1. Chapter 4: Multithreaded Programming
Lecture 3
Hao-Hua Chu

2. I/O Brush (MIT Media Lab)

3. Outline Review what a process is. What is a thread and why?
Multithreading models
Thread libraries
Threading issues

4. Process overview What is a process? What is multi-programming?
A program in execution on the computer
What is multi-programming?
What resources does OS allocate to run a process?
CPU, Memory, file (I/O), network (I/O)
What is a process made up of in memory?
What is the cost in creating & running a process?
What is the cost in context-switching between processes?
Heavy vs. light load

5. Consider a Web Browser
How to load & render these 9+ images files quickly?
Issue parallel HTTP requests, one per process (for example only, not in real implementation).
What is the speedup?
What is the cost using many processes?
How to lower this cost?

6. Single vs. Multithreaded Processes

7. Benefit-Cost Analysis (multithreaded vs. single-threaded processes)
Benefits
Responsiveness
Some threads (image loading threads) blocked, other (UI thread) can continue to run.
Other examples?
Resource sharing
Reuse the same address space, data/code segments
Economy
Thread creation 30x faster than process creation
Thread context switch 5x faster than process context switch
Utilization of multiprocessor architecture
One process running multiple threads on multiple CPUs
What are the costs?

8. Implementing Multithreading
Poor implementation reduces benefit & increases cost 
What are good evaluation metrics for threading implementation?
How to implement multithreading support?
At user-level (via user library): user threads
POSIX Pthreads, Win32 threads, Java threads
At kernal-level: kernel threads
Windows XP, Mac OS X, Solaris, almost all modern OS
Both
What does it mean to implement at the user-level and kernel-level?
Kernel threads are used for VMM, I/O as well as handling syscalls)

9. Multithreading Models
What are the possible mappings between user threads and kernel threads?
Many-to-one
One-to-one
How about one-to-many?

10. Many-to-one Model
All threads from one process map to the same kernel thread
User-level thread library manages context-switching.
Is this implementation good (efficient vs. concurrency)?
Yes, but …
One user thread issues a blocking syscall?
Multi-processor system?
How to improve concurrency?

11. Many-to-one Model Each user thread maps to one kernel thread
Is this implementation good (concurrency vs. efficiency)?
Good concurrency, why? (blocking syscall does not affect other threads)
Expensive, why? (user-thread creation -> kernel-thread creation)
How to have both good concurrency and efficiency?

12. Many-to-many Model
Many user threads are mapped to a smaller or equal number of kernel threads.
Why is this better than Many-to-one? (concurrency & multi-processor)
Why is this better than one-to-one? (efficiency)
Like one-to-one concurrency?
Two-level model

13. 2-Level Model
Allows a user thread to be bound to kernel thread

14. Thread Library APIs: POSIX Pthreads

15. Threading Issues Semantics of fork() and exec() system calls
Thread cancellation
Signal handling
Thread pools
Thread specific data
Scheduler activations

16. Semantics of fork() and exec()
Does fork() duplicate only the calling thread or all threads?
The best answer, when you are not sure, is both.
What would you do if exec() is called immediately after fork()?

17. Thread Cancellation Terminating a thread before it has finished
Two general approaches:
Asynchronous cancellation terminates the target thread immediately
Deferred cancellation allows the target thread to periodically check if it should be cancelled
Why have both?
Concurrency control … (more about this later)

18. Signal Handling
Signals are used in UNIX systems to notify a process that a particular event has occurred
A signal handler is used to process signals
Signal is generated by particular event
Signal is delivered to a process
Signal is handled
Two types of signals: asynchronous vs. synchronous
What is the difference?
How to deliver a signal in a multithreaded process?
Which thread? One or all?

19. Signal Handling Options:
Deliver the signal to the thread to which the signal applies
Applied to Synchronous signals
Deliver the signal to every thread in the process
Deliver the signal to certain threads in the process
Assign a specific thread to receive all signals for the process

20. Thread Pools
Create a number of threads in a pool where they await work
Advantages:
Usually slightly faster to service a request with an existing thread than create a new thread
Allows the number of threads in the application(s) to be bound to the size of the pool

21. Thread Specific Data
Allows each thread to have its own copy of data (not shared!)
Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

22. Scheduler Activations
Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application
Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library
This communication allows an application to maintain the correct number kernel threads

23. Summary Thread overview Multi-threading models Thread issues
Single-thread process vs. multithreaded processes
Multi-threading models
Many-to-one, one-to-one, many-to-many, two-level model
Thread issues
Semantics of fork() and exec() system calls, thread cancellation, signal handling, thread pools, thread specific data, scheduler activations

24. End of Chapter 3