The A5 Architecture & CPU Performance

The original iPhone debuted with a single 412MHz ARM11 core built on a 90nm process. The 3G improved network performance in 2008 but left the SoC untouched. It wasn't until the iPhone 3GS in 2009 that the SoC got a major performance and power update. Apple moved to a 65nm process node, a brand new ARM Cortex A8 based SoC and an upgraded GPU. The A4 released in 2010 once again gave us a process shrink but kept the architecture unchanged.


Apple's A5 SoC

Apple's A5, first introduced with the iPad 2, keeps process technology the same while introducing a brand new CPU and GPU. The A5 integrates two ARM Cortex A9 cores onto a single die. The improvement over the A4 is tremendous. At the single core level, Apple shortened the integer pipeline without reducing clock speed. With a shorter pipeline the A5 gets more done per clock, and without decreasing clock speed the A5 inherently achieves better performance at the same clock. The move to the Cortex A9 also enables out-of-order instruction execution, further improving architectural efficiency. I've heard there's a 20% increase in performance per clock vs. the Cortex A8, but combine that with the fact that you get two A9s vs a single A8 in last year's design and you get a pretty big performance increase.

There are several situations where the A5's two cores deliver a tangible performance benefit over the A4's single core. Like Android, iOS appears to be pretty well threaded. Individual apps and tasks can take advantage of the second core without a recompile or version update. The most obvious example is web browsing.

Mobile Safari is well threaded. Javascript rendering can be parallelized as well as parts of the HTML parsing/rendering process. All of the major Javascript performance benchmarks show a 60 - 70% increase in performance over the A4, which is partially due to the availability of the second core:

SunSpider Javascript Benchmark 0.9.1 - Stock Browser

Rightware BrowserMark

This translates directly into faster page load times. As you can see in the video below, the iPhone 4S (left) loads the AnandTech front page over WiFi in about 5 seconds compared to 9 seconds on the iPhone 4 (right). That's really the best case scenario, the improvement in the next page load time was only about a second (7s vs 8s).

Typical improvements in load time fall in the 10 - 70% range, contributing significantly to the phone feeling snappier than its predecessor. To quantify the improvement I ran through our standard web page loading suite, a test that hits AnandTech, CNN, NYTimes, Engadget, Amazon, Digg and Reddit hosted locally on our lab's network. The average page load time over WiFi for all of the pages is below:

AnandTech Web Page Loading Performance Suite

While web page rendering is a natural fit for multiple cores, I was surprised by how poorly threaded some apps ended up being. For example, although I did see performance improvements in exporting edited videos from the iOS version of iMovie, the gains weren't always evident. A quick profile of the app revealed that much of the export process is still single threaded. Just as we've seen on the desktop, there will be some added work necessary to get all apps to utilize multiple cores on iOS.

It's not always performance within an app that saw improvement with the A5: application install and launch times are also much quicker on the 4S. The time to launch Epic's iOS Citadel demo went from 32 seconds on the iPhone 4 to 22 seconds on the 4S. While the iPhone 4 may feel fast enough for many users, the 4S is noticeably faster.

Photos App - Auto Enhance Image

Much of the faster feel comes from by shaving off of seconds here or there. For example, I noticed apps like Messages pop up just slightly quicker on the 4S and you'll see your listing of messages a hair faster than you would on the 4. In the video above you get a brief idea of the sort of subtle improvements I'm talking about. YouTube launches a fraction of a second quicker on the 4S vs the 4.

These subtle decreases in response time are simply icing on the cake. The move from a 4 to a 4S is one of those upgrades that you'll notice right off the bat but will really appreciate if you go back to an iPhone 4 and try to use it. If you do a lot of web browsing on your phone, you'll appreciate the 4S.

