Windows Phone Performance – Part 7 of many: Lumia 820 performance

In this post, I’ll take a look at the performance of the Nokia Lumia 820 from a developer’s point of view. This post is part of my Windows Phone performance series.

HUGE thanks to Alex Sorokoletov for performing the tests in this article. Since I didn’t have access to a Lumia 820, without him, this post would not have been possible.

The Nokia Lumia 820

The little brother of the flagship Nokia Lumia 920, the 820 has nothing to be ashamed of when it comes to performance. Actually, the 820 has the same dual-core Snapdragon S4 processor as the 920, running at the same 1.5GHz speed. From a developer’s perspective, the big difference is the screen – while the 920 has 1280x768 pixels, the 820 keeps the modest 800x480 pixel resolution of Windows Phone 7. Most of the differences between the 920 and the 820 have to do with how the 920 has to work with 2.5 as much pixels as the 820.

It is worth noting that the 820 only has minimum requirement for 800x480 Windows Phone devices is 512MByte RAM, while the 920 has twice as much. This also boils down to the difference in screen pixels – apps on the the 920 need more RAM simply to decode the high resolution bitmaps and keep them in memory.
Update: the Lumia 820 has 1G RAM

Let’s see how the 820 performs in our little tests!

Application Loading / CPU performance

For this one, I am using the old artificial test from the second part of this series (It’s Full of Stars!).

With Compile in the Cloud being a general Windows Phone 8 feature, the 820 should start applications just as fast as the 920. The first two launches show a bit different picture though, the reasons of which is not entirely clear. Alex performed multiple measurements for each pass, but the 100 star test consistently returned a slower startup time. Regardless of this difference, you can see that the 820 performed roughly as well as the 920 did (almost completely hiding the 920’s line in the chart), even though the later one had to render much more physical pixels.

GPU Performance

To measure the raw CPU performance, once again I used the “Fill Rate Test Sample” from the MSDN article Performance Considerations in Applications for Windows Phone. Here are the numbers in table and chart format:

The Lumia 820 is clearly the top performer here – no wonder, since it has much less pixels to work with than the 920. The 820 keeps a solid 60 FPS up until a Fill Rate of 12, and still doesn’t drop below an acceptable 30 with a Fill Rate of 150. This really shows how the Snapdragon S4’s GPU may even be a bit too powerful for the phone (at least when it comes to Silverlight applications). It can compose 2.5 – 3 times as many full-screen layers as on the 920 before the first signs of lags even show. Not surprisingly, this is the same multiplier as the number of pixels there are between the two phones. Fill Rate is about number of pixels after all.

Summary

In CPU, the two Windows Phone 8 Lumia phones perform similarly. However, when it comes to the GPU, the 820 feels like a race car engine in a common vehicle.

When developing Windows Phone 8 applications and testing on the Lumia 820, you have to keep in mind that high-resolution phones do not tolerate Fill Rates above 4.5 or so. Even though your applications may keep a consistently smooth animation at a Fill Rate of 6 on the 820, other WP phones will struggle. So, keep your Frame Rate Counters turned on. Fortunately, it is not hard to keep the Fill Rate below 5 – we are in a much better situation than what we had with the first generation Windows Phones, where the a value of 2.5 already caused problems, and a lot of time went into making sure we kept the Fill Rate below that level.