CS 471 Operating Systems

Class web site
Fall 2017

Assignment 3: Virtual Memory

Due December 9, 2017, before midnight

[OS/161 Instructions]


Malloc code


You have improved OS/161 to the point that you can now run user processes. However, there are a number of shortcomings in the current system. A process's size is limited by the number of TLB entries (i.e., 64 pages). In this assignment we will adapt OS/161 to take full advantage of the simulated hardware by implementing management of the MIPS software-managed Translation Lookaside Buffer (TLB). You will write the code to manage this TLB. You will also write the code to implement paging -- the mechanism by which memory pages of an active process can be sent to disk when memory is needed, and restored to memory when required by the program. This permits many processes to share limited physical memory while providing each process with the abstraction of a very large virtual memory.

Structure of the TLB entries

In the System/161 machine, each TLB entry includes a 20-bit virtual page number and a 20-bit physical page number as well as the following five fields:

All these bits/values are maintained by the operating system. When the valid bit is set, the TLB entry contains a valid translation. This implies that the virtual page is present in physical memory. A TLB miss occurs when no TLB entry can be found with a matching virtual page and address space ID (unless the global bit is set in which case the address space ID is ignored) with the valid bit set.


The operating system creates the illusion of unlimited memory by using physical memory as a cache of virtual pages. Paging relaxes the requirement that all the pages in a process's virtual address space must be in physical memory. Instead, we allow a process to have pages either on disk or in memory. When the process issues an access to a page that is on disk, a page fault occurs. The operating system must retrieve the page from disk and bring it into memory. Pages with valid TLB entries are always in physical memory. This means that a reference to a page on disk will always generate a TLB fault. At the time of a TLB fault, the hardware generates a TLB exception, trapping to the operating system. The operating system then checks its own page table to locate the virtual page requested. If that page is currently in memory but wasn't mapped by the TLB, then all we need to do is update the TLB. However, the page might be on disk. If this is the case, the operating system must:

  1. Allocate a place in physical memory to store the page;
  2. Read the page from disk,
  3. Update the page table entry with the new virtual-to-physical address translation;
  4. Update the TLB to contain the new translation; and
  5. Resume execution of the user program.

Notice that when the operating system selects a location in physical memory in which to place the new page, the space may already be occupied. In this case, the operating system must evict that other page from memory. If the page has been modified or does not currently have a copy on disk, then the old page must first be written to disk before the physical page can be reallocated. If the old page has not been modified and already has a copy on disk, then the write to disk can be avoided. The appropriate page table entry must be updated to reflect the fact that the page is no longer in memory.

As with any caching system, performance of your virtual memory system depends on the policy used to decide which things are kept in memory and which are evicted. On a page fault, the kernel must decide which page to replace. Ideally, it will evict a page that will not be needed soon. Many systems (such as UNIX) avoid the delay of synchronously writing memory pages to disk on a page fault by writing modified pages to disk in advance, so that subsequent page faults can be completed more quickly.

Your Mission


Consult the ASST3 config file and notice that the arch/mips/mips/dumbvm.c file will be omitted from your kernel. You will undoubtedly need to add new files to the system for this assignment, e.g., kern/vm/vm.c or kern/arch/mips/mips/mipsvm.c. Be sure to update the file kern/conf/conf.kern, or, for machine-dependent files, kern/arch/mips/conf/conf.arch, to include any new files that you create. Take care to place files in the "correct" place, separating machine-dependent components from machine-independent components.

You should also now restrict your physical memory to 512 KB by editing the ramsize line in your sys161.conf file.

Now, config an ASST3 kernel, run make depend, and build it. You are now ready to begin assignment 3; tag your repository asst3-begin.

TLB Handling

In this part of the assignment, you will modify OS/161 to handle TLB faults. Additionally, you need to guarantee that the TLB state is initialized properly on a context switch. One implementation alternative is to invalidate all the TLB entries on a context switch. The entries are then re-loaded by taking TLB faults as pages are referenced. If you do this, be sure to copy any relevant state maintained by the TLB entries back into the page table before invalidating them. (For example, in order for the paging algorithm to know which pages must be written to disk before eviction, you must make sure that the information about whether a page is dirty or not is properly propagated back into the page table.) An alternative to invalidating everything is to use the 6-bit address space IDs and maintain separate processes in the TLB simultaneously.

We recommend that you separate implementation of the TLB entry replacement algorithm from the actual piece of code that handles the replacement. This will make it easy to experiment with different replacement algorithms if you wish to do so. Refer to the kernel config file section of Assignment 2 on how to add configuration options for TLB replacement policies.


In this part of the assignment, you will modify OS/161 to handle page faults. When you have completed this problem, your system will generate an exception when a process tries to access an address that is not memory-resident and then handle that exception and continue running the user process.

You will need routines to move a page from disk to memory and from memory to disk.

