1. Computer Systems/Operating Systems - Class 8
Today’s class
Threads, SMP, and Microkernels
Principles of concurrency
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

2. Computer Systems/Operating Systems - Class 8
Process
Resource ownership - process includes a virtual address space to hold the process image
Scheduling/execution- follows an execution path that may be interleaved with other processes
These two characteristics are treated independently by the operating system
These two characteristics are the essence of a process.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

3. Computer Systems/Operating Systems - Class 8
Process
Dispatching is referred to as a thread or lightweight process
Resource ownership is referred to as a process or task
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

4. Computer Systems/Operating Systems - Class 8
Multithreading
Operating system supports multiple threads of execution within a single process
UNIX supports multiple user processes but only supports one thread per process
Windows, Solaris, Linux, Mach, and OS/2 support multiple threads
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

5. Computer Systems/Operating Systems - Class 8
Process
In a multithreaded environment a process is defined as the unit of resource allocation and a unit of protection
Have a virtual address space which holds the process image
Protected access to processors, other processes, files, and I/O resources
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

6. Computer Systems/Operating Systems - Class 8
Thread
An execution state (running, ready, etc.)
A saved thread context when not running
An execution stack
Some per-thread static storage for local variables
Access to the memory and resources of its process
There may be one or more threads in a process. Each thread has the items on this slide.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

7. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

8. Computer Systems/Operating Systems - Class 8
Benefits of Threads
Takes less time to create a new thread than a process
Less time to terminate a thread than a process
Less time to switch between two threads within the same process
Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

9. Uses of Threads in a Single-User Multiprocessing System
Foreground to background work
Asynchronous processing
Speed of execution
Modular program structure
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

10. Computer Systems/Operating Systems - Class 8
Threads
Suspending a process involves suspending all threads of the process since all threads share the same address space
Termination of a process, terminates all threads within the process
In an OS that supports threads, scheduling and dispatching is done on a thread basis. However, some actions affect all the threads in a process and the OS must manage this at the process level.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

11. Computer Systems/Operating Systems - Class 8
Thread States
Key thread states are Running, Ready, and Blocked
Operations associated with a change in thread state
Spawn
Block
Unblock
Finish
Deallocate register context and stacks
Threads have execution states and may synchronize with one another.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

12. Remote Procedure Call Using Single Thread
Let’s consider performance benefits of threads. Consider a program that performs two remote procedure calls (RPCs) to two different hosts to obtain a combined result.
In a single threaded program the results are obtained in sequence. Thus, the program has to wait for a response from each server in turn.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

13. Remote Procedure Call Using Threads
Rewriting the program so it uses a separate thread for each RPC results in a substantial speedup.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

14. Computer Systems/Operating Systems - Class 8
Multithreading
On a uniprocessor, multiprogramming enables the interleaving of multiple threads within multiple processes. In this example, there are three threads in two processes which are all interleaved on the processor.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

15. Computer Systems/Operating Systems - Class 8
User-Level Threads
All thread management is done by the application
The kernel is not aware of the existence of threads
Two broad categories of threads – user-level threads and kernel-level threads.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

16. Computer Systems/Operating Systems - Class 8
User-Level Threads
P is a process. Any application can be programmed to be multithreaded by using a threads library, which is a package of routines for user-level threads management.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

17. Computer Systems/Operating Systems - Class 8
Kernel-Level Threads
Windows is an example of this approach
Kernel maintains context information for the process and the threads
Scheduling is done on a thread basis
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

18. Computer Systems/Operating Systems - Class 8
Kernel-Level Threads
Kernel-level threads overcome the drawbacks of the user-level thread approach:
The kernel can simultaneously schedule multiple threads from the same process on multiple processors.
If one thread in a process is blocked, the kernel can schedule another thread of the same process.
The main disadvantage to kernel-level threads is that transfer of control from one thread to another within the same process requires a mode switch to the kernel.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

19. Computer Systems/Operating Systems - Class 8
Combined Approaches
Example is Solaris
Thread creation done in the user space
Bulk of scheduling and synchronization of threads within application
Can have a combined user-level/kernel-level thread approach. The user-level threads are mapped onto some number of kernel-level threads.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

20. Computer Systems/Operating Systems - Class 8
Combined Approaches
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

21. Categories of Computer Systems
Single Instruction Single Data (SISD) stream
Single processor executes a single instruction stream to operate on data stored in a single memory
Single Instruction Multiple Data (SIMD) stream
Each instruction is executed on a different set of data by the different processors
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

22. Categories of Computer Systems
Multiple Instruction Single Data (MISD) stream
A sequence of data is transmitted to a set of processors, each of which executes a different instruction sequence. Never implemented
Multiple Instruction Multiple Data (MIMD)
A set of processors simultaneously execute different instruction sequences on different data sets
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

23. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

24. Symmetric Multiprocessing
Kernel can execute on any processor
Typically each processor does self-scheduling form the pool of available process or threads
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

25. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

26. Multiprocessor Operating System Design Considerations
Simultaneous concurrent processes or threads
Kernel routines need to be re-entrant, to allow several processors to execute the same kernel code simultaneously
Scheduling
May be performed by any processor, so conflicts must be avoided
Synchronization
Memory management
Reliability and fault tolerance
A multiprocessor operating system must provide all the functionality of a multiprogramming system plus additional features to accommodate multiple processors.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

