[CSE]  Advanced Operating Systems 
COMP9242 2017/S2 
UNSW
CRICOS Provider
Number: 00098G

PRINTER Printer-Friendly Version

M6: Demand paging

In this milestone you need to implement demand-paging of memory to a backing store. This should build on the earlier milestone where you implemented a pager that used memory to back virtual memory. The backing store for your paging system will be an NFS file called pagefile. You should implement a second-chance page-replacement algorithm.

Design issues

Most of the design in this section will come down to the appropriate choice of data structures. If you haven't already done so, you will need to work out your page table structure. You will also need to decide how to keep track of pages in your paging file. You may want to keep things simple at first by only concentrating on one process, however you should be aware that the next milestone will require you to handle multiple processes.


Assessment

Demonstration

To demonstrate that your code is indeed using the paging system you should artificially reduce the number of free frames, and run a program which uses a large heap and a large stack. You may reduce the number of free frames by modifying ut_find_memory() to return less memory, or by changing the seL4 kernel's view of available memory in kernel/src/plat/imx6/machine/hardware.c

Note: Choose a large difference between RAM and the paging file size, (e.g. at least 10 times the reduced RAM) to ensure you can clearly demonstrate paging extending the available memory to run applications larger than available RAM.

You may want to write a command called thrash which writes data to a given number of pages and then reads it back (verifying that it has been read back correctly).

Show Stoppers

  • Solutions that leak free space in the paging file. For example, using a bump pointer to manage allocation in the page file.
  • Solutions that can't artificially limit the number of frames available in the system to facilitate a demo.

Better Solutions

  • Solutions that do not increase the page table entry size when implementing demand paging.
  • Avoid paging out read-only pages that are already in the page file.
  • In theory, support large page files (e.g. 2-4 GB)
  • Solutions that avoid keeping the entire free list of free page-file space in memory.

Last modified: 04 Aug 2017.