Mapping of ZONE_HIGHMEM in kernel address space in 32bit x86

Question

In a system with 3:1 split, the ZONE_NORMAL with a size of 896MB is permanently mapped to the kernel address space.This leaves a 128MB free space in the Kernel address space and according to my understanding, the ZONE_HIGHMEM pages are mapped temporarily to this 128MB part. If the system actually had a 4GB physical memory you will be mapping(not smultaneously) the HIHGMEM part- which is roughly 3.2GB - to this 128MB part. If that was the case Kernel would have to frequently access HIHGMEM which implicates a frequent change in temporaty mapping and that in my view is a penalty. So what was the reason why ZONE_NORMAL fixed at 896MB and not something really lower?

Answer 1

ZONE_NORMAL memory area is the one where most operations can only take place. The larger it is, the better the whole system performance will be. The penalty you mention may seldom affect the performance.

Please read this article.
http://kerneltrap.org/node/2450