

ARM reveals little dog A7 processor - ssp
http://www.eetimes.com/electronics-news/4229867/ARM-reveals-little-dog-A7-processor

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jws
This is binary compatible with the upcoming A15 processor, but lower speed and
more power efficient. The intent is that a chip will have some A15 cores and
some A7 cores and use them as appropriate to balance speed and energy
consumption.

It could also be used as a standalone processor where its performance is
sufficient and energy efficiency is a win.

The "Cortex A∗" numbering scheme should not be confused with the "Apple A∗"
numbering scheme. They are unrelated. There is in fact an A5 in each scheme at
the moment.

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ori_b
It doesn't help that the instruction set version is numbered similarly, but
their numbers are not the same as the chip version.

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leoc
Or that the A7 will be more recent and more fully-featured than the A8 and A9.

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cpeterso
How much battery life is saved using an asymmetric multicore design as
compared to dynamically downclocking of a "big dog" core? The article does not
address this, but it does say that smaller die size of the "little dog" core
uses 1/3 less power.

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ajross
It's a huge "it depends", and on things that aren't ever going to be publicly
visible. "Shutting down" a core is itself really complicated. You can drop all
the power rails and push the quiescent current all the way to zero, but that
loses the cache; so on resume those cache lines have to be refilled, which is
work you don't have to do if you maintain power to the cache. Which to choose
has to do with how long the CPU will be active for, which depends in sensitive
ways on the architecture of the whole system (how much work can be offloaded
to the hardware vs. requires interrupt handling in the CPU, etc...). But then
of course those other hardware devices have their own power needs. And there's
further complexity due to multiple layers of cache, etc...

Balancing the whole thing is a big guessing game, and the guesses have to be
made by the SoC vendors years before the actual product appears in the market.

My intuition, FWIW, is that the asymmetric idea is a non-starter. It sort of
exists right now: c.f. the "baseband CPU" (or all the DSP cores you find
scattered around these things) in many Android designs, and the extra CPU is
really just a huge resource waste. The hardware people will never get this
right. Putting all the transistors into a single set of SMP cores gives up on
the ability to tune things perfectly, but it puts the OS in control of things.
OS vendors are much closer to the problem and better positioned to make good
guesses.

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leoc
There should be decent public visibility in time, if an A7 ever shows up on a
Nexus phone or on a successor to Beagleboard/Pandaboard. Actually, I don't
have the link just now, but IIRC someone was trying to write software to do
the same thing on the Pandaboard's OMAP4, transferring a running Linux kernel
from the A9 core to one of the OMAP4's built-in M3s. (I don't know what they
were planning to do for memory protection.)

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caf
NVidia's Kal-El SoC implements this approach, although it uses Cortex A9 for
both the Big Dog and Little Dog cores. The Little Dog is produced using low-
leakage transistors, which is where their obtain their power savings (at the
cost of clock speed). In the Kal-El design, the Little Dog communicates with
the Big Dogs through a shared L2 cache.

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Donch
Reminds me of the old BBC coprocessors, which is interesting as ARM's
predecessor was Acorn Computers. Funny to see an idea from the 80's getting a
second run out!

<http://en.wikipedia.org/wiki/BBC_Micro_expansion_units>

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shawn-butler
I doubt this will change process model or application lifecycle already
present in the major smartphone OS. In the "always" on services, long-running
background tasks arena the precious commodity is not cpu usually, it's memory.
There is no "swap space". Interesting to see QNX\RIM listed as an early
partner.

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joenathan
In overall power consumption how high is the CPU on the list of components?
I'd imagine the display, WiFi, cell radios, Bluetooth are the real battery
zappers. If a device was to be built using this big dog-little dog strategy,
realistically how much battery life could expected to be gained?

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Symmetry
It really depends how much of each of those things you're using. If you have
an LCD display that you set to be bright, its going to dominate your power
usage when its on, but not if you're really maxing out the CPU. When you're
not using the phone the cell radio is going to dominate power usage, but if
you're on WiFi range it won't be that bad. Most of the time in that case the
processor is going to be sleeping, but occasionally it will wake up and check
for updates, etc, before going back to sleep. Its in a case like that where
having it be a little A7 instead of the big A15 that wakes up will be a big
advantage.

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ajross
Careful though, that's apples to oranges. Clearly you're right: waking up a
small core for a tiny task is going to require less power than doing the same
on a big core. But the choice here is between a A15-only design (you need the
big core always) and an A15+A7 design. The one with an A7 is a _larger_ chip.
That silicon area has built-in costs in both money and power consumption and
has to be accounted for.

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Symmetry
It certainly has an area cost, but its not going to contribute to power
consumption when its not turned on. Power gating is a pretty well developed
technique these days, and on the high Vt processes that everyone but NVidia
uses you could probably make due with just clock gating for something as tiny
as an A7.

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gsiener
iPhone 5?

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wmf
This is slower than the processor in the iPhone 4S.

Edit: But it could appear in future iPhones as a secondary processor.

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redthrowaway
FTA, it's intended to be used as a standalone in sub-$100 phones, or as a low-
powered processor that could be paired with a more powerful chip and take care
of tasks that aren't performance-intensive. So it's not out of the realm of
possibility that it will see duty in top-of-the-line phones.

The biggest killer for his iPhone 5 question is the fact it's not slated for
release until 2013, which puts it well past the iPhone 5's introduction.

