1. Concurrency, Processes and Threads
© 2004, D. J. Foreman

2. Concurrency
The appearance that multiple actions are occurring at the same time
On a uni-processor, something must make that happen
A collaboration between the OS and the hardware
On a multi-processor, the same problems exist (for each CPU) as on a uni-processor
© 2004, D. J. Foreman

3. Multiprogramming … Combines multiplexing types:
Space-multiplexing - Physical Memory
Time-multiplexing - Physical Processor
Process0
Process1
Processn …
© 2004, D. J. Foreman

4. Multiprogramming-2 Multiprogramming Reason why desired
N programs apparently running simultaneously
space-multiplexed in executable memory
time-multiplexed across the central processor
Reason why desired
Greater throughput (work done per unit time)
More work occurring at the same time
Resources required
CPU
Memory
© 2004, D. J. Foreman

5. The CPU Instruction cycles Modes of execution
Access memory and/or registers
Sequential flow via "instruction register"
One instruction-completion at a time
(Pipelines only increase the # of completions per time unit). They are still sequential!
Modes of execution
Privileged (System)
Non-privileged (User )
© 2004, D. J. Foreman

6. Memory Sequential addressing (0 – n) Partitioned System User
Inaccessible by user programs
User
Partitioned for multiple users
Accessible by system programs
© 2004, D. J. Foreman

7. Processes-1 A Process is
A running program & its address space
A unit of resource management
Independent of other processes
NO sharing of memory with other processes
May share files open at Fork time
One program may start multiple processes, each in its own address space
© 2004, D. J. Foreman

8. Processes-2 Abstraction
Memory
Process-1
Process-n
Instruction stream
CPU
Data stream
Operating System
© 2004, D. J. Foreman

9. Process & Address Space
Resources
Data
Resources
Code
Resources
Stack
Abstract Machine Environment
Address Space
© 2004, D. J. Foreman

10. Processes-3 The Process life-cycle Creation Execution Termination
User or scheduled system activation
Execution
Running
Performing instructions (using the ALU)
Waiting
Resources or Signals
Ready
All resources available except memory and ALU
Termination
Process is no longer available
© 2004, D. J. Foreman

11. Processes-4 Space multiplexing More on this later
Each process operates in its own "address space"
Address space is a sequence of memory locations (addresses) from 0 to 'n' as seen by the application
Process addresses must be "mapped" to real addresses in the real machine
More on this later
© 2004, D. J. Foreman

12. Processes-5 Time multiplexing
Each process is given a small portion of time to perform instructions
O/S controls the time per process and which process gets control next
Many algorithms for this
No rules (from user's/programmer's view) on which process will run next or for how long
Some OS's dynamically adjust both time and sequence
© 2004, D. J. Foreman

13. Processes-7 FORK (label) QUIT() JOIN (count) (an atomic operation)
Starts a process running from the labeled instruction – gets a copy of address space
QUIT()
Process terminates itself
JOIN (count) (an atomic operation)
Merges >=2 processes
Really more like "quit, unless I'm the only process left"
© 2004, D. J. Foreman

14. Threads-1
A unit of execution within a process (like a lightweight process – an "lwp") also called a "task"
Share address space, data and devices with other threads within the process
Private stack, status (IC, state, etc)
Multi-threading
>1 thread per process
Limited by system to some max #
Per system
Per process
© 2004, D. J. Foreman

15. Thread Models JRE DOS Classic UNIX WinXX, Solaris, Linux, OS/2
© 2004, D. J. Foreman

16. Threads-2 Several thread API's
Solaris: kernel-level threads & pthreads
Windows: kernel-level threads & pthreads
OS/2: kernel-level threads
Posix (pthreads) – full set of functions
#include <pthread.h> // for C, C++
Allows porting without re-coding
Java threads implemented in JVM, independent of OS support
Like multiprogramming implementation in Win3.1
Uses underlying kernel support where available
© 2004, D. J. Foreman

17. Threads-3 Windows (native) POSIX (Linux, Solaris, Windows)
CreateThread( DWORD dwCreateFlags = 0, UINT nStackSize = 0, LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL );
POSIX (Linux, Solaris, Windows)
iret1 = pthread_create( &thread1, NULL, (void*)&print_message_function, (void*) message1);
© 2004, D. J. Foreman

18. Threads-4 Advantages of kernel-supported threads:
May request resources with or without blocking on the request
Blocked thread does NOT block other threads
Inexpensive context switch
Utilize MP architecture
Thread library for user threads is in user space
Thread library schedules user threads onto LWP’s
LWP’s are:
implemented by kernel threads
scheduled by the kernel.
© 2004, D. J. Foreman

19. Notes on Java The JVM uses monitors for mutual exclusion
provides wait and notify for cooperation
© 2004, D. J. Foreman

20. Java & Threads-1 Thread creation – 2 ways
import java.lang.*; public class Counter extends Thread {                         public void run() 
//overrides Thread.run                            {                       ....                     } }
extension from the Thread class
© 2004, D. J. Foreman

21. Java & Threads-2
import java.lang.*; public class Counter implements Runnable {         Thread T;                                 public void run()                               {                                       ....                     } }
Instance of the Thread class as a variable of the Counter class – creates an interface
Can still extend the Counter class
© 2004, D. J. Foreman

22. Java & Threads-3 Difference between the two methods
Implementing Runnable, -> greater flexibility in the creation of the class counter
Thread class also implements the Runnable interface
© 2004, D. J. Foreman

23. Wait & Signal - semaphores
Classical definitions
Wait - P (s) // make me wait for something
DO WHILE (s<=0)
END
s=s-1 // when s becomes > 0, decrement it
Signal - V (s) // tell others: my critical job is done
s=s+1
These MUST appear as ATOMIC operations to the application
© 2004, D. J. Foreman