CSE 120 Nachos Project 3: Virtual Memory
Due: December 6, 2004 at Midnight
In this lab you will extend Nachos to support demand paged virtual
memory. This new functionality gives processes the illusion of a
virtual memory that is larger than the available machine memory.
You will implement and debug virtual memory in two steps. First,
you will implement demand paging using page faults to dynamically load
process virtual pages on demand, rather than initializing page frames
for each process in advance at Exec time as you did in Project
2. Next, you will implement page replacement, enabling your kernel to
evict a virtual page from memory to free up a physical page
frame to satisfy a page fault. Demand paging and page replacement
together allow your kernel to "overbook" memory by executing more
processes than would fit in machine memory at any one time, using page
faults to "juggle" the available physical page frames among the larger
number of process virtual pages. If it is implemented correctly,
virtual memory is undetectable to user programs unless they monitor
their own performance.
The operating system kernel works together with the machine's
memory management unit (MMU) to support virtual memory. Coordination
between the hardware and software centers on the page table structure
for each process. You used page tables in Project 2 to allow your
kernel to assign any free page frame to any process page, while
preserving the illusion of a contiguous memory for the process. The
indirect memory addressing through page tables also isolates each
process from bugs in other processes that are running concurrently. In
this project, you will extend your kernel's handling of the page
tables to use three special bits in each page table entry (PTE):
This project has four parts:
- Valid bit: The kernel sets or clears the valid bit in each
PTE to tell the machine which virtual pages are resident in
memory (a valid translation) and which are not resident (an
invalid translation). If a user process references an address
for which the PTE is marked invalid, then the machine raises a
page fault exception and transfers control to your kernel's
- Use bit: The machine sets the use bit (aka reference bit) in
the PTE to pass information to the kernel about page access
patterns. If a virtual page is referenced by a process, the
machine sets the corresponding PTE reference bit to inform the
kernel that the page is active. Once set, the reference bit
remains set until the kernel clears it.
- Dirty bit: The machine sets the dirty bit in the PTE
whenever a process executes a store (write) to the corresponding
virtual page. This informs the kernel that the page is dirty; if
the kernel evicts the page from memory, then it must first
"clean" the page by writing its contents to disk. Once set,
the dirty bit remains set until the kernel clears it.
- [35 pts] Implement demand paging. In the first part, you will
continue to preallocate a page frame for each virtual page of each
newly created process at Exec time, just as in Project 2. As before,
return an error from the Exec system call if there are not enough free
page frames to hold the process' new address space. But for this
part, you need to make the following changes to AddrSpace:
- In your AddrSpace initialization method, initialize all the PTEs
- In the same method, remove the code that (1) zeros out the
physical page frames, and (2) preloads the address space with the code
and data segments from the file. You will do this on demand when the
process causes a page fault -- you will continue to allocate physical
page frames in AddrSpace for each virtual page, but delay loading the
frames with data until they are actually referenced by the process.
- Handle page fault exceptions in ExceptionHandler. When the
process references an invalid page, the machine will raise a page
fault exception (if a page is marked valid, no fault is generated).
Modify your exception handler to catch this exception and handle it by
preparing the requested page on demand.
- To prepare the requested page on demand, add a method to
AddrSpace to page in the faulted page from the executable file. Note
that faults on different address space segments are handled in
different ways. For example, a fault on a code page should read the
corresponding code from the executable file, a fault on a data page
should read the corresponding data from the executale file, and a
fault on a stack frame should zero-fill the frame.
- Once you have paged in the faulted page, clear the page fault
exception by marking the PTE as valid. Then let the machine
restart execution of the user program at the faulting instruction --
when you return from the exception, do not increment the PC as you did
when handling a system call so that the machine will reexecute the
If you set up the page (by initializing it) and page table (by
setting the valid bit) correctly, then the instruction will execute
correctly and the process will continue on its way, none the wiser.
As you make the changes above, keep the following points in mind:
- Remember, a virtual page may contain portions of two segments,
such as the end of the code segment and the beginning of the data
segment. A fault on that page will require you to load from both the
code and data segments into that page. You need to handle this
boundary case for all situations where two segments can overlap on a
page (code, data, stack, and argument).
