Re: General questions about virtual memory
- From: Max Laier <max@xxxxxxxxxxxxxx>
- Date: Wed, 30 Jul 2008 19:28:32 +0200
On Wednesday 30 July 2008 18:07:51 FreeBSD Hackers wrote:
This suggest that you don't understand virtual memory at all. Go back to
the
start of the chapter and re-read. The page directories and page tables
describe a *virtual* address space. For a given architecture the
*virtual* address space has a fixed size (4GB for i386), so the page
table structure is
always the same size (though it might be sparsely populated). Inside the
page
Ack! As soon as I read this I realized the mistake I had made in my
thinking. This was a dumb question, and I knew better than to ask.
Somehow I had confused myself.
----- 8< -----
If a read request is made to a virtual address who's data has been swapped
out, the CPU traps to the OS to fix the problem. Assuming there are no
free page frames for the new data, a page frame is selected and evicted to
make room for the new page. Whatever page was chosen belongs to a process
somewhere in the system. When that page frame gets swapped, the PTE
pointing to that page frame must be updated to indicate that that data is
no longer in RAM. How does the OS find that PTE? Does it search through
every entry of every page table for every process in the system until it
finds it?
You should have quoted (and I suppose read) my entire message:
.. you need additional bookkeeping
to track that (see core map, free lists, ...)
Wikipedia really does a good job explaining all this.
--
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