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Process Synchronization
Indranil Gupta
Lecture 8
Sep 12, 2005
CS423UG Operating Systems
CS 423UG - Operating Systems, Indranil Gupta 2
A Small Puzzle
x:=x+1; x:=x-
â˘x initially is == 1 â˘x is shared between the two processes
- What is the value of x after both processes complete?
Producer process Consumer process
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Agenda
Data Races
Critical region and mutual exclusion
Mutual exclusion using busy waiting
Disabling Interrupts
Lock Variables
Strict Alternation
Petersonâs solution
TSL
Sleep and Wakeup
Quiz Discussion
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A few things we know nowâŚ
1. âProcessâ: a program in execution
2. âThreadâ: a light weight process
3. âSchedulingâ:
Which process/thread (from ready queue)
should run?
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Inter-Process Communication (IPC)
Communication
Processes pass information to each other
Mutual exclusion & Synchronization
Correct coordination among processes
Also applies to threads
âCorrect Coordinationâ?! Whatâs the
problem, exactly?
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Data Races: A simple game
Two volunteers:
Producer: produce 1 card per iteration
Step1: increment a (shared) counter
Step2: put a card on the table
Consumer:
Step1: check the counter to see if it is zero
Step2a: if the counter is zero, go back to step
Step2b: if the counter is nonzero, take a card from
the table
Step3: decrement counter
I am the OS, and I schedule
I decide who should run when, who should stop
when
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What can Go Wrong?
Scheduler Stops Producer before Step 2 and let
Consumer run.
What happens?
Two volunteers
Producer: produce 1 card per iteration Step1: increment the counter Step2: put the card on the table Consumer: Step1: check the counter to see if it is zero Step2a: if the counter is zero, go back to step Step2b: if the counter is nonzero, take a card from the table Step3: decrement the counter
Context switch
Consumer gets stuck in Step 2b! CS 423UG - Operating Systems, Indranil Gupta 8
Letâs Reorder Producerâs Instructions
Scheduler Stops Producer before Step 2 and let
Consumer run.
What happens?
Two volunteers
Producer: produce 1 card per iteration
Step1: put the card on the table Step2: increment the counter
Consumer:
Step1: check the counter to see if it is zero Step2a: if the counter is zero, go back to step Step2b: if the counter is nonzero, take a card from the table Step3: decrement the counter
What could go wrong with this example?
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âIncrement/Decrement the Counterâ
x:=x+1; compiles to
Load Reg, x
Inc Reg
Save x, Reg
x:=x-1; compiles to
Load Reg, x
Dec Reg
If the initial value of x was 1, after Save x, Reg this intervleaving, it will be 0 (should have been left as 1)
Producer Consumer
Suppose the counter is at memory address âxâ
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Those were Examples of Data Races
Reason: data sharing
Previous game: producer and consumer
Share the counter
Share the cards
Examples
Thread: two threads spawned within Apache
web server both try to post (http) to a common
webpage
Process: two processes both try to print by
accessing a queue with in and out variables
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Spooling Example: Correct
Shared memory (queue)
abc Prog.c Prog.n
F
âŚ
⌠out
in
Process 1 Process 2
next_free = in;
next_free = in
int next_free;
Store F1 into next_free;
Store F2 into next_free;
int next_free;
in=next_free+
F
3 in=next_free+
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Spooling Example: Wrong (Race)
Shared memory
abc Prog.c Prog.n
âŚ
⌠out
in
Process 1
next_free = in;
next_free = in /* value: 7 */
int next_free;
Stores F1 into next_free;
Stores F2 into next_free;
in=next_free+
F
4 in=next_free+
5
F
Process 2 int next_free;
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III. Strict Alternation
thread me; /* For two threads */
{ while (true) {
while ( turn != my_thread_id) { }; Access shared variables; // Critical Section; turn = other_thread_id;
Do other work } }
Satisfies Mutual Exclusion but not Progress.
Why?
