问题
I am running into weird performance issues on older device (iPad Mini 2 of year 2014) ever since migrating to MTKView from OpenGLES based views. To summarize, I am rendering live video preview using MTKView and also optionally record videos (like in RosyWriter sample code). My new app version is written in Swift and supports both Metal & OpenGLES. So I have both Open GL and Metal rendering code to compare. I also have a legacy version of app written in Objective C & uses OpenGLES for all preview and rendering. Here are performance issues I see on iPad Mini 2:
When using MTKView, if I debug the app in XCode and zoom video (using AVCaptureDevice zoom API, no Metal code to crop/zoom frames), the frame rate drops significantly (from 30 to 10 fps). If I record video while zooming (which means additional video encoding pipeline in another thread), there are frame drops in the recorded video as well.
When I profile the app using time profiler, MTKView is smooth enough with zoom. However, if I start recording video now , MTKView fps again drops or frames get delayed in the preview, but there is no frame drop issue in recorded video -- the end recorded video is still smooth.
If I select Release version of app for Debug, behavior is same as while profiling,
If I switch back to GLKView instead of MTKView, there are no issues in preview even in Debug version. There are still issues when recording -- preview gets delayed when zooming. But if I profile the app, there are NO delays or frame drops!!!!
Finally, the original legacy version in Objective C that uses OpenGLES for all purposes has no issues at all.
Now the question is what tools in Instruments can I use to nail down the exact issues for frame drops in preview and recording. Can Metal not match performance of OpenGLES on older devices?
来源：https://stackoverflow.com/questions/58362049/mtkview-vs-glkview-performance-on-old-devices