Physical vs. virtual memory allocation.

Physical vs. virtual memory allocation.

Post by Lars the parallelize » Thu, 02 Oct 1997 04:00:00

Hi !

I am looking at page coloring for performance improvement and
would like the OS to do the practical work. To be able to do
this I have to control the physical placement of data based on
virtual address.

To do be able to do this I have some basic question on
page allocation in Linux (2.0.30) on a Pentium :

Are pages allocated on other places than __get_free_pages()
(in mm/page_alloc.c)  ?

User memory seems to be allocated at first access to a page,
but what is the calling path (to __get_free_pages) ?

Does map_nr in  struct page (in include/linux/mm.h) reflect
the physical page number of the page ?
If not how can I get the physical address of an non-allocated page ?


1. mmap, virtual memory, physical memory

    Help I'm confused! From my readings on the Net and elsewhere it
would appear that the mmap system call is used to map a file into a
process's virtual memory, adding a new region to the address space. But
it seems also that this virtual mapping implies physical mapping  if the
corresponding physical pages are not already loaded (by some other
process), except for the MAP_ANONYMOUS case.
If this is so how can we have a page in a process's address space
(virtual memory) that is not present in physical memory? Of course this
can happen after swapping but initially when loading a program how do
you create its address space (virtual memory) without having all the
regions in physical memory? i.e. How do you provide for demand-paging?

                                Any help appreciated.

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