Why is Android(安卓)laggy, while iOS, Windows Phone 7, QNX, and WebOS are fluid?

The Root Cause

It’s not GC pauses. It’s not because Android runs bytecode and iOS runs native code. It’s because on iOS all UI rendering occurs in a dedicated UI thread with real-time priority . On the other hand, Android follows the traditional PC model of rendering occurring on the main thread with normal priority.

This is a not an abstract or academic difference. You can see it for yourself. Grab your closest iPad or iPhone and open Safari. Start loading a complex web page like Facebook. Half way through loading, put your finger on the screen and move it around. All rendering instantly stops. The website will literally never load until you remove your finger. This is because the UI thread is intercepting all events and rendering the UI at real-time priority.

If you repeat this exercise on Android, you’ll notice that the browser will attempt to both animate the page and render the HTML, and do an ‘ok’ job at both. On Android, this a case where an efficient dual core processor really helps, which is why the Galaxy S II is famous for its smoothness.

On iOS when an app is installing from the app store and you put your finger on the screen, the installation instantly pauses until all rendering is finished. Android tries to do both at the same priority, so the frame rate suffers. Once you notice this happening, you’ll see it everywhere on an Android phone. Why is scrolling in the Movies app slow? Because movie cover thumbnails are dynamically added to the movie list as you scroll down, while on iOS they are lazily added after all scrolling stops.

Other Reasons

The fundamental reason Android is laggy is UI rendering threading and priority, but it’s not the only reason. First, hardware acceleration, despite Dianna’s reservations, does help. My Nexus S has never been snappier since upgrading to ICS. Hardware acceleration makes a huge difference in apps like the home screen and Android market. Offloading rendering to the GPU also increases battery life, because GPUs are fixed-function hardware, so they operate at a lower power envelope.

Second, contrary to what I claimed earlier, garbage collection is still a problem, even with the work on concurrent GC in Dalvik. For example, if you’ve ever used the photo gallery app in Honeycomb or ICS you may wonder why the frame rate is low. It turns out the frame rate is capped at 30 FPS because without the cap, swiping through photos proceeds at 60 FPS most of the time, but occasionally a GC pause causes a noticeable “hiccup”. Capping the frame rate at 30 fixes the hiccup problem at the expense of buttery smooth animations at all times.

Third, there are the hardware problems that Dianne discussed. The Tegra 2, despite Nvidia’s grandiose marketing claims, is hurt by low memory bandwidth and no NEON instruction set support (NEON instructions are the ARM equivalent of Intel’s SSE, which allow for faster matrix math on CPUs). Honeycomb tablets would be better off with a different GPU, even if it was theoretically less powerful in some respects than the Tegra 2. For example, the Samsung Hummingbird in the Nexus S or Apple A4. It’s telling that the fastest released Honeycomb tablet, the Galaxy Tab 7.7, is running the Exynos CPU from the Galaxy S II.

Fourth, Android has a ways to go toward more efficient UI compositing. On iOS, each UI view is rendered separately and stored in memory, so many animations only require the GPU to recomposite UI views. GPUs are extremely good at this. Unfortunately, on Android, the UI hierarchy is flattened before rendering, so animations require every animating section of the screen to be redrawn.

Fifth, the Dalvik VM is not as mature as a desktop class JVM. Java is notorious for terrible GUI performance on desktop. However, many of the issues don’t carry over to the Dalvik implementation. Swing was terrible because it was a cross platform layer on top of native APIs. It is interesting to note that Windows Phone 7’s core UI is built in native code, even though the original plan was to base it entirely on Silverlight. Microsoft ultimately decided that to get the kind of UI performance required, the code would have to be native. It’s easy to see the difference between native and bytecode on Windows Phone 7, because third party apps are written in Silverlight and have inferior performance (NoDo and Mango have alleviated this problem and the Silverlight UIs are generally very smooth now).

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