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IIIb. Using Flags
int flag[2]= {false, false};
thread me;
while (true)
flag[my_thread_id] = true;
while (flag[other_thread_id] ) { };
Access shared variables; // Critical Section;
flag[my_thread_id] = false;
Do other work
No Progress. Could block indefinitely. Why?
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IV. Petersonâs Solution int flag[2]={false, false}; // shared array int turn; // shared variable { while (true) { flag[my_thread_id] = true; turn = other_thread_id; while (flag[other_thread_id] and turn == other_thread_id ) { }; Access shared variables; // Critical Section flag[my_thread_id] = false; Do other work } }
This works!!! Why?
Questions to ask: What happens whenâŚ
- One process tries to enter, and the other process tries to enter later?
- Both processes try to enter simultaneously?
EXIT
ENTER
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V. Test & Set (TSL)
Requires hardware support
Atomically tests if a variable is 0, and sets
it to 1 (one instruction, not separable, i.e.,
atomic)
char test_and_set (Rx, char* target); // All done atomically { Rx=*target; *target = 1; }
TSL Rx, target
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Using the TSL
instruction
Does this work? Questions to ask: What happens whenâŚ
- One process tries to enter, and the other process tries to enter later?
- Both processes try to enter simultaneously?
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Other Similar Hardware Instructions
swap
void swap (char* x,* y);
//*y==1. All done atomically
{ char temp = *x;
*x = *y;
*y = temp
}
Also â˘Intel x86 XCHG (exchange) instruction â˘SPARC CASA and CASXA instructions Similar to TSL, but with deals with memory
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VI. Sleep and Wakeup
Problems with previous solutions:
Busy waiting is wasteful
Wastes CPU resources and time
Could cause Priority Inversion (aha, Mars
Pathinder problem!):
a high priority process waits for a low priority process
to leave the critical section
the low priority process can never execute since the
high priority process is not blocked
Solution: sleep and wakeup (syscall
supported)
When blocked, go to sleep
Wakeup when it is OK to retry entering the critical
section CS 423UG - Operating Systems, Indranil Gupta^26
- In a computer, there are 20
ready'' processes, 5blocked''processes and 4 ``running'' processes. The computer has: A. 1 CPU B. 2 CPU's C. 3 CPU's D. 4 or more CPU's - Which of the following instructions should be privileged(checked thoroughly before allowing a process to execute)? A. Read value of timer B. Read the clock C. Clear the memory D. Yield the CPU to scheduler
- When a CPU divides by zero, what happens IMMEDIATELYafterward? A. The computer crashes. B. A TRAP is executed. C. The CPU kills the process. D. None of the above.
- Which of the following is NOT a process? A. A Firefox browser. B. A Command shell. C. A File. D. Acrobat PDF reader.
- Which of the following is common to two different user-level threads that belong to the same process? A. Program Counter. B. Stack. C. Address Space. D. None of the above.
- Which of the following will IMMEDIATELY change the PCB of a process? A. The CPU context switches the process out from the CPU. B. The process executes an instruction. C. The process initiates a system call. D. The CPU, while executing the process, receives an interrupt from the disk device for a request by another process.
- Consider a system where no users are logged in and whereno useful processes have been started up by the OS. What is the CPU doing? A. The CPU may be sleeping. B. The CPU is busy-waiting (a while(1) loop). C. The CPU is running a dummy OS process that does nothing useful. D. Any of the above.
- One of the 3 threads in a process calls fork(). When thesystem call returns successfully, how many NEW threads does the system contain? All threads are kernel-levelthreads (assume Solaris-style threads). A. 6 B. 3 C. 1 D. None of the above.
- Which of the following metrics is the LEAST important for batch systems? A. Turnaround Time B. Response Time C. Waiting Time D. Efficiency
- Which of the following processes is typically the MOST I/O- bound? A. Internet Explorer Browser (text pages) B. Unix Command shell C. Emacs editor window D. Windows Media Player (MPEG player)
QUIZ 1 Solutions
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Reminder
Reading for this lecture was: Sections
2.3.0-2.3.
Reading for next lecture: Sections 2.3 &
More useful solutions to synchronization
MP
