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

You’ll be able to 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.

Fashionable 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, operating 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 maintain 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 so as to velocity 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 creating sense of all of the efficiency counter information. To do that, you employ the Metallic System Hint and Metallic Debugger built-in to Xcode and Devices.

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

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

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.

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

You should utilize 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 bear in mind that System Stage Caches may 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 implies that switch is slowing down your rendering. To alleviate these bottlenecks there are a number of issues you are able to do.

One option 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 means 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 dimension.

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 presently executing out of the full thread pool. 100% occupancy means a given GPU is presently maxed out by way of the variety of threads and general work it could actually 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 Elimination often happens someplace in the course of every render go earlier than fragment processing – shortly after the Tiled Vertex Buffer is distributed to the GPU to be rasterized.

Depth buffers and hidden floor removing are used to get rid of 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 must be drawn.

For instance, surfaces on the backsides of opaque 3D objects do not must 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 removing 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 may also be used to reduce mixing, which additionally incurs a efficiency price.

To optimize drawing with hidden floor removing, you will need to draw objects by order of visibility state – specifically testing whether or not or not objects are opaque, testing by translucency, and making an attempt to keep away from interleaving opaque and non-opaque meshes.

To get began with Metallic optimization, make sure to 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 positively need to spend a number of 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’ll be able to’t get a detail-level view of what is really occurring 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 enormous and sophisticated topic – we have simply barely gotten began and can additional discover this subject in future articles.