Re: The hall of bloat

[Alan Cox <alan lxorguk ukuu org uk>]

On Llu, 2005-05-16 at 18:21, Soeren Sandmann wrote:
> Alan Cox <alan lxorguk ukuu org uk> writes:

> Isn't it always the case that you have to fill in the page table
> hierarchy down to the pages that you are actually using, regardless of
> the size of the mapping? Why would a mapping of 8K with both pages
> used require more page table entries than a mapping of 10M with two
> pages used?

The mapping is tree structured so you would typically need to fill two
or three levels of table when you map a page. If two pages are close
then they will use the same mappings, if they are distant they may
occupy entirely different parts of the tree.

Its thus possible that 1024 pages might require 12K of kernel page
tables, or even a bit over 2Mb if you allocate precisely non optimally.
This generally speaking only bites people doing sparse arrays when they
should be using hashes or computing values instead of doing lookup.