Nachos Instructional OS and Project 1

CS 170, Tao Yang

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Why Nachos OS? Learn by reading and modifying simple

operating system code Extremely important OS experience Deep understanding of OS features/system calls

Where/when they can fail, or have poor performance

A skeletal OS that supports kernel threads, user-level processes MIPS instruction execution as a virtual

machine ~9K lines of C++ code.

System Layers

Base Operating System (Linux for our class)

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)

User processUser process

Project 1B Tasks 1-3

Linux

Nachos threads

Thread 1 Thread 2 Thread N

Nachos OS modules

Simulated MIPS Machine

main()

ThreadTest()

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Steps to Install Nachos Obtain and install Nachos source code.

Copy the source code from ~cs170/nachos2015.tar

Compile the source code with Makefile Run Nachos demo

Under the threads subdirectory (run threads)

Under the userprog subdirectory (compile/execute binary code)

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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 3A). filesys -- file system (Project 3B) test -- binary code to be executed in this virtual

machine network -- networking protocol (not used in this class) bin -- utilities/tools (binary format conversion)

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Source code reading and Project 1

Objectives of next 2 weeks Scan through ~1,000 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

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();}

Sample Example of Nacho Threads

Start to fork and execute a function in each child thread.

Parent also executes the same function

Function executed by threads

Create 2 new threads.

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Nachos Threads

Each thread has the following TCB (thread control block) from thread.h

int *stackTop; // stack pointerint machineState[18]; // copy of registersint *stack // bottom of stackThreadStatus status; //ready, running, or blockedchar *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()

Nachos Thread States and Transitions

running(kernel)

readyblocked

Scheduler::Run

Scheduler::ReadyToRun

Thread::Sleep

Thread::Yield

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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.

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.

Semaphore Implementation with no Busy waiting (Cont.)

P(semaphore *S) Disable-interrupt

S->value--; if (S->value < 0) { Add this thread to waiting-queue;

sleep(); } Enable-interrupt

V(semaphore *S) Disable-interrupt

S->value++; if (S->value <= 0) {

Remove x from waiting-queue;

wakeup(x); } Enable-interrupt

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Reading source code

Have a picture of overall components and their roles.

Be able to compile and run a simple case. 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

Project 1A: Concurrent Hash Table

Many pthreads

What each thread test calls?Many pthreads

void *tfunc(void *arg){long k= (long) arg; //thread IDinti, b=NUMKEYS/NumberofThread;for (i= b *k; i< b*k+b && i<NUMKEYS; i++)

{test1(&hash, NUMKEYS, i,2);

}}void test1(HashMap *htable, int n, int k, int w){

int i, errorflag=0;for (i=0; i<n; i++)

(*htable).get(i);for (i=k-w; i<k+w; i++) (*htable).put(i,i);if((*htable).get(k)==-1)

errorflag=1;(*htable).remove(k);for (i=0; i<n; i++)

(*htable).get(i);(*htable).put(k,k);for (i=0; i<n; i++)

(*htable).get(i);}

get()put()remove()

Coarse-grain synchronization of concurrent hash table access

Many pthreads

Fine-grain synchronization

Many pthreads

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Part B of 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 concurrent access of a hash table

Project 1B Tasks 1-3

Linux

Nachos threads

Thread 1 Thread 2 Thread N

Nachos OS modules

Simulated MIPS Machine

main()

ThreadTest()

Hashtable

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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.

TA Varun provides a sample change template for threadtest.cc

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.

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Tasks of Project 1 (Part B)

Task 1 Synchronize hash table (fine-grain) with

semaphore Task 2.

Implement Lock code in synch.cc similar as Nachos semaphore code

Follow the Nachos semaphore implementation Synchronize hash table (fine-grain)

Task 3 Implement conditional variables Synchronize hash table (fine-grain)

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Key steps when Nachos executes

After you type ``nachos'' under threads subdirectory:

It is executing as a single Linux 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.