- If you use ReadMem (or WriteMem) to implement a system call, it
is entirely possible for those functions to reference a page that has
yet to be loaded into memory (since you can give them an arbitrary
address in the process virtual address space). If this happens,
ReadMem will return FALSE because it triggered a page fault when it
tried to access the address you gave it. You should not
consider this an error. Instead, you should retry the operation
assuming that the referenced page was successfully loaded. If it is
FALSE again, then return an error.
- StartProcess and Exec closed the executable file after creating
the address space. You no longer have this luxury, and will have to
keep it open during the life of the process.
Testing: Start by testing with one process running at a
time. During debugging, you will probably want to print out the
arguments that you are giving to ReadAt and bzero when initializing a
page during a page fault to make sure that you are loading the correct
parts of the executable file into the virtual page. Then test with
multiple processes; this should work automatically since these changes
should be independent of the number of processes running.
- [35 pts] Now implement demand paged virtual memory with page
replacement. In this second part, not only do you delay initializing
pages, but now you delay the allocation of physical page frames until
a process actually references a virtual page that is not already
loaded in memory.
- Start by completing the gutting of your code that creates an address
space. In part one, you removed the code that initialized the virtual
pages. Now, remove the code that (1) allocates page frames and (2)
preinstalls virtual-physical translations when setting up the page
table. Instead, merely mark all the PTEs as invalid.
- Extend your page fault exception handler to allocate a page frame
on-the-fly when a page fault occurs. In part one, you just
initialized the virtual page when a page fault occurred. In this
part, allocate a physical page for the virtual page and use your code
from part 1 to initialize it, mark the PTE as valid, and return from
You can get the above two changes working without having page
replacement implemented for the case where you run a single program
that does not consume all of physical memory. Before moving on, be
sure that the two changes above work for a single program that fits
into memory (e.g., array).
Now implement page replacement to free up a physical page frame
to handle page faults:
- Extend your page fault exception handler to evict pages once
physical memory becomes full. First, you will need to select a victim
page to evict from memory; for now, keep this simple and just choose a
convenient page. Then mark the PTE for that page as invalid.
- Evict the victim page. If the page is clean (i.e., not dirty),
then the page can be used immediately; you can always recover the
contents of the page from disk. If the page is dirty, though, the
kernel must save the page contents in backing store on disk.
- Read in the contents of the faulted page either from the
executable file or from backing store (see below).
- Implement a BackingStore class to handle page in and page out
operations. An important part of this project is to use the Nachos
file system interface to allocate and manage the backing store.
Implement methods to allocate space on backing store, locate pages on
backing store, push pages from memory to backing store (for page out),
and pull from backing store to memory (for page in).
- Use the FileSystem class to create files for backing store (see
filesys/filesys.h). After creating the backing store file, use the
FileSystem class to open it. Opening the file will return an OpenFile
object, which allows you to do reads and writes (see
filesys/openfile.h). The Makefiles are setup to compile with
FILESYS_STUB defined, so be sure to look at that version of the
As you implement the above operations, keep the following points
- As in the first part of the project, the first time a page is
touched it needs to be initialized (from the executable file for code
and data, bzero'd otherwise; see part 1 above). If this page is
subsequently evicted to backing store, it will be read from there on
further page faults.
- You are not limited to one file for backing store for the entire
system, and might find it more convenient to have a backing store file
for each process (doing so makes locating evicted pages convenient).
However, do not create backing store files at finer granularities;
e.g., do not use one file per page.
- Be sure to clear the dirty bit when you mark a PTE for the victim
page as invalid.
- When running multiple processes, you may select a victim page
from another process. As a result, you will need to update the PTE in
the page table for that process, not the faulting one.
Finally, you should only do as many page reads and writes as
necessary to execute the program, and as dictated by your page
replacement algorithm. You will soon discover that the first page
fault is different than subsequent ones on code and data pages. On
the first fault you need to read from the executable file, and on the
second you need to read from the backing store. Your implementation
needs to be able to handle this situation. It might be tempting to
just copy the pages from the executable file to the backing store when
the process is first created, or on a page-by-page basis when the
first fault occurs, but both of these cases introduce extra unecessary
disk I/Os and should not be used.
