[Q&A][KERNEL]Kernel questions - Galaxy S II Q&A, Help & Troubleshooting

I have made this Q&A thread to talk about the kernel settings. Feel free to post about battery stats, kernel questions and other stuff!
Q: How do I flash a kernel?
A: There are two main (and popular) ways of doing this, via recovery and via odin. Flash a kernel via recovery is like flashing a rom except that you don't have to do any wipes. Whereas flashing via ODIN requires you to flash the .tar file under 'PDA' or 'AP'.
Q: What is the best CPU Governor?
A: It depends what do you mean about 'best', saying 'best' could refer to the battery life, performance, stability, etc. Elaborate your question to a more clear question.
Q: What is the best I/O Scheduler?
A: Same thing as the previously answered question. However, often people talk about the speed, reliability and stability of an I/O Scheduler
Q: My kernel doesn't have the option to change any of these settings, help!
A: You will have to change your kernel in order to change these settings
Q: Why can't my phone allow me to overclock or undervolt?
A: Most likely the kernel you are using doesn't support these functions. Read the kernel OP before flashing!
Q: What does the kernel version in the phone's settings tell me?
A: Like all operating systems, there must be some layer of software the provides all of the drivers for your phone. A newer version of a kernel (Linux for our case) doesn't necessarily mean that the kernel is better, usually it could provide bug fixes but most developers don't bother updating this anyway.
Q: Is there an app to change my kernel settings?
A: If you read the kernel OP, you would have known the compatible apps for your kernel.
Q: Why does my battery drain faster when I flash a new kernel?
A: Kernels aren't developed to drain your battery, this could be caused by an inconsistency in code that may have broke something in the kernel causing battery drain (highly unlikely these days). You will have to go through a few power cycles (drain battery to a low percentage, recharge and repeat).
Q: What is the best kernel for XYZ rom?
A: Rule from XDA, no best ROM or best kernel, etc. Only recommendations can be given by users. Experiment yourself or find reviews on some combinations.
Q: Why should I switch kernels when the stock is good enough?
A: Custom kernels provide extra functionality for your device that could improve performance and battery life. Sometimes installing a custom recovery can bring bug fixes earlier than a rom update (fixes on kernel side like the FPbug). If you think that playing with a custom rom is too technical already, don't use a custom kernel then.
Q: The kernel I am using has an old recovery version, how do I update the recovery?
A: You can update the recovery in many ways such as by updating the kernel, switching kernels, flashing a newer custom recovery like Philz recovery, etc.
Q: What CPU governor and I/O scheduler do you use?
A: Have a look at my hidden signature section
Q: Can I revert to stock kernel once I flash a custom recovery?
A: Usually you can only do this if you reflash the ROM that you were using.
Q: Why does flashing a ROM remove my custom recovery?
A: It is because most custom ROMs come with their own kernels
Q: Why does my screen turn black after the samsung logo after flashing a kernel?
A: You have flashed the wrong version of kernel on your phone. Check the kernel OP before flashing!
Will be updated with more info soon.