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Current Nachos Threadtest.cc

void SimpleThread(int which) { int num, val;

for(num = 0; num < 5; num++) {printf("*** thread %d looped %d times\n", which,

num); c currentThread->Yield(); }} *** thread 0 looped 0 times

*** thread 1 looped 0 times*** thread 0 looped 1 times*** thread 1 looped 1 times*** thread 0 looped 2 times*** thread 1 looped 2 times*** thread 0 looped 3 times*** thread 1 looped 3 times*** thread 0 looped 4 times*** thread 1 looped 4 timesNo threads ready or runnable, and no pending interrupts.Assuming the program completed.Machine halting

void ThreadTest1() { Thread *t = new Thread("forked thread"); t->Fork(SimpleThread, 1); SimpleThread(0); }

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Key calling graph when Nachos executes under thread directory

All files are in threads directory.

main() inmain.cc

Initialize()in system.cc

ThreadTest ()in threadtest.cc

Thread:Yield ()in thread.cc

Thread:Fork ()in thread.cc

StackAllocate()in thread.cc

FindNextToRun ()in scheduler.cc

ReadyToRun ()in scheduler.cc

Run ()in scheduler.cc

SWITCH ()in switch.s

ThreadRoot ()in switch.s

func() such as SimpleThread()in ThreadTest.cc

currentThread->Finish ()in threadtest.cc

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Key Calling graph for Project 1 Task 1

All files are in thread directory.

main() inmain.cc

Initialize()in system.cc

ThreadTest ()in threadtest.cc

Thread:Yield ()in thread.cc

Thread:Fork ()in thread.cc

currentThread->Finish ()in threadtest.cc

Spawn multiple threads similar to ptest.cc in Part A(hashtable test)

Insert some Yield()

Semaphore P()/V()in synch.cc

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Nachos 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 queue 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)

Nachos scheduler

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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.

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Key operations of Nachos’ function SWITCH()

SWITCH ()in switch.s

Thread->Fork(func(), arg)

func(arg)

Call ThreadRoot ()in switch.s

Save current thread context

Load targetthread context

CallThreadFinish()

Call InterruptEnable()

Callfunc(arg)

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TCB of Nachos Threads

Each thread has the following TCB (thread control block) from thread.h

int *stackTop; // stack pointerint machineState[18]; // copy of registersint *stack // bottom of stackThreadStatus status; //ready, running, or blockedchar *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()

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Thread Switching

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.

int *stackTop;int machineState[18]int *stack ThreadStatus status;

int *stackTop;int machineState[18]int *stack ThreadStatus status;

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SWITCH(oldThread a0, nextThread a1)Save register values to current thread’s TCB

# a0 -- pointer to old thread’s TCB

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

sp

s0

s1

s2

…

stackTopmachineState[0]machineState[1]machineState[2]…

TCB

save

save

save

save

MIPS Registers

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SWITCH(oldThread a0, nextThread a1)

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

sp

s0

s1

s2

…

stackTopmachineState[0]machineState[1]machineState[2]…

TCB

save

save

save

save

MIPS Registers

switch.h#define SP 0#define S0 4#define S1 8#define S2 12#define S3 16#define S4 20#define S5 24#define S6 28#define S7 32#define FP 36#define PC 40

What is value of SP and S0?

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Load register values from new 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();

sp

s0

s1

s2

s3

stackTopmachineState[0]machineState[1]machineState[2]machineState[3]…

machineState[9]

ra

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Key operations of Nachos’ function SWITCH()

SWITCH ()in switch.s

Thread->Fork(func(), arg)

func(arg)

Call ThreadRoot ()in switch.s

Save current thread context

Load targetthread context

CallThreadFinish()

Call InterruptEnable()

Callfunc(arg)

ThreadRoot

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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();

sp

s0

s1

s2

s3

stackTopfunc() argThreadFinish()InterruptEnable()…

ThreadRoot() addr

ra

a1

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Which routine fills values of the new thread

TCB?

StackAllocate() in Thread::Fork()

machineState[PCState] = (int) ThreadRoot; // PCState=9machineState[StartupPCState] = (int) InterruptEnable;

//StartupPCState=3 =>s3machineState[InitialPCState] = (int) func; //InitialPCState=0 =>s0machineState[InitialArgState] = arg; //InitialArgState=1 =>s1machineState[WhenDonePCState] = (int) ThreadFinish;

//WhenDonePCState=2 =>s2

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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