At CES this year Samsung introduced the oddly named Exynos 5 Octa SoC, one of the first Cortex A15 SoCs to implement ARM's big.LITTLE architecture. Widely expected to be used in the upcoming Galaxy S 4, the Exynos 5 Octa integrates 4 ARM Cortex A7 cores and 4 ARM Cortex A15 cores on a single 28nm LP HK+MG die made at Samsung's own foundry. As we later discovered, the Exynos 5 Octa abandons ARM's Mali GPU for Imagination's PowerVR SGX 544MP3, which should give it GPU performance somewhere between an iPad 3 and iPad 4.

The quad-core A7 can run at between 200MHz and 1.2GHz, while the quad-core A15 can run at a range of 200MHz to 1.8GHz. Each core can be power gated independently. The idea is that most workloads will run on the quad-core A7, with your OS hot plugging additional cores as performance demands increase. After a certain point however, the platform will power down the A7s and start switching over to the A15s. Both SoCs implement the same revision of the ARM ISA, enabling seamless switching between cores. While it's possible for you to use both in parallel, initial software implementations will likely just allow you to run on the A7 or A15 clusters and switch based on performance requirements.

What's most interesting about Samsung's ISSCC presentation is we finally have some hard power and area data comparing the Cortex A15 to the Cortex A7. The table above puts it into numbers. The quad-core A15 cluster occupies 5x the area of the quad-core A7 cluster, and consumes nearly 6x the power in the worst case scenario. The area difference is artificially inflated by the fact that the A15 cluster has an L2 cache that's 4x the size of the A7 cluster, but looking at the die photo below you can get a good feel for just how much bigger the A15 cores are themselves:

In its ISSCC presentation, Samsung stressed the value of its custom libraries, timing tweaks and process technology selection in bringing the Exynos 5 Octa to market. Samsung is definitely marching towards being a real player in the SoC space and not just another ARM licensee.

The chart below is one of the most interesting, it shows the relationship between small integer code performance and power consumption on the Cortex A7 and A15 clusters. Before switching from the little CPU to the big one, power consumption is actually quite reasonable - south of 1W and what you'd expect for a smartphone or low power tablet SoC. At the lower end of the performance curve for the big CPU things aren't too bad either, but once you start ramping up clock speed and core count power scales linearly. Based on this graph, it looks like it takes more than 3x the power to get 2x the performance of the A7 cluster using the Cortex A15s.


Comments Locked


View All Comments

  • UpSpin - Wednesday, February 20, 2013 - link

    According to the chart, the Quad Core A15 part consumes about 5W! Probably CPU only. If this SoC is put inside a smartphone, the battery would be dead in less than an hour if you also consider the power consumption of the display and PowerVR GPU..
    This SoC is for tablets with large enough batteries and large enough surface to passivly cool the 5W+GPU waste power. Maybe in the next Galaxy Note 10 it will find a use or in a boosted Nexus 10, but never in a smartphone.
  • tempestglen - Wednesday, February 20, 2013 - link

    4xA15=4.5 watt
    4xA7=0.75 watt

    80% of phone's running time is low performance, so battery life of exynos octa will be good.
  • Aenean144 - Thursday, February 21, 2013 - link

    It really depends on the 80%.

    The A7 turns back the clock about 3 years back to the Cortex-A8 days in terms of DMIPS/Hz. I can easily see many an app, process or thread wanting more. Will be interesting to see where running a web browser will land. It's not going to be pretty if it stays on the A7.
  • UpSpin - Thursday, February 21, 2013 - link

    A7 has 1.9 DMIPS/MHz, A9 has 2.5 DMIPS/MHz.
    The Galaxy Nexus has a 1.2 GHz Dual Core A9 --> 6000 DMIPS
    This SoC has a 1.2 GHz Quad Core A7 --> 9120 DMIPS
    Really, the LITTLE part should handle any normal tasks easily. Video playback gets done by hardware decoders, GUI rendering gets done by the GPU, website parsing and other processing stuff gets done by the CPU.

    The Galaxy Nexus runs fluid. This A7 quad core is at least 30% faster, a smartphone could live without the A15 easily.
  • Death666Angel - Thursday, February 21, 2013 - link

    All very true. :D
    My Galaxy Nexus has some "think pauses" (I'm running a custom everything, so not sure if that happens on plain Android). But when that happens I often wonder if it is a CPU issue or a memory one. It mostly happens when starting/switching between memory intensive apps (big emails, video, browser). Would the noticeable performance increase be bigger from an A15 upgrade or from getting a midway decent SSD with >200MB/s seq r/w and >30MB/s rnd r/w. :)
  • UpSpin - Thursday, February 21, 2013 - link

    That's the idea behind big.LITTLE. In low demanding tasks use the A7 in high performance task use the A15.
    But the device must be able to handle the A15 power consumption.
    But if the A15 consume 5W and you start a game which will most probably make a use of the A15 power, your smartphone battery will be dead in an hour, just because of the CPUs.
    Yes, in standby the battery life will be good, but I never denied this. That's what big.LITTLE is made for.
    In heavy use however, this SoC will, with CPU and GPU full power, consume, most probably, 10W. A smartphone battery has <10Wh. A smartphone surface is too small to dissipate 10W.
    This SoC won't find a use in a smartphone. It's physically impossible, except you never make a use of the A15 cores, which defies the purpose of this SoC!
  • Aenean144 - Wednesday, February 20, 2013 - link

    I think that's just CPU.

    If you assume 3.5 DMIPS/MHz for Cortex-A15, a quad-core A15 running at 2 GHz is 3500*2*4 = 28000 DMIPS. That's quite close to the point in the upper right in the plot, which is actually a little over 5 Watts. Maybe 5.2 W.

    Even in a tablet, the SoC may be prevented from maxing out the CPU and the GPU at the same time. This could be an 8 to 10 W SoC with both the GPU and CPU maxed out.
  • xaml - Saturday, February 23, 2013 - link

    It is prominently claimed so for this very reason, here:
  • xaml - Saturday, February 23, 2013 - link

    That was @UpSpin, for the prehistoric lack of editing.
  • lmcd - Sunday, February 24, 2013 - link

    Disappointing. Exynos 5 Octa should have made it in there. And as some commenter noted, a 720p SAMOLED+ would be preferable to the 1080p SLCD cited.

Log in

Don't have an account? Sign up now