The problem with A15 is that it's a very huge performance jump compared to A9 but at the same time it also uses significantly more power.
There's a cool architecture from ARM that consists of full fledged, fast A15 cores (which is faster than Krait by 20-30% per clock cycle and is fine with running at over 2GHZ but uses more energy than Krait) and A7 (which is a low power core, almost equal to A8 in performance but uses up far less energy). That can come in any core configurations (for example 4 A15 cores and 1 A7 core on one chip).
The idea is to run UI and basic OS processes using the A7 core, but when you launch demanding apps they would run on the A15 cores. A single A7 at 1ghz is just a bit faster than the A8, which for example powered the Galaxy S/ Nexus S which also should be enough for stand by and basic stuff and when needed the A15 would kick in.
ARM had a reference design ready last year and it was supposed to power the Galaxy S3 but Samsung apparently wasn't ready with their Exynos chips using that architecture. I'm pretty sure they'll have something like that by the time the Galaxy S4 comes out.
Nvidia kind-of tried that with Tegra 3 but their fifth core is still A9 (which uses over twice as much energy as A7), it's just undervolted and clocked lower.
So imagine this (which is how it works in any Tegra 3 device, like the Nexus 7):
but with a lower power "companion core" using half of that power and the "proper" 4 cores being at least twice as fast (but more power hungry as well). That's what you get in ARM's big.little architecture.
Details here:
http://www.arm.com/products/processors/technologies/bigLITTLEprocessing.php
Compared to Krait, A15 alone doesn't offer enough power gain in relation to its energy consumption but coupled with that A7 companion core it might in the end offer superior performance and similar battery life, if not longer stand by times.
Krait cores were optimized for highest possible performance per Watt of energy - they're perfectly balanced so there's no need for that companion core, but A15 offers more raw computing power and A7 offers lower energy use.
There's a cool architecture from ARM that consists of full fledged, fast A15 cores (which is faster than Krait by 20-30% per clock cycle and is fine with running at over 2GHZ but uses more energy than Krait) and A7 (which is a low power core, almost equal to A8 in performance but uses up far less energy). That can come in any core configurations (for example 4 A15 cores and 1 A7 core on one chip).
The idea is to run UI and basic OS processes using the A7 core, but when you launch demanding apps they would run on the A15 cores. A single A7 at 1ghz is just a bit faster than the A8, which for example powered the Galaxy S/ Nexus S which also should be enough for stand by and basic stuff and when needed the A15 would kick in.
ARM had a reference design ready last year and it was supposed to power the Galaxy S3 but Samsung apparently wasn't ready with their Exynos chips using that architecture. I'm pretty sure they'll have something like that by the time the Galaxy S4 comes out.
Nvidia kind-of tried that with Tegra 3 but their fifth core is still A9 (which uses over twice as much energy as A7), it's just undervolted and clocked lower.
So imagine this (which is how it works in any Tegra 3 device, like the Nexus 7):
but with a lower power "companion core" using half of that power and the "proper" 4 cores being at least twice as fast (but more power hungry as well). That's what you get in ARM's big.little architecture.
Details here:
http://www.arm.com/products/processors/technologies/bigLITTLEprocessing.php
Compared to Krait, A15 alone doesn't offer enough power gain in relation to its energy consumption but coupled with that A7 companion core it might in the end offer superior performance and similar battery life, if not longer stand by times.
Krait cores were optimized for highest possible performance per Watt of energy - they're perfectly balanced so there's no need for that companion core, but A15 offers more raw computing power and A7 offers lower energy use.