- [15 pts] Testing. In this project, programs can use more virtual
memory than physical memory, and pages are brought into memory only if
they are actually referenced by the user program. To show that you
only initialize pages that are referenced on demand, write one test
program that references all of the pages in memory, and a second test
program that only references some of the pages. You can use the
pagein and pageout counters (below) to convince yourself that you are
only initializing the pages that are referenced by the process. For
example, accessing a two-dimensional array selectively (e.g., all
rows, some rows) can give different page reference behavior, and not
calling a function will only execute a subset of the code in an
executable file. We do not particularly care if the test programs do
anything useful (like multiplying matrices), but they should generate
memory reference patterns of interest.
Your test programs should also demonstrate the page replacement
policy and correct handling of dirty pages (e.g., allocating backing
storage and preserving the correct contents for each page on backing
store). Implement additional test programs that (1) generate good
locality (most references are to a small subset of pages), and (2)
generate poor locality (everything is referenced repeatedly in a
pattern), and (3) random locality (pages are effectively referenced
randomly). If necessary, try reducing the amount of physical memory
to ensure more page replacement activity.
Here is an example test program that is larger than physical
memory and uses the console to draw a "snake" wandering around on the
- [15 pts] Maintain counters of page faults and pagein and pageout
events. Print out these counters when Nachos exits. This will aid in
debugging, and indicate how well the page replacement algorithm is
working with the processes running in the system. You might also want
to print out a DEBUG message when each of these events occurs as the
process is running (and identify the process that caused the event so
that you can disambiguate among multiple executing processes).
In particular, add two new counters to the Statistics class in
machine/stats.h, numPageOuts and numPageIns, and
update the constructor and Print methods in stats.cc to initialize and
report the values of those counters (ignore the top of the file where
it says to not change the file). Increment the "PageOut" counter
whenever you write a page to the backing store (e.g., "clean" pages
should not cause a PageOut). Increment the "PageIn" counter whenever
you read a page from (1) the original executable file and (2) your
backing store. Print the values of those counters on the same line as
the numPageFaults counter.
With backing store implemented, your operating system will use
main memory as a cache over a slower and cheaper backing store. As
with any caching system, performance depends largely on the policy used
to decide which pages are kept in memory and which to evict. As you
know, we are not especially concerned about the performance of your
Nachos implementations (simplicity and correctness are paramount), but
in this case we want you to experiment with one of the page
replacement policies discussed in class. For project 3, use a simple
algorithm to implement page replacement such as FIFO or random
replacement. For extra credit, you can implement LRU or an LRU
approximation (see below). In any case, document your replacement
algorithm choices in the project writeup.
In your project writeup, create a table that reports the values of
the paging counters for all of the test programs that you used to test your
page replacement algorithm. If you implemented more than one
replacement algorithm, report the values for all of the algorithms
that you implemented. Here is an example:
Physical memory size: 32 pages.
Page replacement policy: Random.
Program PageFaults PageOuts PageIns
halt 3 0 2
matmult 112 45 74
sort 755 624 722
Note that the exact numbers of page faults and disk I/Os is
implementation dependent, so do not be surprised if your
implementation results in different values.
- (Extra Credit) [4 pts] Implement LRU or an LRU approximation for
your page replacement algorithm, examining and clearing the use bit in
each PTE in order to gather information to drive the policy. Make it
possible to run your nachos executable with your original algorithm as
well your LRU algorithm (e.g., with a new command line switch to
main.cc). Using the counters and experiments with test programs,
demonstrate that LRU does a better job of replacing pages than your
original (FIFO or random). Extend your page fault table in your
writeup to include the values for your LRU algorithm. Design a test
case where LRU does a worse job.
As with projects 1 and 2, we would like a writeup
describing what changes you made, how well they worked, how you tested
your changes, and how each group member contributed to the project.
For project 3, also include a discussion of the page replacement
algorithm that you used, and a description of how well that page
replacement algorithm worked on your test programs (e.g., the test
program name, the number of pages it requires, and the faults it
generated). Please include this writeup in your userprog directory.
And as with the previous projects, use CVS to turn in your project.