When
it comes to performance and power there is no device so widely
misunderstood as the iPhone. The new iPhone 6 (and iPhone 6 Plus) is no
exception - you’d find bashful comments about its comparatively low
clock speed, ‘only’ two CPU cores, low amount of RAM, lack of expandable
storage, and what not in practically every online forum.
Looking
at numbers without fully understanding them, though, is a dangerous
business. This iPhone 6 performance review aims to clear some of the
widespread misunderstandings and give a more detailed overview of the
state of mobile CPUs, and how Apple’s efforts compare to that of the
main rival: the mostly Qualcomm-powered Android fleet.
Apple A8 and ARM's architecture license
When
it comes to the CPU, it’s worth starting off with a quick refresh on
the facts. The overwhelming majority of mobile devices - be it Android,
Windows Phone, or iOS ones - are based on ARM-derived architectures. ARM
offers two types of licenses to its clients: a processor license and an
architecture license.
Most manufacturers use the processor
license that grants them the right to take an ARM-designed core and use
it in their SoC. An example for ARM-designed cores include the
battery-optimized Cortex A7 (and its newer, 64-bit Cortex A53 successor)
and the Cortex A15 (with its newer, Cortex A57 64-bit heir). Phone
makers like Samsung, for instance, take those two cores and combine them
in various big.LITTLE combinations to come with SoCs like the Exynos
5430 in the Galaxy Alpha where the company combines four power-efficient
A53s running at lower clock speeds and four performance-driven A57 that
can go up to higher clocks, but also draw more battery.
The
other type of licensees, those under ARM’s architecture license program,
take a totally different approach by just using the ARM instruction
set, while building their own CPU core. The most prominent companies
that do that are Qualcomm and… Apple. Apple used to operate under an ARM
processor license all the way until the iPhone 4s, but decided to
switch to an architecture license for the iPhone 5, and has building its
own CPU cores ever since then.
The state of 64-bit
Looking
at this timing, you see how this coincides with Apple’s industry-first
introduction of 64-bit chips - the first 64-bit phone, the iPhone 5s,
arrived two years after Apple introduced its first processor, and Apple
has clearly used this time slot to outpace the industry. To this day,
Apple remains uniquely positioned in the transition to 64-bit on mobile -
all first-party apps were 64-bit-ready on iOS 7 launch date, and the
company has given developers an ample timeline and great tools to
optimize their app quickly and effortlessly to 64-bit. With extremely
low levels of fragmentation in Apple’s ecosystem (where by fragmentation
we mean that iOS adoption rates are high and happen in days, while on
Android transitions span months, if not years), the company is one year
away from having a lineup consisting of 64-bit devices only. This will
happen next year when the Apple iPhone 5 is expected to go out of
production, and the 64-bit iPhone 5s with Apple A7 (or as speculated, a
plastic derivative of the 5s with similar hardware) takes the lowest
place in Apple’s ecosystem.
Looking over to the Android camp,
we’re seeing that the platform lags behind a full year and more. To this
date, in late 2014, the biggest Android vendors like Samsung, HTC, LG,
and others, are all releasing their flagships with 32-bit chips like the
Snapdragon 805 and Snapdragon 801. Both those chips are based on the
now 3-year old Krait core (with some tweaks, of course), and later on in
this article you’d be able to spot the difference in compute power.
Naturally, using the 32-bit 805 translates into those flagships not
being able to benefit from ART optimizations in Android L.
The
earliest this could (and likely would) change is in spring of 2015 when
the first wave of Android flagships for next year is expected to arrive.
Some (and hopefully most) of those devices are said to feature the
Snapdragon 810, Qualcomm’s first top-level 64-bit SoC. In just over a
year time, Qualcomm has overhauled its portfolio to consist of 64-bit
chips on practically all levels, from the low to the high-end. However,
the Snapdragon 810 does not ship with a custom Qualcomm core (such a
core would likely take more time for development) - instead, the company
goes back to using an ARM processor license and equips the 810 with a
big.LITTLE setup with four low-power Cortex A53 and four
performance-driven Cortex A57 cores.
