Methods to optimize code for Apple’s Metallic graphics framework

Apple GPUs are Tile-Based mostly Deferred Renderers – which implies they use two major passes: tiling, and rendering. The general rendering pipeline is proven under.

You possibly can consider these two phases as one when geometry is calculated and created, and one other when all pixel rendering is processed.

In most fashionable Apple GPU software program, geometry is calculated and damaged down into meshes and polygons, then rendered to a pixel-based picture, one picture per body.

Trendy Apple GPUs have particular subsections in every core that deal with shaders, textures, a pixel backend, and devoted tile reminiscence. Every core makes use of these 4 areas throughout rendering.

Throughout every body render, a number of passes are used, working on a number of GPU cores, with every core processing a number of duties. Normally, the extra cores, the higher the efficiency.

To measure this efficiency, GPU counters are used.

GPU counters hold monitor of every GPU’s load and measure if every does or would not have sufficient work. Additionally they discover efficiency bottlenecks.

Lastly, GPU counters optimize the instructions that take the longest with a view to pace up efficiency.

There are over 100 and fifty varieties of Apple GPU efficiency counters, and protecting all of them is past the scope of this text.

There may be the issue of constructing sense of all of the efficiency counter information. To do that, you utilize the Metallic System Hint and Metallic Debugger built-in to Xcode and Devices.

There are 4 Metallic GPU Counters which embody essential methods to optimize Metallic in your apps and video games. They’re:

1. Efficiency limiters
2. Reminiscence Bandwidth
3. Occupancy
4. Hidden Floor Removing

Efficiency limiters, or limiter counters measure the exercise of a number of GPU subsystems by discovering the work being executed, and discovering stalls that may block or decelerate parallel execution.

Trendy GPUs execute math, reminiscence, and rasterization work in parallel (on the identical time). Efficiency limiters assist establish efficiency bottlenecks that decelerate your code.

You should use Apple’s Devices app to make use of efficiency limiters to optimize your code. There are half a dozen completely different efficiency limiters in Devices.

Apple’s Devices app.

Reminiscence bandwidth GPU counters measure transfers between GPU and system reminiscence. The GPU accesses system reminiscence each time buffers or textures are accessed.

However remember that System Degree Caches will also be triggered, which implies you could sometimes discover small bursts of upper reminiscence throughput than precise DRAM switch speeds. That is regular.

Should you see a reminiscence bandwidth counter with a excessive worth it probably signifies that switch is slowing down your rendering. To alleviate these bottlenecks there are a number of issues you are able to do.

One method to scale back reminiscence bandwidth slowdowns is to cut back the scale of working information units. This speeds issues up as a result of much less information is being transferred from system reminiscence.

One other approach is to solely load information wanted by the present render go, and to solely retailer information wanted by future render passes. This additionally reduces the general information measurement.

You too can use block texture compression (ASTC) to cut back texture asset sizes, and lossless compression for textures generated at runtime.

Occupancy measures what number of threads are at present executing out of the full thread pool. 100% occupancy means a given GPU is at present maxed out when it comes to the variety of threads and general work it might deal with.

The Occupancy GPU counter measures the share of whole thread capability utilized by the GPU. This whole is the sum of the compute, vertex, and fragment occupancy.

Hidden Floor Removing normally happens someplace in the course of every render go earlier than fragment processing – shortly after the Tiled Vertex Buffer is shipped to the GPU to be rasterized.

Depth buffers and hidden floor elimination are used to eradicate any surfaces that aren’t seen to the view’s digital camera within the present scene. This hastens efficiency as a result of these surfaces do not should be drawn.

For instance, surfaces on the backsides of opaque 3D objects do not should be drawn as a result of the digital camera (and the viewer) by no means see them – so there is no level in drawing them.

Surfaces hidden by different 3D objects in entrance of them relative to the digital camera are additionally eliminated.

GPU counters can be utilized throughout hidden floor elimination to seek out the full variety of pixels rasterized, the variety of fragment shaders (really the variety of calls to fragment shaders), and the variety of pixels saved.

GPU counters will also be used to reduce mixing, which additionally incurs a efficiency value.

To optimize drawing with hidden floor elimination, you may wish to draw objects by order of visibility state – specifically testing whether or not or not objects are opaque, testing by translucency, and attempting to keep away from interleaving opaque and non-opaque meshes.

To get began with Metallic optimization, you should definitely take a look at the WWDC movies Optimize Metallic apps and video games with GPU counters from WWDC20, Harness GPUs w Metallic additionally from WWDC20, and Delivering Optimized Metallic Apps + Video games from WWDC19.

Subsequent, learn Capturing a Metallic workload in Xcode and Metallic Debugging Sorts on the Metallic Debugger pages on Apple’s Developer Documentation web site.

There may be additionally Analyzing your Metallic workload within the Metallic Debugger documentation.

You will undoubtedly wish to spend numerous time with Xcode’s Metallic Debugger and Hint documentation to be taught in-depth how the completely different GPU counters and efficiency graphs work. With out these, you may’t get a detail-level view of what is really happening in your Metallic code.

For compressed textures, it is also worthwhile to learn up on Adaptive Scalable Texture Compression (ASTC) and the way it works in fashionable rendering pipelines.

Metallic efficiency optimization is an unlimited and complex topic – we have simply barely gotten began and can additional discover this subject in future articles.