Maximizing iPhone App Performance by Michael Jurewitz (jurewitz@apple.com)

Tuning for Performance

• iPhone and iPod Touch cannot even be compared to a Mac in terms of hardware
• Instruments is the best tool for memory analysis
• Shark can be used as well
• Choose the right tuning environment
  • Always check for graphics performance on the device

Drawing and Scrolling

• Use built-in drawing whenever possible

  • UIKit views are already optimized
  • Take advantage of animatable view properties

• Custom drawing

  • Mark views as opaque
  • Compositing is one of the most expensive processes on the device
  • Draw minimally
    • Call setNeedsDisplayInRect
    • Check the rectangle passed to drawRect

• Cache static drawing objects

  • Be prepared to release them in low memory conditions

• Invalidate timers before creating new timers

  • When an application gets suspended, invalidate your timers

• Use PNG files

  • Optimized at build time
  • Converted from unmultiplied RGBA to premultiplied BGRA
  • Unnecessary data removed
  • Compression rate may be increased

• Improving scrolling performance

  • Use opaque views whenever possible
  • Avoid allocating views during scrolling
    • Allocating views is very expensive
  • Reuse table cells
    • Tag cells with reuse ID’s
    • Dequeue reusable cells
  • Collapse cell view hierarchy
    • Create a single view that does all drawing and layout

Application Launch

• Design apps for fast launch, short use
  • Consider the user’s immediate needs
  • Make your app launch and quit REALLY fast
  • Get your UI on screen as quickly as possible
    • Don’t perform network operations in applicationDidFinishLaunching
    • Use a small initial NIB file
    • Don’t load images that you won’t use
    • Make your UI responsive as soon as possible

Memory Usage

• Overview: 70MB already allocated before your app launches

  • 12MB for graphics
  • 32MB for kernel (wired)
  • 12MB for daemons
  • 10MB for springboard
  • 4MB for phone

• Virtual memory system

  • No swap file
  • Writable pages stay in memory
  • Read-only pages can be evicted and reloaded
  • No garbage collection
  • Standard reference counting is the only way

• Static memory

  • Thumb instruction set (works on all devices)
    • No floating point access
  • Thumb-2 instruction set (only on 3GS)
    • Has floating point access (much better for games)
  • Don’t use unnecessary compiler options
    • Avoid C++ exceptions
    • Avoid Runtime Type Identification (RTTI)

• Dynamic memory

  • Avoid autoreleasing objects
    • Autorelease can be expensive
    • Use release whenever possible
  • Use direct allocation instead of convenience methods
  • Cache the right objects
    • Cache frequently used objects
    • Be prepared to release them when there is a low memory warning
  • Declare properties as nonatomic
    • Access to nonatomic objects is up to 10 times faster (vs atomic)
    • However, nonatomic objects are NOT thread safe
  • Image and layer objects are small by themselves, but backing store is usually quite large
    • Use appropriately sized images (don’t scale unless you have to)
    • Images are decompressed in memory to display them (4 bits per px)
      • 320px x 480px x 48/px = 614.4KB
  • Use UIImage imageNamed for small, frequently used images
  • UIImage initWithData retains the NSData, on a memory warning, release the NSData if you can recreate it
  • Use NSData initWithContentOfMappedFile for large files
  • UIImage initWithContentsOfFile only retains the path, and doesn’t load it until it’s needed for display. It doesn’t cache any data.

• Low Memory Warnings

  • One foreground app, many processes
  • Each process consumes memory
  • Background processes are killed before the frontmost app is exited
  • Any app may create low memory conditions
  • Expect memory warnings
  • You MUST respond to low memory warnings
    • Release any objects that can be reconstructed
    • Implement applicationDidReceiveMemoryWarning in your app delegate
      • Create accessor method to return the object
      • Release the property, then set it to nil
    • Implement didReceiveMemoryWarning in your view controller and viewDidUnload in your views
      • Release references to retained outlets
    • Listen to memory warning notifications through the app delegate

• Autorelease pools

  • Use nested autorelease pools around loops to make sure that the objects created during the loop are all drained when the process is complete

Files and Data

• Use memory mapping for large files

  • Avoid loading large files into memory

• For large data sets, use Core Data

  • Doesn’t keep entire data set in memory

• Don’t include unnecessary files

• Data in ~/Documents is backed up, don’t use this for caches

  • Use ~/Library/Caches for data that can be reacquired

• plists are converted to compact binary format by Xcode

  • Use plists for structured data files when bundling data (instead of XML)

Power and Battery Life

• Radios: 3G

  • Very expensive to send data
  • 3G networks require phones to stay in high power state for a few seconds after lat packet is sent or received
  • Coalesce data into large chunks, rather than a thin stream
  • Minimize amount of data transmitted, use compact data formats

• Radios: Wi-Fi

  • Uses less power than 3G
  • Detect when you are on Wi-Fi rather than on cell

• Core Location

  • Use least amount of accuracy necessary, default is the most expensive
    • If you only care about where the user is in general, ask for a wider location area
  • Set the distance filter property appropriately
  • Turn it off when you are done

• CPU/GPU

  • Improving performance results in better battery life (fast code = less power)
  • iPhone OS is event based, DO NOT poll
  • Subscribe to events whenever possible
  • If you must poll, use a timer with a low frequency
  • Consolidate CPU usage into short bursts
    • Allows CPU to enter idle state
    • Use Instruments: CPU Sampler to check CPU activity level
  • When using OpenGL ES
    • Pick a fixed frame rate
    • If frame hasn’t changed, don’t redraw

Download this file