension

The `VK_MVK_moltenvk` *Vulkan* extension provides functionality beyond standard *Vulkan* functionality, 
to support configuration options and behaviour that is specific to the **MoltenVK** implementation of *Vulkan* 
functionality. You can access this functionality by including the `vk_mvk_moltenvk.h` header file in your code. 
The `vk_mvk_moltenvk.h` file also includes the API documentation for this `VK_MVK_moltenvk` extension.

The following API header files are included in the **MoltenVK** package, each of which 
can be included in your application source code as follows:

	#include <MoltenVK/HEADER_FILE>

where `HEADER_FILE` is one of the following:

- `vk_mvk_moltenvk.h` - Contains declarations and documentation for the functions, structures, 
  and enumerations that define the behaviour of the `VK_MVK_moltenvk` *Vulkan* extension.

- `mvk_vulkan.h` - This is a convenience header file that loads the `vulkan.h` header file
   with the appropriate **MoltenVK** *Vulkan* platform surface extension automatically 
   enabled for *macOS*, *iOS*, or *tvOS*. Use this header file in place of the `vulkan.h` 
   header file, where access to a **MoltenVK** platform surface extension is required.
   
   The `mvk_vulkan.h` header file automatically enables the `VK_USE_PLATFORM_METAL_EXT` 
   build setting and `VK_EXT_metal_surface` *Vulkan* extension.
  
- `mvk_datatypes.h` - Contains helpful functions for converting between *Vulkan* and *Metal* data types.
  You do not need to use this functionality to use **MoltenVK**, as **MoltenVK** converts between 
  *Vulkan* and *Metal* datatypes automatically (using the functions declared in this header). 
  These functions are exposed in this header for your own purposes such as interacting with *Metal* 
  directly, or simply logging data values.

>***Note:*** Except for `vkGetMoltenVKConfigurationMVK()` and `vkSetMoltenVKConfigurationMVK()`, 
 the functions in `vk_mvk_moltenvk.h` are not supported by the *Vulkan SDK Loader and Layers*
 framework. The opaque Vulkan objects used by the functions in `vk_mvk_moltenvk.h` (`VkPhysicalDevice`, 
 `VkShaderModule`, `VKImage`, ...), must have been retrieved directly from **MoltenVK**, and not through 
 the *Vulkan SDK Loader and Layers* framework. The *Vulkan SDK Loader and Layers* framework often changes 
 these opaque objects, and passing them from a higher layer directly to **MoltenVK** will result in 
 undefined behaviour.


<a name="moltenvk_config"></a>
### Configuring MoltenVK

The `VK_MVK_moltenvk` *Vulkan* extension provides the ability to configure and optimize 
**MoltenVK** for your particular application runtime requirements.

There are three mechanisms for setting the values of the **MoltenVK** configuration parameters:

- Runtime API via the `vkGetMoltenVKConfigurationMVK()/vkSetMoltenVKConfigurationMVK()` functions.
- Application runtime environment variables.
- Build settings at **MoltenVK** build time.

To change some of the **MoltenVK** configuration settings at runtime using a programmatic API, 
use the `vkGetMoltenVKConfigurationMVK()` and `vkSetMoltenVKConfigurationMVK()` functions to 
retrieve, modify, and set a copy of the `MVKConfiguration` structure.

The initial value of each of the configuration settings can be established at runtime 
by a corresponding environment variable, or if the environment variable is not set, 
by a corresponding build setting at the time **MoltenVK** is compiled. The environment 
variable and build setting for each configuration parameter share the same name.

See the description of the `MVKConfiguration` structure parameters and corresponding environment 
variables in the `vk_mvk_moltenvk.h` file for more info about configuring and optimizing 
**MoltenVK** at runtime or build time.


<a name="shaders"></a>
*Metal Shading Language* Shaders
--------------------------------

*Metal* uses a different shader language than *Vulkan*. *Vulkan* uses the new 
*SPIR-V Shading Language (SPIR-V)*, whereas *Metal* uses the *Metal Shading Language (MSL)*.
**MoltenVK** uses **Runtime Shader Conversion** to automatically convert your *SPIR-V* shaders 
to their *MSL* equivalents, during loading your *SPIR-V* shaders, using the standard *Vulkan* 
`vkCreateShaderModule()` function.


<a name="spv_vs_msl"></a>
### Troubleshooting Shader Conversion

The shader converter technology in **MoltenVK** is quite robust, and most *SPIR-V* shaders 
can be converted to *MSL* without any problems. In the case where a conversion issue arises, 
you can address the issue as follows:

- Errors encountered during **Runtime Shader Conversion** are logged to the console.

- To help understand conversion issues during **Runtime Shader Conversion**, you can enable the 
  logging of the *SPIR-V* and *MSL* shader source code during shader conversion, by turning on 
  the `MVKConfiguration::debugMode` configuration parameter, or setting the value of the `MVK_DEBUG` 
  runtime environment variable to `1`. See the [*MoltenVK Configuration*](#moltenvk_config) 
  description above.

  Enabling debug mode in **MoltenVK** includes shader conversion logging, which causes both 
  the incoming *SPIR-V* code and the converted *MSL* source code to be logged to the console 
  in human-readable form. This allows you to manually verify the conversions, and can help 
  you diagnose issues that might occur during shader conversion.

