Reducing the memory footprint of Metal apps
Learn best practices for using memory efficiently in iOS and tvOS.
Overview
Efficient memory usage is a critical consideration for resource-intensive Metal apps in iOS and tvOS. Create apps so they use as little memory as possible, making more memory available for other apps and system services.
iOS and tvOS monitor your app’s total memory usage at runtime, and if the amount exceeds a predetermined limit, the system terminates your app. This limit varies by device model, so test your app on all supported devices.
Optimize memory usage in your Metal app
Here are some tips to understand how your Metal app is using memory, and best practices to reduce memory usage.
Measure memory consumption with Xcode 11 and later. Build your app with the iOS or tvOS SDK, then use Xcode Memory Report to observe your app’s total memory footprint during execution. For more information about Xcode Memory Report, see Gathering information about memory use.
Use these tools to gain a deeper understanding of your app’s memory footprint.
Take an Instruments trace and look at the Metal Resource Allocations Instrument, which is part of the Metal System Trace template. For more information on working with the Metal Resource Allocations Instrument, see Delivering Optimized Metal Apps and Games.
Capture a GPU trace with Metal debugger and see a visual representation of all of a frame’s GPU resources and a table of properties for each resource with the Memory view. For detailed information about the Memory view, see Delivering Optimized Metal Apps and Games.
Observe and take action on suggestions highlighted in the Memory insights section of the GPU trace summary. For more information on Memory insights, see Gain insights into your Metal app with Xcode 12.
Use the memory graph tool to help identify leaks and abandoned memory. For more information, see Memory Graph Debugging.
Query the amount of memory available to your app at runtime using the os_proc_available_memory API to help you identify memory spikes.
Make your assets as small as possible. Avoid loading a large resource when a smaller one works. Use compressed texture formats. Load lower-resolution textures when running on memory-constrained devices. Consider reducing the fidelity of 3D models and compressing per-vertex data.
Simplify memory-intensive special effects. Some effects, like shadows or motion blur, require large offscreen buffers for temporary image data. Consider lowering the resolution of these image buffers or applying fewer effects when running on memory-constrained devices.
Avoid loading unused resources. Xcode can help you identify Metal objects that aren’t in use. Use Metal Debugger to get a GPU trace. Then navigate to the Memory view, and select the Unused filter.
Designate resources as volatile when possible. Use MTLPurgeableState.volatile and setPurgeableState(_:) for textures and buffers the operating system can safely discard under low-memory conditions, and be subsequently recreated or reloaded by your app as needed. This design means your app keeps a cache of idle resources in memory but doesn’t count them toward the memory limit.
Use memoryless texture storage for temporary-render targets. MTLStorageMode.memoryless avoids allocating regular system memory, allowing apps to store the contents of temporary render targets directly in tile memory on the GPU.
Use Metal resource heaps. Using MTLHeap, your app can have Multiple Metal resources backed by the same memory allocation. For example, use resource heaps when transient resources are produced and consumed for each frame but aren’t all used together. Those not used together share the same memory, backed by a heap.
Additional resources
The WWDC video Gain insights into your Metal app with Xcode 12 provides additional information about the latest memory-debugging tools and best practices to help you optimize the memory footprint of your Metal apps and games.