I wasn't entirely sure whether or not I could attribute all of these performance improvements to the faster CPU. It's possible that some of the tests I mentioned are IO bound and Apple could have used faster NAND in the 4S. To find out I rounded up a bunch of iPhone 4Ses at all available capacities and measured sequential write speed:

NAND Performance

Apple uses multiple sources for NAND so it's possible that you'll see these numbers move around a bit depending on your particular phone. It looks like the iPhone 4S' NAND is no faster than what Apple shipped last year - at least in sequential write speed. The target appears to be 20MB/s and Apple does its best to stay around there. My iPhone 4 is actually pretty quick at 22MB/s but the advantage isn't significant enough to make a huge deal about. I don't have a good way of measuring random IO performance yet but application launch time is largely governed by sequential IO so I don't expect we're seeing gains from anything outside of the CPU and memory bandwidth in the earlier tests.

Faster Throughput on WCDMA The Memory Interface
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  • metafor - Tuesday, November 01, 2011 - link

    Fair enough. But that really doesn't take away from the fact that the A5 is a relatively large chip and from the UV-scans of it, looks to use quite a bit of that die area for the GPU.

    I don't know if a similar scan has been done of Exynos but one can't safely say both chips are far bigger than SoC's traditionally used in this space.

    Though that trend appears to be moving forward with MSM8960 and Tegra 3.....
    Reply
  • PeteH - Tuesday, November 01, 2011 - link

    That leads to an interesting question: will Apple always have the largest SoCs, and thus (most likely) the highest performance in the mobile space?

    The reason I could see this happening is that Apple doesn't have to sell their SoC's at a profit, so they're paying closer to cost for the chips (excluding the fab mark up). Other manufacturers (like NVIDIA) need to make a profit on their chips.
    Reply
  • name99 - Thursday, November 03, 2011 - link

    "I'm not entirely sure why they had to use such a powerful GPU, though. "

    And you know EXACTLY how Apple use the GPU do you?
    Does Siri run some of its workload on the GPU? Does the faster camera stuff (eg fast HDR) run on the GPU? Does Apple already have OpenCL running (for internal use) on iOS?
    Reply
  • doobydoo - Friday, December 02, 2011 - link

    He must be an Android fan.

    Androids new marketing campaign will offer a revolutionary 'new' feature - the ability to have a slower GPU than other phones!!!

    Magical.
    Reply
  • InternetGeek - Monday, October 31, 2011 - link

    They might give AMD and nVidia a run for their money if they ever tried creating desktop products... Reply
  • sprockkets - Monday, October 31, 2011 - link

    Kyro 2 was a good chip, but obviously went to focus on the desktop market. Reply
  • tipoo - Tuesday, November 01, 2011 - link

    Maybe, but there's a reason such crossovers usually take so long. Look at Intel trying to get into this space, I don't doubt they will be good at it but it takes years of development. Imagination specializes in low power, it would take lots of development effort to get into the high power desktop game. Reply
  • _tangent - Tuesday, November 01, 2011 - link

    I think this might be intentionally ironic given they got out of that game a long time ago :P

    On point though, anyone would given AMD and nVidia a run for their money with the right up front cash and expertise. I imagine the barrier to entry into that market is truly colossal though. Point is, the SGX543 MP2 is no evidence one way or the other.
    Reply
  • lurker22 - Monday, October 31, 2011 - link

    Before buying many people who got a 4s on AT&T told me how much better it was than their prior AT&T iphones.

    Anand, thanks for confirming and explaining the reasons.
    Reply
  • LordSojar - Monday, October 31, 2011 - link

    Can't we have reviews as detailed as this for the really big name Android phones? They are always far less detailed and lack a lot of the testing put into this.... thing....

    Apple makes a few adjustments, tweaks a few things, adds in the same processor that's in the iPad 2, and we have a highly detailed, scientific review that covers every single aspect, even if said aspects are the same. Samsung releases a new phone that has overall better features, faster CPU, faster NAND, a different and arguably better (or at least equal) screen, and mums the word?

    The bias is getting a bit out of hand at this point... We get that you're big time Apple fans, but cmon... At least do a major review of this caliber for the Droid RAZR and the Samsung Galaxy Nexus and the Galaxy S2 Skyrocket (LTE on AT&T!). Even if you combine them into one review, just make it THIS detailed for once instead of giving Apple the huge, super detailed ultra review!
    Reply

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