What is Kernel ?
A kernel in an operating system—in this case Android—is the component responsible for helping your applications communicate with your hardware. It manages the system resources, communicates with external devices when needed, and so on. Android uses a variation of the Linux kernel.
A kernel is not the same as a ROM, even though you install them in mostly the same way. A ROM is a bit more all-encompassing. It’s the operating system you use on your phone, the software your phone uses to get things done—the kernel is the bridge between that ROM and your hardware. All ROMs come with a kernel installed, but you can install a third-party one if you like—and that’s what this post is about.
What is Custom Kernel ?
Custom Kernels are nothing but modified stock kernel. Its done in order to achieve some features which is not present in the kernel provided by Manufactures. You can find one for your device if anybody has created a custom kernel for your “device”. Kernels are device specific too.
Why Should you care about it ?
Custom kernel can give you :-
Better Performance and Battery Life
This is the big change a new kernel can bring to your device. I’d separate these into two categories, but they’re so intertwined that you really need to consider both when picking a kernel. There are a bunch of different kernel features that contribute to this:
Clock Speeds: In a very basic sense, higher clock speeds will improve performance on your phone. Flashing a new kernel allows you to overclock your phone, using higher clock speeds than the manufacturer intended. They can also let you reach lower clock speeds, so you can underclock your phone when you aren’t using it, thus saving battery life.
CPU Governors: Different kernels can support different CPU Governers, which manage the way your phone ramps up or down its clock speeds as you use it. There are a few different kinds you’ll see, including Conservative, which focuses on battery life by ramping up your CPU very gradually when needed; Interactive, which focuses more on performance and smoothness by scaling up the CPU faster; InteractiveX, which is like Interactive but scales the CPU down when your screen is off (for better battery life); and Smartass, which is similar to Conservative but takes more factors into account when ramping up the CPU.
Task Scheduler: Kernels come with two different types of task schedulers: the Completely Fair Scheduler (CFS) and the Brain F**k Scheduler (BFS). CFS kernels are designed for regular phone use, like texting, web browsing, and otherwise multitasking apps on your phone. Most stock kernels are CFS kernels. BFS kernels focus more on whatever app is in the foreground, which is great for things like games but can be a bit laggier and a bit less stable.