Lecture 5

Question 1

Many-to-many

Answer 1

• Mapping is not fixed, need a thread scheduler
• When a user thread blocks, kernel notifies thread scheduler (upcall) to select another thread to use a free kernel thread

Question 2

Pros and cons of many-to-many

Answer 2

• Pro: Increased concurrency; flexible (users may create as many user threads as they want)
• Con: Very complex to implement

Question 3

POSIX Pthreads

Answer 3

• API specification, implementation is up to OS

- Common in UNIX systems

Question 4

Windows Threads

Answer 4

• Implementation of the one-to-one model in kernel

Question 5

Java Threads

Answer 5

• Java threads are managed by JVM, which is run on top of OS
• JVM specifies the interface, not the implementation
• JVM uses thread services provided by the host OS

Question 6

Threading Issues:

Answer 6

• Semantics of fork() and exec() system calls
• Signal handling
• Thread pools

Question 7

If a process has multiple threads and one of them calls fork(), should the entire process be duplicated or just the calling thread?

Answer 7

It depends. If exec() is called immediately after fork(), there’s no need to duplicate all threads as they will be overwritten anyway. Otherwise, all threads should be duplicated.

Question 8

Signals are used in UNIX to…

Answer 8

…notify a process that an event has occured

Question 9

Two types of signal:

Answer 9

• Synchronous (delivered to the same process that performed the operation that caused the signal)
• Asynchronous (Signal generated by an external event, usually delivered to a different process)

Question 10

Signal Sequence

Answer 10

• Signal is generated by an event (eg. Ctrl-C pressed or packet arrived)
• Signal is delivered to a process
• Signal handler processes the signal (default by OS, can be user-defined)

Question 11

Should OS deliver a signal to: all threads, one thread, or specific thread of a process?

Answer 11

• Synchronous: deliver signal to thread

- Asynchronous: deliver signal to all threads belonging to the process

Question 12

True or false: Threads cannot block signals

Answer 12

False

Question 13

Interrupts vs Signals

Answer 13

• Interrupts started by CPU, I/O, or software; signal started by kernel or process
• Interrupts handled by ISR in kernel, signals handled by default signal handler in kernel, or by process that overrides it
• ISR must be simple, signal handler can be complex and call other functions in the kernel
• Kernel may map specific interrupts to signals and deliver them to processes for specific handling

Question 14

Thread Pools

Answer 14

Create a set of threads that wait for work

Question 15

Advantages of Thread Pools

Answer 15

• Faster to service a request with existing thread than to create a new one
• Limit number of threads in an application to pool size (further requests are queued until a thread is free)
• Easy to manage resources

Question 16

Linux refers to threads and processes as…

Answer 16

…as tasks

Question 17

Thread creation is done through…

Answer 17

…clone system call

Question 18

Clone can be parameterized to allow different resource sharing responsibilities:

Answer 18

• No sharing of resources between parent and child, the entire process is duplicated (same as fork)
• OR, sharing of all resources between parent and child

Question 19

For resource sharing, only ____ are created for child

Answer 19

Pointers to shared resources

Question 20

Pros and Cons of Linux not differentiating between processes and threads

Answer 20

• Good: Simplify scheduling

- Bad: Complex to correlate threads with their processes