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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  • tipoo - Monday, October 31, 2011 - link

    Anyone know if there is a reason this hasn't made it into any Andriod phone yet? Does Google specify compatible GPU's, or is it cost, or development time, etc? Looks like it slaughters even the Mali 400 which is probably the next fastest. Reply
  • zorxd - Monday, October 31, 2011 - link

    The only reason is that no one used it yet. The TI OMAP 4470 will use the 544 which is probably a little faster.
    The SGS2 is using the slower Mali 400, however it was released 6 months ago. Yet it's not that bad, even beating the 4S in Glbenchmark pro.
    Reply
  • zorxd - Monday, October 31, 2011 - link

    I meant no SoC vendor is using it. Reply
  • djboxbaba - Monday, October 31, 2011 - link

    The numbers were incorrect and have been updated, the 4S is ~2x faster than the GS2 on the GLBenchmark Pro. Reply
  • freezer - Thursday, November 03, 2011 - link

    But not when running at phone's native resolution. Thats what people will use while running games on their phone.

    iPhone 4S has much more pixels for GPU to draw while having much smaller screen. Not very optimal for gaming right?

    http://glbenchmark.com/result.jsp?benchmark=glpro2...
    Reply
  • djboxbaba - Thursday, November 03, 2011 - link

    Correct, but we're comparing the GPU's by standardizing the resolution. Of course in the native resolution this will change. Reply
  • thunng8 - Monday, October 31, 2011 - link

    I don't see any GL benchmark that the Mail 400 beats the 4S??? Reply
  • freezer - Thursday, November 03, 2011 - link

    That's because Anandtech review shows only the 720p offscreen results.

    This gives very different numbers compared to running GL Benchmark Pro in phone's native resolution.

    iPhone 4S has about 60% more pixels than Galaxy S2, and so its GPU has to draw much more pixels in every frame.

    Go to glbenchmark.com and dig database yourself.
    Reply
  • Ryan Smith - Monday, October 31, 2011 - link

    The 544 should be identical to the 543 at the same clock and core configuration. It's effectively a 543 variant with full D3D feature level 9_3 support. The primary purpose of the 544 will be to build Windows devices, whereas for non-Windows devices the 543 would suffice. We don't have access to PowerVR's pricing, but it likely costs more due to the need to license additional technologies (e.g. DXTC) to achieve full 9_3 support. Reply
  • Penti - Tuesday, November 01, 2011 - link

    Who will use it to support Windows Phone though? Qualcomm uses their own AMD/ATi based Adreno GPU. I guess it will be TI's attempt off getting Microsoft to support Windows Phone on their SoC in order to supply say partners of theirs like Nokia. Or might just be a later purchase/contract date for the other SoC vendors. Getting the IP-blocks later, but many did opt for the Mali-400 so why wouldn't they opt for the successor too? It seems to have worked out good. Samsung is just one of the vendors that usually did use PowerVR. I guess ST-E will use it in order to support Windows Phone on Nova A9540 SoC too. While Android vendors might opt for the older A9500 still.

    Interesting to see how Nvidia do lag in this field though.
    Reply

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