Given the long period of
time it takes for the Android install base to switch to an ART-enabled
version of the platform in meaningful numbers (let’s keep in mind that
we don’t have a minimum target for ART, and chances are that it won’t be
KitKat, but Android L), it is clear that Android is in a much less
favorable position in terms of 64-bit-readiness.
Apple A8 die break-down
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Both TSMC and Samsung are said to be making the A8 in a 40-60 ratio
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Being
as secretive as Apple is (the company does not disclose processor
details in the way Intel does) hides a little joy for us, tech
reviewers, to try and reverse-engineer its efforts.
We’re not
completely in the dark, though: in the past two release cycles, Apple
has been disclosing the number of transistors in the Apple A8: there’s
now a whopping 2 billion of them, double the number from the A7. As far
as we can tell, this is the most ever in a smartphone chip - in
comparison, some estimates claim that the Snapdragon 805 chip features
700 million transistors.
From here on, the journey towards a
better understanding of the Apple A8 starts with a teardown of the
iPhone 6 and images of the A8 die from Chipworks. Those images give us a
detailed breakdown of the Apple A8 die and the location of its various
components.
Despite (or rather because of) the doubling of
transistor count, the die size has grown smaller and comes in at 89mm in
the A8, down from 102mm in the A7. Apple has switched the places of
components on the die, and the CPU is now on the bottom left (it was on
the bottom right), with a large block of L3 cache above it. Despite a
20% decrease in the size of the SRAM block (cells have shrunk in third
from 0.12µm to 0.08µm), it’s likely that more advanced circuitry makes
up for the difference and we’re still dealing with 4MB of L3 cache
memory. At the time of this writing, we have seen the first benchmarks
showing that memory latency has indeed improved by a hefty 20ns when we
go out to L2 $ and further.
The most drastic change in size,
however, seems to be in the CPU die size: the new CPU measures 12.2mm,
nearly 30% smaller than the 17.1mm CPU die in the Apple A7. By all
visible clues, the rest of the architecture remains the same: we have
64KB/64KB of L1 instruction/data $ (L1 is the fastest cache, located on
the CPU die), and a 1MB block of L2 cache shared between the cores.
Apple
has provided a few important details about the CPU performance of its
new A8: first, the company says the new CPU comes a 25% performance
improvement, and illustrates this with a chart showing generational
improvement all the way since the 2G iPhone (the 25% number is derived
by comparing the iPhone 5s’s 40x CPU overhead over the 2G iPhone and the
50x peek in the iPhone 6).
On clock speeds and deceptive marketing
With a modest
boost in CPU clock speeds from 1.3GHz to 1.4GHz (an 8% speed-up), the
25% improvement obviously comes from various other tweaks and tricks.
Before diving deeper in benchmarks, though, here is the place for a
quick insert about clock speeds and the state of the industry.
Commentators in forums are quick to point out the apparent inferiority
of Apple clock speeds in comparison to the much faster speeds declared
in rival Snapdragon and Exynos chips, for instance. The most up-to-date
example is the Snapdragon 805 with a declared clock speed of ‘up to
2.7GHz’. At first sight, Apple’s Cyclone core looks like a sore loser
with its declaration for just half that at 1.4GHz.
Most people
would call it a day at this point - the Snapdragon outperforms the A8
hugely, case closed. This, however, would be naïve: running real-world
applications and games shows instantly that the 2.7GHz speeds can only
be achieved for a very short periods of time, but after those short
outbursts, the chip quickly throttles back to the much more sane
~1.3GHz. Put simply, the 2.7GHz number that you read about is not the
nominal frequency, but maxed out turbo speeds that are not sustainable
for the long term. In fact, Apple is being much more truthful as it
declares actual nominal (and not turbo) speeds for its chip, plus, the
company goes on to disclose a second big thing about its chip: sustained
performance times. Apple actually claims its A8 is capable of running
flat at its nominal speeds for (at least) 20 minutes.
This is the
right place to note that ARM, the licensee company for both the
Snapdragon and the Apple A8 CPU cores, has actually claimed that the
current generation of its processors works best in terms of thermal
output/performance at around 1.2GHz. Going up above that ensues big
consequences - AnandTech has earlier shared estimates that going above
the 1.5GHz threshold by just 100MHz brings up a shocking, quadratic
increase in voltage and power consumed by the chip.
Apple may have finally succumbed to common sense: 
