Chapter 4: Multithreaded Programming Lecture 3 10.7.2008 Hao-Hua Chu

I/O Brush (MIT Media Lab)

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

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

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?

Single vs. Multithreaded Processes

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?

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)

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

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?

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?

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

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

Thread Library APIs: POSIX Pthreads

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

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()?

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)

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?

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

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

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)

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

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

End of Chapter 3