Overview
Process creation is heavy-weight while thread creation is light-weight
can simplify code, increase efficiency
Benefits: responsiveness, sharing, economy, scalability
multicore, multiprocessor, parallelism, concurrency
Data parallelism: same data distributed across multicores, same operation on each
Task parallelism: each thread performing unique operation on different cores
Amdahl's Law
Multithreading Models
User/Kernel
User threads: management by user-level threads library
Kernel threads: support by kernel
Kernel have a thread table for kernel threads, while Process have own thread table for user threads
Why user thread? no kernel intervention, efficient
Why not user thread? one blocked, all blocked
Why kernel thread? can on different processors
Why not kernel thread? slow
Models
Many-to-One (one block all block, can not parallel, few now)
One-to-One (more concurrency, number restricted)
Many-to-Many (sufficient)
Two-Level (allow 1:1 and M:M)
M:M for server, 1:1 for PC
Thread Libraries
Library: user space or kernel-level
Pthreads: either as user-level or kernel level, a specification not implementation
Implicit Threading
creation and management of threads done by compilers
thread pools: create a number of threads in a pool where they await work (faster, large number)
Threading Issues
fork() and exec()
two version of fork(): copy all threads or copy one thread
exec() replace all thread
if exec() immediately after fork() then copy all is unnecessary
Cancellation
async: immediately
deferred: allow target thread to periodically check if cancelled
disabled: remains pending until enables it
Signal Handling
signals are used to notify a process that a event has occurred
where should a signal be delivered for multi-threaded?
all threads in a process share a same handler
Thread-Local Storage
each thread have its own data, across functions, similar to static data
Why not use thread stack? life cycle reasons.
Scheduler Activations
How to maintain an appropriate number of kernel threads allocated to the app?
Intermediate data structure between user and kernel threads: lightweight process (LWP), a virtual processor on which process can schedule user thread to run, each LWP attached to kernel thread
Upcalls: from kernel to the upcall handler in the thread library
When blocked, the attached LWP also blocks. The kernel makes an upcall and then allocates a new LWP to the application.