Nachos Instructional OS

1. Nachos Instructional OS CS 170, Tao Yang, Fall 2012

2. Why Nachos OS? • Learn by reading and modifying simple operating system code • Extremely important OS experience • A skeletal OS that supports • kernel threads, user-level processes • MIPS instruction execution as a virtual machine • ~9K lines of C++ code.

3. System Layers User process User process Nachos kernel threads Thread 1 Thread 2 Thread N Nachos OS modules (Threads mgm, File System, Code execution/memory mapping, System calls/Interrupt) Simulated MIPS Machine (CPU, Memory, Disk, Console) Base Operating System (Linux for our class)

4. Project 1 Tasks 1-3 main() Nachos threads ThreadTest() Thread 1 Thread 2 Thread N Nachos OS modules Simulated MIPS Machine Linux

5. Steps to Install Nachos • Obtain and install Nachos source code. • Copy the source code from ~cs170/nachosSept20.tar.gz • Compile the source code with Makefile • Run Nachos demo • Under the threads subdirectory (run threads) • Under the userprog subdirectory (compile/execute binary code) • Project 0: Form a group/turn-in proj 0. Due on Oct 10

6. Nachos code directory • machine --- Basic machine specification (MIPS simulator as a virtual machine). • threads --- threads management (Project 1). • userprog -- binary code execution and system calls (Project 2). • vm -- virtual memory (empty, Project 3). • filesys -- file system (Project 3) • test -- binary code to be executed in this virtual machine • network -- networking protocol (not used in this class) • bin -- utilities/tools (binary format conversion)

7. Source code reading and Project 1 Objectives of next 2 weeks • Scan through ~1,000-2,000 lines of code under threads directory • Learn how context switch is accomplished among threads. • Learn how thread scheduling is done. • Learn how locks/synchronization are implemented and used. • Use Linux pthreads. • Complete Project 1

8. Sample Example of Nacho Threads int shared=1; main () { Thread *t1 = new Thread("forked thread1"); Thread *t2 = new Thread("forked thread2"); t1->Fork(SimpleThread, 1); t2->Fork(SimpleThread, 2); SimpleThread(3); } SimpleThread(int i) { printf(“Hello %d. Shared %d\n”, i, shared); currentThread->Yield(); } Create 2 new threads. Start to fork and execute a function in each child thread. Parent also executes the same function Function executed by threads

9. Nachos Threads • Each thread has the following TCB (thread control block) from thread.h int *stackTop; // stack pointer int machineState[18]; // copy of registers int *stack // bottom of stack ThreadStatus status; //ready, running, or blocked char *name; • Thread operations: • Thread(char *debugName). Create a thread. • Fork(VoidFunctionPtr func, int arg). Let a thread execute a function. • Yield(). Suspend the calling thread and select a new one for execution. • Sleep(). Suspend the current thread, change its state to BLOCKED, and remove it from the ready list • Finish()

10. Nachos Thread States and Transitions Thread::Yield running (kernel) Thread::Sleep Scheduler::Run blocked ready Scheduler::ReadyToRun

11. Semaphore object for thread synchronization • Disable and re-enable interrupts to achieve mutual exclusion (e.g., by calling Interrupt::SetLevel()). • Operations for a Semaphore object: • Semaphore(char* debugName, int initialValue) • P(): Decrement the semaphore's count, blocking the caller if the count is zero. • V() :Increment the semaphore's count, releasing one thread if any are blocked waiting on the count.

12. Semaphore Implementation • Atomic  critical section • P(S) { while ( S <= 0) ; // wait S--; } • V(S) { S++; } • Issue in busy waiting • Waste time • Applications may spend lots of time in such a critical section

13. Semaphore Implementation with no busy waiting • Each semaphore has a waiting queue. • Two internal operations: • Sleep– place the thread invoking the operation on the waiting queue. • Nachos: Thread:Sleep() • wakeup– remove one thread in the waiting queue and place it in the ready queue.

14. Semaphore Implementation with no Busy waiting (Cont.) • P(semaphore *S) { S->value--; if (S->value < 0) { add this thread to waiting-queue; sleep(); } } • V(semaphore *S) { S->value++; if (S->value <= 0) { Remove x from waiting-queue; wakeup(x); }}

15. Reading source code • Have a picture of overall components and their roles. • Be able to compile and run a simple case. • Follow Project 0 and execute two cases. • Trace the code execution through a simple case • Basic/high level mechanism involved in execution. • Walk through a simple thread test case in Nachos. Week 2 link in http://www.cs.ucsb.edu/~cs170/refer.html

16. Project 1 Tasks 1-3 main() Nachos threads ThreadTest() Thread 1 Thread 2 Thread N Nachos OS modules Simulated MIPS Machine Linux

17. Key steps when Nachos executes After you type ``nachos'' under threads subdirectory: • It is executing as a single Unix process. • The main() calls • Initialize() to start up interrupt handling, create a scheduler for managing the ready queue. • ThreadTest() • currentThread->Finish() to let other threads continue to run.