You will need to decide how to implement backing store (the place on disk where you store virtual pages not currently stored in physical memory). The default sys161.conf includes two disks; you can use one of those disks for swapping. Please do swap to a disk and not somewhere else (such as a file). Also, be sure not to use that disk for anything else! To help prevent errors or misunderstandings, please have your system print the location of the swap space when it boots.

You will need to store evicted pages and find them when you need them. You should maintain a bitmap that describes the space in your swap area. Think of the swap area as a collection of chunks, where each chunk holds a page. Use the bitmap to keep track of which chunks are full and which are empty. The empty chunks can be evicted into. You also need to keep track, for each page of a given address space, of which chunk in the swap area it maps onto. When there are too many pages to fit in physical memory, you can write (modified) pages out to swap.

When the time comes to bring a page into memory, you will need to know which physical pages are currently in use. One way to manage physical memory is to maintain a core map, a sort of reverse page table. Instead of being indexed by virtual addresses, a core map is indexed by its physical page number and contains the virtual address and address space identifier for the virtual page currently backed by the page in physical memory. When you need to evict a page, you look up the physical address in the core map, locate the address space whose page you are evicting and modify the corresponding state information to indicate that the page will no longer be in memory. Then you can evict the page. If the page is dirty, it must first be written to the backing store. In some systems, the writing of dirty pages to backing store is done in the background. As a result, when the time comes to evict a page, the page itself usually clean (that is, it has been written to backing store, but not modified since then). You could implement this mechanism in OS/161 by creating a thread that periodically examines pages in memory and writes them to backing store if they are dirty.

Your paging system will also need to support page allocation requests generated by kmalloc(). You should review kmalloc() to understand how these requests are generated, so that your system will respond to them correctly.

Testing malloc() and free()

OS/161 provides you with a malloc implementation. We will be getting a patch out with instructions soon, but if you want to tackle integrating malloc manually (not that hard) you can grab the source file by copying it from ~cs161/malloc/malloc.c. You'll need to add that file to lib/libc. (Don't forget to "cvs add".) Modify include/stdlib.h to add function declarations for malloc() and free(), and add the line SRCS+=malloc.c to lib/libc/Makefile. For the purposes of these problems, looking at ~cs161/malloc/malloc.c should be enough.

Now that OS/161 has paging, you can support applications with larger address spaces. The malloc() and free() library functions are provided in the standard C library. Read the code and answer the following questions in a file called malloc.txt:

Note that the operation of malloc() and free() is a standard job interview question -- you should understand this code!

You are responsible for making the malloc() we give you work; this will involve writing an sbrk() system call.

Instrumentation and Tuning

In this section, we ask you to tune the performance of your virtual memory system. Use the file performance.txt as your "lab notebook" for this section. You will undoubtedly want to implement some additional software counters. As a start, we suggest:

You should add the necessary infrastructure to maintain these statistics as well as any other statistics that you think you will find useful in tuning your system.

Once you have completed all the problems in this assignment and added instrumentation, it is time to tune your operating system.

At a minimum, use the matmult and sort programs provided to determine your baseline performance (the performance of your system using the default TLB and paging algorithms). Experiment with different TLB and paging algorithms and parameters in an attempt to improve your performance. As before, predict what will happen as you change algorithms and parameters. Compare the measured to the predicted results; obtaining a different result from what you expect might indicate a bug in your understanding, a bug in your implementation, or a bug in your testing. Figure out where the error is! We suggest that you tune the TLB and paging algorithms separately and then together. You may not rewrite the matmult program to improve its performance.

You should add other test programs to your collection as you tune performance, otherwise your system might perform will at matrix multiplication and sorting, but little else. Try to introduce programs with some variation in their uses of the memory system.

Provide a complete summary of the performance analysis you conduct. Include a thorough explanation of the performance benefits of your optimizations.

What to hand in

Design Document

Your design document must include:

Assignment code


The first step is understanding how TLB and page faults occur. To make this task easier, one person in your group should study TLB faults and the other should study page faults.

However, the time required to implement page fault handling is longer than the time required to implement TLB fault handling. You should divide the virtual memory implementation into several small and well-defined modules so that you can both work on it as soon as one of you has completed the TLB implementation. Get together as early as possible to share what you each have discovered.

Look at the code system161/src/mipseb/mips.c to see how TLB faults are generated. Then examine the vm_fault() handler in os161/src/kern/arch/mips/mips/dumbvm.c. What changes must you add to support TLB and page faults?

Some of the key issues are:

When you have completed your initial implementation of the TLB and virtual memory, one partner can begin experimenting with matmult while the other writes other test programs. Stay in touch and test each other's code.

Do your best to break your partner's implementation, then help him/her fix it. When you write test programs, think about verifying that your replacement algorithms are working correctly (i.e., "If I run this program it should generate exactly n TLB faults with your algorithm; does it?").

Review each other's designs. Think about how you might have implemented the different parts and compare ideas.

As your system begins to stabilize, begin to work on the performance tuning. Continue to test and fix bugs. Take turns testing and tuning. Be sure to keep a careful log of your performance experiments so you can trade it back and forth.