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iOS Scroll Performance

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Kyle Sherman
Kyle Sherman

This presentation by Kyle Sherman, LinkedIn iOS Developer for the SlideShare iOS app, goes over fixing issues with jittery scroll performance in iOS applications. The presentation goes over the basics of using Instruments to measure and fix problems, tips for using Instruments, and a concrete example from the new LinkedIn iOS flagship application.

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iOS Scroll Performance Kyle Sherman LinkedIn iOS Developer
Don’t just blame a technology (i.e. Autolayout)
Measure!
Step 1: Use Instruments to measure Scroll Performance
Rule #1: Always profile on device!
Setting Up Instruments • Hit Command-I to Profile • Or Product -> Profile • Or Hold down Play button; select Profile • After done building (long time), select Core Animation • Turn on Device Utilization %
Setting Up Instruments • Hit Command-I to Profile • Or Product -> Profile • Or Hold down Play button; select Profile • After done building (long time), select Core Animation • Turn on Device Utilization %
Setting Up Instruments • Hit Command-I to Profile • Or Product -> Profile • Or Hold down Play button; select Profile • After done building (long time), select Core Animation • Turn on Device Utilization %
Profiling • Tip #1: Decrease sample rate • Hit the Record Button • Tip #2: Set flags using Command-Down arrow • Tip #3: Double click flag to name it
Profiling • Tip #1: Decrease sample rate • Hit the Record Button • Tip #2: Set flags using Command-Down arrow • Tip #3: Double click flag to name it
Profiling • Tip #1: Decrease sample rate • Hit the Record Button • Tip #2: Set flags using Command-Down arrow • Tip #3: Double click flag to name it
Profiling • Tip #1: Decrease sample rate • Hit the Record Button • Tip #2: Set flags using Command-Down arrow • Tip #3: Single click flag to name it
Step 2: Analyze results
Examine Frame Rate • Examine the frame rate • Look at the CPU and GPU usage at and around large frame rate drops
Examine Frame Rate • If Render/Device utilization is high, requires more investigation (details later) • If not, and CPU usage is high: • Click Time Profiler so CPU usage is highlighted • Drag around peak • Look at heaviest stack trace
Step 3: Fix the problem
GPU Issues • GPU Issues require further diagnosis • Easiest way is to use Simulator • Offscreen-Rendered is worst performance hit (details to come) • Watch WWDC 2014 Session 419: Advanced Graphics and Animations for iOS Apps
Step 4: Measure Again!
Step 5: Repeat Steps 2, 3, & 4 until satisfied
Which brings us to our concrete example…
Performant Circular Images Concrete Example:
Applying the skills learned from the previous part, we diagnose the issue(s)
Problem Extremely Choppy Scrolling in this view
Analyzing The Data • Both the GPU utilization was high and CPU was spiking • I decided to investigate CPU usage first, because it was possibly simpler to understand • Saw suspicious methods and fixed issues in code
Results Frame rate increased very slightly
Re-measure!
GPU Issues • Now it was time to resolve the GPU issues • After watching WWDC 2014 Session 419, learned about rendering pipeline and how bad offscreen rendering is • The following is a screenshot with Color Offscreen-Rendered turned on
Color Offscreen-Rendered On
Analysis • Every single circle is offscreen-rendered • There are tons of circles on the screen • Need a more performant way to make the circle
Test Hypothesis Turn off circle creation
Yep! After remeasuring, scrolling is now smooth. And look at Offscreen- Rendered now!
Solutions
Solution #1 • In the WWDC session, Apple suggests creating a mask like the figure to the left of the canvas • Create the mask by drawing a circle inside of a square of the same size of the image • Only fill outside of the circle • The fill color of the mask must be the same as the view (white in this case) • Place this mask on top of the image (as a subview) Image
Doesn’t work in this case Reason: Circles overlap
Solution #2 Retrieve circular images from API
Valid solution, but… Adds to the number of images we serve (storage $$)
Solution #3 • Create a circular image mask and apply to the image • Perform on a background thread, so it doesn’t block UI thread
Solution #3 (code) https://gist.github.com/drumnkyle/c2ae34ea2422514c45d5
Solution #3 Tips • Only do it once per image if possible • You can process when image downloads or first time it is used • Add a circular version of the image to your cache
Resources • I highly recommend watching the WWDC video mentioned. Maybe even a few times as it goes into very good detail. • Apple TV tech talks video Tuning your tvOS Apps, also goes over these topics https://developer.apple.com/videos/play/wwdc2014-419/ https://developer.apple.com/videos/play/techtalks-apple-tv/9/

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