18. Key Calling graph when Nachos executes under thread directory StackAllocate() in thread.cc All files are in threads directory. Thread:Fork () in thread.cc Initialize() in system.cc SWITCH () in switch.s FindNextToRun () in scheduler.cc Thread:Yield () in thread.cc main() in main.cc ThreadRoot () in switch.s ReadyToRun () in scheduler.cc ThreadTest () in threadtest.cc func() such as SimpleThread() in ThreadTest.cc Run () in scheduler.cc currentThread->Finish () in threadtest.cc

19. Key Calling graph for Project 1 Task 1 All files are in threads directory. Thread:Fork () in thread.cc Spawn multiple threads. Each executes a sample loop. Finally print shared variable at the end Initialize() in system.cc Thread:Yield () in thread.cc main() in main.cc ThreadTest () in threadtest.cc Semaphore P()/V() in synch.cc currentThread->Finish () in threadtest.cc

20. Key Calling graph for Project 1 Task 3 All files are in threads directory. Thread:Fork () in thread.cc Spawn multiple threads as test stations Each executes allocate() and release() operations Initialize() in system.cc main() in main.cc ThreadTest () in threadtest.cc Lock/conditions in synch.cc currentThread->Finish () in threadtest.cc

21. Scheduler object for thread scheduling • Decide which thread to run next • Invoked when the current thread gives up CPU. • The current Nachos scheduling policy is round-robin: • Select the front of ready list. • Append new threads to the end. • Key operations: • ReadyToRun(Thread *thread). Make thread ready to run and add to ready list. • Thread *FindNextToRun() • Run(Thread *nextThread)

22. Thread Switching • Suspend current thread, save its state, and restore the state of new thread. • Switch(oldThread, newThread): • Save all registers in oldThread's TCB. • to be used when the old thread is resumed. • Load new values into the registers from TCB of the new thread.

23. Single-threaded vs multithreaded Process

24. Key operations of SWITCH() Thread->Fork(func(), arg) Save current thread context Call InterruptEnable() SWITCH () in switch.s Load target thread context func(arg) Call func(arg) Call ThreadRoot () in switch.s Call ThreadFinish()

25. SWITCH(oldThread, nextThread)Save register values to current thread’s TCB • # a0 -- pointer to old thread’s TCB a1 -- pointer to new thread’sTCB sw sp, SP(a0) # save new stack pointer sw s0, S0(a0) sw s1, S1(a0) sw s2, S2(a0) sw s3, S3(a0) sw s4, S4(a0) sw s5, S5(a0) sw s6, S6(a0) sw s7, S7(a0) sw fp, FP(a0) sw ra, PC(a0) # save return address a0 TCB Registers save sp stackTop machineState[0] machineState[1] machineState[2] … save s0 save s1 save s2 …

26. Load register values from new thread’s TCB • # a0 -- pointer to old thread’s TCB a1 -- pointer to new thread’sTCB lw sp, SP(a1) # load the new stack pointer lw s0, S0(a1) lw s1, S1(a1) lw s2, S2(a1) … lw s7, S7(a1) lw fp, FP(a1) lw ra, PC(a1) # load the return address j ra #Call ra which is ThreadRoot(); a1 sp stackTop machineState[0] machineState[1] machineState[2] machineState[3] … machineState[9] s0 s1 s2 s3 ra

27. How TCB is initialized? • # a0 -- pointer to old thread’s TCB a1 -- pointer to new thread’sTCB lw sp, SP(a1) # load the new stack pointer lw s0, S0(a1) lw s1, S1(a1) lw s2, S2(a1) … lw s7, S7(a1) lw fp, FP(a1) lw ra, PC(a1) # load the return address j ra #Call ra which is ThreadRoot(); a1 sp stackTop func() arg ThreadFinish() InterruptEnable() … ThreadRoot() addr s0 s1 s2 s3 ra

28. Which routine fills values of the new thread TCB? • StackAllocate() in Thread::Fork() machineState[PCState] = (int) ThreadRoot; // PCState=9 machineState[StartupPCState] = (int) InterruptEnable; //StartupPCState=3 =>s3 machineState[InitialPCState] = (int) func; //InitialPCState=0 =>s0 machineState[InitialArgState] = arg; //InitialArgState=1 =>s1 machineState[WhenDonePCState] = (int) ThreadFinish; //WhenDonePCState=2 =>s2 • bb

