Linux – the Zen kernel

Some extra debug information like getting the call stack and some other things like that that are needed by gdb for debugging will be enabled.

This will have slight impact in the performance. But you will see this mainly by using the tools using which most of the code runs in the kernel space. e.g. check the speed of a file copy, creating multiple threads and switching between then etc.

Linpack is a software library (user space) for performing numerical linear algebra. Mostly this makes very minimal or no system calls. i.e. minimal interaction with the kernel. Thereby you don't see much change in performance measured using Linpack.

Swap is only valid during a given boot, so all the tracking information is kept in memory. Swapping pages in and out is handled entirely by the kernel, and is transparent to processes. Basically, memory is split up into pages, tracked in page tables; these are structures defined by each CPU architecture. When a page is swapped out, the kernel marks it as invalid; thus, the next time anything tries to access the page, the CPU will fault, which will cause a handler in the kernel to be invoked; it’s this handler’s responsibility to restore the page’s contents.

In Linux, there’s a swap_info structure which describes each swap device or file. Within that structure, a swap_map maps memory pages to blocks in the swap device or file. When a pages is swapped out, the kernel stores the swap_info index and swap_map offset in the corresponding page table entry, which allows it to find the page on disk when necessary. (All supported architectures provide enough space for this in their page tables, but there are limits — e.g. the available space means that Linux can manage at most 64GiB of swap on x86.)

You’ll find a much more detailed description of all this in the “Swap Management” chapter of Mel Gorman’s Understanding the Linux Virtual Memory Manager.