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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  • dacramer - Tuesday, November 01, 2011 - link

    The browser benchmarks show the 4s as fastest, but this benchmark is FLAWED as a measurement of CPU performance.

    The benchmark compares Safari to the Android stock browser. The stock Android browser does not support 2 cores, only 1. So all Android dual cores phones would be nearly twice as fast with another browser such as Opera.

    I'm surprised Anand did not include this discussion in the review. He seams fixated on the idea that stock to stock browser is a valuable comparison when it totally misleads readers on CPU performance.
    Reply
  • Lucian Armasu - Wednesday, November 02, 2011 - link

    In case you missed it, the Droid RAZR scored 10% faster than iPhone 4S in the SunSpider test.

    But yes, this is still mostly a software issue, and I expect the Android 4.0 browser to be even faster on the same chip.
    Reply
  • thunng8 - Wednesday, November 02, 2011 - link

    FYI, Google says the Galaxy Nexus scores 1983ms in Sunspider running ICS Reply
  • doobydoo - Wednesday, November 02, 2011 - link

    "Where did it show faster CPU?"

    Where did it in my comment?
    Reply
  • Breit - Thursday, November 03, 2011 - link

    I just run SunSpider 0.9.1 on my Galaxy S2, international model (everything stock, with Android 2.3.5) and it came in at 1626.5ms with Opera Mobile browser and 3354.8ms with the stock Android browser. So saying the CPU on the Galaxy S2 is weaker than the CPU on the 4S is just not right (and that by a big margin!).
    At least make comparisons with the best software there is and not just 'some' browser you eventually found on your phone...
    I'll bet there is someone programming his own browser just for the kicks or as an exercise for his programming skills and that browser scores over 9000ms in SunSpider. Does that mean the CPU in that phone is slow? ;)
    Reply
  • doobydoo - Friday, December 02, 2011 - link

    You could achieve the same results on the iPhone too..

    The comparison is (rightly) comparing stock browser to stock browser, and the iPhone 4S is faster. It would be even faster again using say, Opera.

    As for some guy posting 9000ms - you can filter the results to 'best' times so this is a non issue.
    Reply
  • Tetracycloide - Tuesday, November 01, 2011 - link

    Isn't there more to displays than just white level in nits, black level in nits (on a chart were I can't seem to find the Galaxy S II or the Infuse), and contrast (again, don't see the Samsung Galaxy S II or the Infuse on the contrast chart either)? Don't get me wrong, I don't agree in the slightest with the summary "this is apple cheerleading" but honestly the display comparisons are pretty lacking IMO. Where are the rest of them? There's more to a screen than just nits. Reply
  • medi01 - Thursday, November 03, 2011 - link

    Last time iphone disappeared from comparison photo, from the one where it would pale in comparison, when asked, author "explained' that he simply "forgot" it in the pocket. (!!!). Now they didn't even bother. Round of but kissing on "retina" hype and phones disappearing from charts.

    Find contrast of Galaxy phones on the charts pretty please...
    Reply
  • steven75 - Friday, November 11, 2011 - link

    Could you be any more of an Fandroid? Seriously. Reply
  • Stuka87 - Tuesday, November 01, 2011 - link

    Wait what?

    Almost every single page has a comparison to Android phones. Several of them have graphs showing the differences!

    And your comment on the displays may be your opinion, but actual measurements show otherwise. but hey, if you like your over saturated display with a much lower pixel density, thats your decision.
    Reply

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