27. Computer Systems/Operating Systems - Class 8
Microkernels
Small operating system core
Contains only essential core operating systems functions
Many services traditionally included in the operating system are now external subsystems
Device drivers
File systems
Virtual memory manager
Windowing system
Security services
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

28. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

29. Benefits of a Microkernel Organization
Uniform interface on request made by a process
Don’t distinguish between kernel-level and user-level services
All services are provided by means of message passing
Extensibility
Allows the addition of new services
Flexibility
New features added
Existing features can be subtracted
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

30. Benefits of a Microkernel Organization
Portability
Changes needed to port the system to a new processor are changed in the microkernel - not in the other services
Reliability
Modular design
Small microkernel can be rigorously tested
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

31. Benefits of Microkernel Organization
Distributed system support
Message are sent without knowing what the target machine is
Object-oriented operating system
Components are objects with clearly defined interfaces that can be interconnected to form software
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

32. Computer Systems/Operating Systems - Class 8
Microkernel Design
Low-level memory management
Mapping each virtual page to a physical page frame
The bulk of memory management, including the protection of the address space of one process from another, can be implemented outside the kernel.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

33. Computer Systems/Operating Systems - Class 8
Microkernel Design
Interprocess communication
Basic mechanism is a message
A port is a queue of messages destined for a particular process
I/O and interrupt management
Hardware interrupts handled as messages
I/O ports included in address space
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

34. Computer Systems/Operating Systems - Class 8
Windows Processes
Implemented as objects
An executable process may contain one or more threads
Both processes and thread objects have built-in synchronization capabilities
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

35. Windows Process Object
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

36. Computer Systems/Operating Systems - Class 8
Windows Thread Object
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

37. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

38. Linux Task Data Structure
State
Scheduling information
Identifiers
Interprocess communication
Links
Times and timers
File system
Address space
Processor-specific context
In Linus a process is called a task.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

39. Computer Systems/Operating Systems - Class 8
The Running state is either Executing or Ready to execute.
The Interruptible state is a blocked state, in which the process is waiting for an event.
The Uninterruptible state is also a blocked state, but the process is waiting directly on hardware conditions and therefore will not handle any signals.
In the Stopped state, the process has been halted and can only be resumed by action from another process (e.g. a process that is being debugged).
In the Zombie state, the process has been terminated but for whatever reason still must have its task structure in the process table.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

40. Computer Systems/Operating Systems - Class 8
Concurrency
Multiple applications
Multiprogramming
Structured application
Application can be a set of concurrent processes
Operating-system structure
Operating system is a set of processes or threads
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

41. Computer Systems/Operating Systems - Class 8
Concurrency
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

42. Difficulties of Concurrency
Sharing of global resources
Operating system managing the allocation of resources optimally
Difficult to locate programming errors
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

43. Computer Systems/Operating Systems - Class 8
A Simple Example
void echo() {
chin = getchar();
chout = chin;
putchar(chout); }
This simple procedure provides a character echo mechanism; it inputs a character from the keyboard and puts it out on the display. Note that chin and chout are global variables.
Consider the following event sequence:
Process P1 invokes echo() and is interrupted after getchar() returns and stores its value in chin. Say the character is ‘A’.
Process P2 is activated and invokes echo() which runs to conclusion. Suppose the character is ‘B’.
Process P1 is resumed. What is the output? It’s ‘B’.
Need to restrict the procedure to having only one process at a time in it.
Process P1 invokes echo() and is interrupted after getchar() returns. It’s input is ‘A’.
Process P2 is activated and invokes echo(). Because P1 is still inside echo(), P2 is blocked and thus suspended.
At some later time P1 is resumed and completes execution of echo(). The character ‘A’ is displayed.
When P1 exits echo() the block on P2 is removed.
At some later time P2 is resumed and successfully invokes echo().
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

44. Computer Systems/Operating Systems - Class 8
A Simple Example
Process P1 Process P2
chin = getchar(); .
chin = getchar();
chout = chin; chout = chin;
putchar(chout); .
putchar(chout);
Now consider a multiprocessor system. Events on the same line take place in parallel.
If there is no protection of the procedure from multiple processes entering it, then we will again lose the input data from P1 and both putchar(chout) statements will output the same character, the one input for process P2.
Can handle this the same way as before. P2 will be blocked until P1 finishes with echo().
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

45. Computer Systems/Operating Systems - Class 8
Race Condition
A race condition occurs when multiple processes or threads read and write data items so that the final result depends on the order of execution of instructions in the multiple processes or threads.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

46. Operating System Concerns
Keep track of various processes
Allocate and deallocate resources
Processor time
Memory
Files
I/O devices
Protect data and resources
Output of process must be independent of the speed of execution of other concurrent processes
These are design and management issues for operating systems that are raised by the existence of concurrency.
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

47. Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

48. Competition Among Processes for Resources
Mutual Exclusion
Critical sections
Only one program at a time is allowed in its critical section
Example only one process at a time is allowed to send command to the printer
Deadlock
Starvation
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

49. Requirements for Mutual Exclusion
Only one process at a time is allowed in the critical section for a resource
A process that halts in its noncritical section must do so without interfering with other processes
No deadlock or starvation
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8

50. Requirements for Mutual Exclusion
A process must not be delayed access to a critical section when there is no other process using it
No assumptions are made about relative process speeds or number of processes
A process remains inside its critical section for a finite time only
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8
Monday, September 24, 2007
Computer Systems/Operating Systems - Class 8