03/25/22 1 extension for the rest of quarter) Project 2 final deadline is Tuesday midnight May 19 Project 0 resubmission for autograding : June 1 (changed) Project 0 score =max(old score, old score *0.10 + new score *0.90). Donot print “shell>” prompt. Project 3A (May 29). Harness code is released and will be updated. Optional Project 3B (June 4). -- You can use Project 3B to replace midterm OR one of project scores: Project 1, 2, 3A.
Transcript
04/19/23 1
Announcement (No deadline extension for the rest of quarter)
Project 2 final deadline is Tuesday midnight May 19 Project 0 resubmission for autograding : June 1
(changed) Project 0 score =max(old score, old score *0.10 +
new score *0.90). Donot print “shell>” prompt.
Project 3A (May 29). Harness code is released and will be updated.
Optional Project 3B (June 4).-- You can use Project 3B to replace midterm OR one of project scores: Project 1, 2, 3A.
Project 3A
CS 170, Tao Yang
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High-level Summary Objective: Virtual memory management for program
execution when memory is not enough. vm directory for 3A is empty
Need to come up a design and code you donot have to follow harness code
Test C programs ~cs170/nachos-projtest/proj3a Amount of work
~300 lines or less of code in vm ( few changes in userprog)
Simplification is allowed using Linux file I/O instead of Nachos file I/O in swap space management
Initially allocate 0 memorypage to program B.Store B’s pages in swap space
Sawp in/swap out
Virtual Memory Managerfree page management
replacement managementswap-in/out
Disk
DirectoryFiles
Swap Space512 sectors(128B/sector)
Swap Space Managerget/put/free
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Example 1 Assume SWAP has 10 free sectors. Memory has 3 free
pages. Program A’s virtual pages: 0, 1, 2,3 Use a random replacement policy Load binary of program A:
Allocate 4 sectors in SWAP: 4, 5, 6,7. Copy virtual page content of A to sectors 4, 5, 6, 7 in SWAP Notice NO memory pages are allocated to program A.
Execute program A and read code in virtual addr 0. Page fault need to fetch virtual page 0 of A Allocate one free memory page to virtual page 0.
Allocate Frame #1 Load sector #4 from disk to Frame #1.
Resume the instruction execution at virtual addr 0.
Example 1 with 3-page memory and10-sector SWAP
DirectoryFiles Program A binary
SWAP with 10 sectors4,5,6,7 usedfor Program A: Vpage 0, 1, 2,3
0
1
2
Memory frames
0 Invalid1Invalid2Invalid3Invalid
Page table A
Disk
Program A needs 4 virtual pages
Example 1 with 3-page memory and10-sector SWAP
DirectoryFiles Program A binary
SWAP with 10 sectors4,5,6,7 usedfor Program A: Vpage 0, 1, 2,3
0
1
2
Memory frames
0 11Invalid2Invalid3Invalid
Page table A
Disk
Program A needs 4 virtual pages
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Example 2 Assume SWAP has 6 free sectors left.
Program A’s pages occupy Sectors #4,5,6,7. Memory is used by Program A’s virtual pages: 0, 1, 2
Occupy frames 1, 0, 2 Access virtual address of A in 3*128 +2:
That is virtual page #3. Identify frame #0 as victim Copy out content of Frame #0 (virtual page #1 of A) to Sector #5 in
SWAP Copy Sector #7 from SWAP to Frame #0. Set up page table entry for the victim address space and current
address space properly. Resume the instruction execution to access virtual addr
3*128+2.
Example 2: Program A needs Virtual Page 3
DirectoryFiles Program A binary
SWAP with 10 sectors4,5,6,7 usedfor Program A: Vpage 0, 1, 2,3
0
1
2
Memory frames
0 11 02 23Invalid
Page table A
Disk
Frame #0 (for page #1 of A)is victim
Example 2 after virtual page 3 is loaded
DirectoryFiles Program A binary
0
1
2
Memory frames
0 11 invalid2 23 0
Page table A
Swap out
Swap inSWAP with 10 sectors4,5,6,7 usedfor Program A: Vpage 0, 1, 2,3
Disk
Frame #0 (for page #1 of A)is victim
Example 3: Program B is loaded
Directory Program B binary Program A binary
0
1
2
Memory frames
0 11 invalid2 23 0
Page table A
SWAP with 10 sectors0, 1, 2 usedfor program B.4,5,6,7 used for Program A
Disk
0 invalid1 invalid2 invalid
Page table B
Program B needs 3 virtual pages
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Example 3 Assume Program B is loaded by Exec()
Program B needs 3 pages. Allocate 3 free sectors in SWAP. Program B’s pages occupy Sectors #0, 1,2
Access virtual address 0 of B: That is virtual page #0 of B Identify Frame #2 as victim Copy out content of Frame #2 (virtual page #2 of A) to Sector #6 in
SWAP Copy Sector #0 from SWAP to memory frame #2. Set up page table entry for the victim address space and
current address space properly Resume the instruction execution to access virtual addr 0 of
program B
Example 3: after addr 0 of Program B is accessed
Directory Program B binary Program A binary
0
1
2
Memory frames
0 11 invalid2 invalid3 0
Page table A
SWAP with 10 sectors0, 1, 2 usedfor program B.4,5,6,7 used for Program A
Disk
0 21 invalid2 invalid
Page table B
Frame #2 (for page #2 of A)is victim
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How to access a sector in SWAP? Page size = sector size =128 bytes Access sector x
Open the SWAP file Seek position: x*128 in the SWAP file Length 128. Functions to write/read sector content
Use Nachos OpenFile’s ReadAt(), WriteAt() Or Linux file read/write.