- For some issues, you may be able to adjust your *SPIR-V* code so that it behaves the same 
  under *Vulkan*, but is easier to automatically convert to *MSL*.

- You are also encouraged to report issues with shader conversion to the 
  [*SPIRV-Cross*](https://github.com/KhronosGroup/SPIRV-Cross/issues) project. **MoltenVK** and 
  **MoltenVKShaderConverter** make use of *SPIRV-Cross* to convert *SPIR-V* shaders to *MSL* shaders. 


<a name="performance"></a>
Performance Considerations
--------------------------

This section discusses various options for improving performance when using **MoltenVK**.


<a name="shader_load_time"></a>
### Shader Loading Time

A number of steps is required to load and compile *SPIR-V* shaders into a form that *Metal* can use. 
Although the overall process is fast, the slowest step involves converting shaders from *SPIR-V* to
*MSL* source code format.

If you have a lot of shaders, you can dramatically improve shader loading time by using the standard
*Vulkan pipeline cache* feature, to serialize shaders and store them in *MSL* form offline.
Loading *MSL* shaders via the pipeline cache serializing mechanism can be significantly faster than 
converting from *SPIR-V* to *MSL* each time.

In *Vulkan*, pipeline cache serialization for offline storage is available through the 
`vkGetPipelineCacheData()` and `vkCreatePipelineCache()` functions. Loading the pipeline cache 
from offline storage at app start-up time can dramatically improve both shader loading performance, 
and performance glitches and hiccups during runtime code if shader loading is performed then.

When using pipeline caching, nothing changes about how you load *SPIR-V* shader code. **MoltenVK** 
automatically detects that the *SPIR-V* was previously converted to *MSL*, and stored offline via 
the *Vulkan* pipeline cache serialization mechanism, and does not invoke the relatively expensive
step of converting the *SPIR-V* to *MSL* again.


<a name="swapchains"></a>
### Swapchains

*Metal* supports a very small number (3) of concurrent swapchain images. In addition, *Metal* can
sometimes hold onto these images during surface presentation.

**MoltenVK** supports using either 2 or 3 swapchain images. For best performance, it is recommended
that you use 3 swapchain images (triple-buffering), to ensure that at least one swapchain image will 
be available when you need to render to it. 

Using 3 swapchain images is particularly important when rendering to a full-screen surface, because 
in that situation, *Metal* uses its *Direct to Display* feature, and avoids compositing the swapchain
image onto a separate composition surface before displaying it. Although *Direct to Display* can improve 
performance throughput, it also means that *Metal* may hold onto each swapchain image a little longer 
than when using an internal compositor, which increases the risk that a swapchain image will not be a
vailable when you request it, resulting in frame delays and visual stuttering.


<a name="xcode_config"></a>
### Xcode Configuration

When a *Metal* app is running from *Xcode*, the default ***Scheme*** settings reduce performance. 
Be sure to follow the instructions for configuring your application's ***Scheme*** within *Xcode*,
found in the  in the [installation](#install) section above.


<a name="trace_tool"></a>
### Metal System Trace Tool

To help you get the best performance from your graphics app, the *Xcode Instruments* profiling tool 
includes the *Metal System Trace* template. This template can be used to provide detailed tracing of the
CPU and GPU behaviour of your application, allowing you unprecedented performance measurement
and tuning capabilities for apps using *Metal*.


<a name="limitations"></a>
Known **MoltenVK** Limitations
------------------------------

This section documents the known limitations in this version of **MoltenVK**.

- On *macOS* versions prior to *macOS 10.15.6*, native host-coherent image device memory is not available.
  Because of this, changes made to `VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` device memory by the CPU 
  or GPU will not be available to the GPU or CPU, respectively, until the memory is flushed  or unmapped by 
  the application. Applications using `vkMapMemory()` with `VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` 
  device memory on *macOS* versions prior to *macOS 10.15.6* must call either `vkUnmapMemory()`, or
  `vkFlushMappedMemoryRanges()` / `vkInvalidateMappedMemoryRanges()` to ensure memory changes are coherent 
  between the CPU and GPU.  This limitation does **_not_** apply to `VKImage` device memory on *macOS* 
  starting with *macOS 10.15.6*, does not apply to `VKImage` device memory on any version of *iOS* or *tvOS*, 
  and does **_not_** apply to `VKBuffer` device memory on any platform.

- Image content in `PVRTC` compressed formats must be loaded directly into a `VkImage` using 
  host-visible memory mapping. Loading via a staging buffer will result in malformed image content. 

- Pipeline statistics query pool using `VK_QUERY_TYPE_PIPELINE_STATISTICS` is not supported.

- Application-controlled memory allocations using `VkAllocationCallbacks` are ignored.

- Since **MoltenVK** is an implementation of *Vulkan* functionality, it does not load 
  *Vulkan Layers* on its own. In order to use *Vulkan Layers*, such as the validation layers, 
  use the *Vulkan Loader and Layers* from the *[Vulkan SDK](https://vulkan.lunarg.com/sdk/home)*. 
  Refer to the *Vulkan SDK [Getting Started](https://vulkan.lunarg.com/doc/sdk/latest/mac/getting_started.html)* 
  document for more info.