29. ThreadRoot() uses registers s0, s1,s2,s3 to execute targeted func() jal StartupPC # call InterruptEnable() stored in register s3 move a0, InitialArg #move argument in s1 to a0 jal InitialPC # call main thread procedure func() stored in s0 jal WhenDonePC # when done, call ThreadFinish() in s2

30. Project 1 Objectives • Tasks 1-3 under threads subdirectory. • Gain experience with simple thread programming (execute multiple Nachos threads with synchronization). • Implement locks and condition variables (missing from the file synch.cc). • Use for laundromat application • Task 4 under hw1_task1_pthreads subdirectory • Show that Pthreads code works similarly

31. Tasks 1/2/3: Nachos Files involved Key files to read and modify (modify 3 files in red) • main.cc, threadtest.cc -- a simple test of our thread routines. • thread.h thread.cc -- Nachos thread data structure and operations. • scheduler.h scheduler.cc -- The thread ready list. • synch.h synch.cc -- synchronization routines. • system.h, system.cc -- Nachos startup initialization /shutdown • switch.h, switch.s -- assembly code for thread switching. Other related files: • synchlist.h, synchlist.cc -- synchronized access to lists using locks/conditions (useful examples for your programming). • list.h list.cc -- generic list management. • utility.h utility.cc -- some useful definitions and debugging routines. • interrupt.h interrupt.cc -- manage interrupts. • time.h timer.cc -- clock emulation. • stats.h stats.cc -- collect interesting statistics.

32. Sample/Makefile, and test script [cs170@csil PROJECT1]$ more testscript make clean; make threads/nachos > Task1-1_trace make clean; make "DEFINES = -DTHREADS -DHW1_SEMAPHORES" threads/nachos > Task1-2_trace make clean; make "DEFINES = -DTHREADS -DHW1_LAUNDRY_LC" threads/nachos > Task3_trace cd hw1_task1_pthreads make; make run >Task4_trace

33. Sample Code for Tasks 1/2/3 ~cs170/sample/PRJECT1/ • Has an incomplete sample solution for Project 1 • <~50 lines of sample code are removed. • Removed positions are marked • Caveat: • Not well documented, not fully tested. Possibly awkward style/design • Still your responsibility to produce good solutions (correctness, performance, style) with at least 50% difference

34. Task 1 (1): Use Nachos threads and explain behavior • int SharedVariable; void SimpleThread(int which) { int num, val; for(num = 0; num < 5; num++) { val = SharedVariable; printf("*** thread %d sees value %d\n", which, val); currentThread->Yield(); SharedVariable = val+1; currentThread->Yield(); } val = SharedVariable; printf("Thread %d sees final value %d\n", which, val); }

35. Task 1 (2): Use Nachos semaphores to synchronize • int SharedVariable; void SimpleThread(int which) { int num, val; for(num = 0; num < 5; num++) { val = SharedVariable; printf("*** thread %d sees value %d\n", which, val); currentThread->Yield(); SharedVariable = val+1; currentThread->Yield(); } val = SharedVariable; printf("Thread %d sees final value %d\n", which, val); } Critical section needs a mutual execution Need a barrier so every thread sees the same value of “SharedVariable”

36. Task 2: Lock/condition variable implementation • Implement Lock code in synch.cc similar as Nachos semaphore code • Sample solution is available • Implement conditional variables • A few lines were removed from the sample code.

37. Task 4: Use Pthreads instead Nachos for Task 1 (2) Use pthread_create() to execute • int SharedVariable; void SimpleThread(int which) { int num, val; for(num = 0; num < 5; num++) { val = SharedVariable; printf("*** thread %d sees value %d\n", which, val); currentThread->Yield(); SharedVariable = val+1; currentThread->Yield(); } val = SharedVariable; printf("Thread %d sees final value %d\n", which, val); } Use pthread_yield() Use pthread_mutex_lock/unlock(), Barrier using pthread_mutex_lock()/unlock() pthread_cond_wait(),/cond_broadcast()

38. Implementation Strategy for Project 1 • Read key Nachos files listed in the previous slide. • Find difference between sample solution with the existing code: main.cc, threadtest.cc, synch.cc. Find what is removed (search “removed”) • Implement Task 1 using semaphore • Implement Task 2 using code similar to semaphore implementation in synch.cc (check how sample solution implements lock). • Implement Task 3 using lock/conditions with 2 stations (sample solution). Then test with 3 stations. • For Task 4, study related Pthread functions, model Task 1 using Pthreads, and synchronize with its lock/condition variables