How to determine a sector is available? Use a bitmap. Or any method you want.
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How to set up the initial content of virtual page?
See Project 2 implementation/solution on how to set up n virtual pages of an address space (addrspace.cc) Read the binary from disk Set up code/data/stack etc.
For project 3A, instead of allocating n memory frames for this address space Allocate n free disk sectors in SWAP Copy content of n virtual pages one by one to these
sectors using Write(). If duplicating a child address space from a parent space, make
sure you use the latest in-memory copy of parent pages if they have been modified (dirty).
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Project 3A: Virtual Memory Work on vm subdirectory mainly
+ addrspace.cc/.h and exception.cc in userprog Create/manage a backing store (a file called SWAP
using the OpenFile class). Implement a page fault handler with dirty bit handling
and a page replacement policy (LRU or second chance) Test under various conditions:
One process with an address space larger than physical memory.
Concurrent processes with combined address space larger than physical memory.
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Report to be submitted P3A_WRITEUP1. Summarize what is completed, what is
not.2. describe the design of VM
Describe design options and their trade-offs. List/explain main components and code
modification in implementing your design
3. Summarize the test effort (what is passed, the purpose of each test)
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Start with Sample Code Makefiles + harness code
SWAP space manager (but Dani combines this manager with VM page manager)
Used to maintain information about pages that are currently in swap space.
Flag dirty pages. VM page manager
Handle a page fault at particular address. Identify a victim page by LRU or second-
chance code. Swap-out/swap-in
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Questions How many pages to allocate when handling
Exec(binary) or Fork(function)? 0, proportionally, completely? If not, where are new or updated pages stored?
When will a page fault exception be generated? Where to check if a page is not in memory?
Find a physical page Who manages a set of memory pages used? How to find a free page (victim page)? When to actually do a swap out (write to disk)? Where to get page content from disk?
Getting Started Read machine/translate.cc and machine.cc:
In Machine:Translate() for virtual address translation, PageFaultException is detected when the desired page is not in memory.
In Machine:ReadMem, Translate() is called for translating the desired virtual memory address and machine->RaiseException() is called with PageFaultException error.
In Machine:RaiseException() registers[BadVAddrReg] stores bad address. Change to system mode. Call ExceptionHandler
Assembly code: read data
Translate Addr
Page fault
What is next
Read mipssim.cc Machine->ReadMem() is called in executing
each instruction. If PageFaultException is detected, the
exception handler should load the desired page. The hardware will try again.
Need to expand exception.cc to handle PageFaultException. Once handled, return to user mode and restart
the instruction caused the fault
User Instruction Execution
Machine:Translate()
ExceptionHandler()Deal with
PageFaultException
Cannot find this page?Raise PageFaultException
Page writing?
Set dirty bit
Machine:Run ()
OneInstruction ()
ReadMem () WriteMem ()
Re-execute if Exception is raised
Files to be modified for Part A
New files in directory vm Virtual memory manager Swap space manager
Directory userprog (extension to Project 2) exception.cc
Extension to handle PageFaultException Addrspace.cc/.h
Prepare code/data for SWAP backstore. Virtual address translation -> paging if needed
Page Fault Handling
Write a Swap Manager to facilitate free sector allocation in Swap File (may be combined with VM manager) Initialize SWAPSIZE (512) as the total free sectors
available. Allocate a sector. Free a sector.
Virtual memory manager Find a free memory page or a victim page Swap out a selected page to SWAP if needed load a page containing the virtual address (copy
from the SWAP file to an allocated memory page). Adjust the page table.
Modify AddSpace.cc
In translating a virtual user address for kernel, if it is not in memory, bring data from SWAP.
When allocating a new address space, Allocate a proper number of sectors from
SWAP for this process. Copy binary data to the allocated SWAP
sectors. Initial # of pages for this process can be 0
Synchronization issues
• Two system calls may be processed concurrently and the synchronization is needed. Any time a process sleeps in the
kernel, another process can run Two processes try to initiate I/O on the same
page at the same time May need a lock for each physical
memory page, and possibly also for each virtual memory page.
