7/3/20151 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 * 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

7/3/20153 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

Test program that needs more than 32 pages #include "syscall.h" char array[128*32]; main() { char *str = "Hello world!\n"; array[0]=‘a’; Write(hello_str, strlen(str)+1, 1); } C program binary Project 2 code VM management Execution result nachos –x binary

Sawp in/swap out Bring a page into memory ONLY when it is needed Disk Initially allocate 0 memory page to program B. Store B’s pages in swap space

Sawp in/swap out Virtual Memory Manager free page management replacement management swap-in/out Disk Directory Files Swap Space 512 sectors (128B/sector) Swap Space Manager get/put/free

7/3/20157 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 and 10-sector SWAP Directory Files Program A binary SWAP with 10 sectors 4,5,6,7 used for Program A: Vpage 0, 1, 2, Memory frames 0 Invalid 1Invalid 2Invalid 3Invalid Page table A Disk Program A needs 4 virtual pages

Example 1 with 3-page memory and 10-sector SWAP Directory Files Program A binary SWAP with 10 sectors 4,5,6,7 used for Program A: Vpage 0, 1, 2, Memory frames 0 1 1Invalid 2Invalid 3Invalid Page table A Disk Program A needs 4 virtual pages

7/3/ 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 Directory Files Program A binary SWAP with 10 sectors 4,5,6,7 used for Program A: Vpage 0, 1, 2, Memory frames Invalid Page table A Disk Frame #0 (for page #1 of A) is victim

Example 2 after virtual page 3 is loaded Directory Files Program A binary Memory frames invalid Page table A Swap out Swap in SWAP with 10 sectors 4,5,6,7 used for 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 Memory frames invalid Page table A SWAP with 10 sectors 0, 1, 2 used for program B. 4,5,6,7 used for Program A Disk 0 invalid 1 invalid 2 invalid Page table B Program B needs 3 virtual pages

7/3/ 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 Memory frames invalid 2 invalid 3 0 Page table A SWAP with 10 sectors 0, 1, 2 used for program B. 4,5,6,7 used for Program A Disk invalid 2 invalid Page table B Frame #2 (for page #2 of A) is victim

16 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.

17 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).

7/3/ 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.

7/3/ Report to be submitted P3A_WRITEUP 1. 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)

7/3/ 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

7/3/ 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.