COP 4600 Operating Systems Spring 2011

Dan C. Marinescu

Office: HEC 304

Office hours: Tu-Th 5:00-6:00 PM

Last time: Virtualization for the three abstractions

Threads Virtual Memory Bounded buffer

The kernel of an operating syste Today:

Threads State

Processor switching Semaphores Deadlocks

Next time Communication with a bounded buffer

Lecture 16 – Tuesday, March 22, 2011

Lecture 16 2

Thread and VM management – virtual computer

The kernel supports thread and virtual memory management Thread management:

Creation and destruction of threads Allocation of the processor to a ready to run thread Handling of interrupts Scheduling – deciding which one of the ready to run threads should be allocated

the processor

Virtual memory management maps virtual address space of a thread to physical memory.

Each module runs in own address space; if one module runs multiple threads all share one address space.

Thread + virtual memory virtual computer for each module.

Lecture 16 3

Threads and the Thread Manager

Thread virtual processor - multiplexes a physical processor a module in execution; a module may have several threads. sequence of operations:

Load the module’s text Create a thread and lunch the execution of the module in that thread.

Scheduler system component which chooses the thread to run next

Thread manager implements the thread abstraction. Interrupts processed by the interrupt handler which interacts with

the thread manager Exception interrupts caused by the running thread and processed by

exception handlers Interrupt handlers run in the context of the OS while exception

handlers run in the context of interrupted thread.

Lecture 16 4

The state of a thread; kernel versus application threads

Thread state: Thread Id unique identifier of a thread Program Counter (PC) -the reference to the next computational step Stack Pointer (SP) PMAR – Page Table Memory Address Register Other registers

Threads User level threads/application threads – threads running on behalf of users Kernel level threads – threads running on behalf of the kernel

Scheduler thread – the thread running the scheduler Processor level thread – the thread running when there is no thread ready to run

Lecture 16 5

Lecture 16 6

Virtual versus real; the state of a processor

Virtual objects need a physical support. In addition to threads we should be concerned with the processor or

cores running a thread. A system may have multiple processors and each processor may

have multiple cores The state of the processor or core:

Processor Id/Core Id unique identifier of a processor /core Program Counter (PC) -the reference to the next computational step Stack Pointer (SP) PMAR – Page Table Memory Address Register Other registers

Lecture 16 7

Thread Layer

Processor Layer

Thread 1 Thread 2 Thread 7

Processor A Processor B

ID

SP

PC

PMAR

ID

SP

PC

PMAR

ID

SP

PC

PMAR

ID

SP

PC

PMAR

ID

SP

PC

PMAR

8Lecture 16

Processor and thread tables – control structures that must be locked for serialization

Lecture 16 9

Primitives for processor virtualization

Lecture 16 10

Memory CREATE/DELETE_ADDRESS SPACEALLOCATE/FREE_BLOCKMAP/UNMAPUNMAP

Interpreter ALLOCATE_THREAD DESTROY_THREADEXIT_THREAD YIELDAWAIT ADVANCETICKETACQUIRE RELEASE

Communication channel

ALLOCATE/DEALLOCATE_BOUNDED_BUFFERSEND/RECEIVE

Thread states and state transitions

Lecture 16 11

The state of a thread and its associated virtual address space

Lecture 16 12

Switching the processor from one thread to another

Thread creation: thread_id ALLOCATE_THREAD(starting_address_of_procedure, address_space_id); YIELD function implemented by the kernel to allow a thread to wait for an

event. Save the state of the current thread Schedule another thread Start running the new thread – dispatch the processor to the new thread

YIELD cannot be implemented in a high level language, must be implemented in the machine

language. can be called from the environment of the thread, e.g., C, C++, Java allows several threads running on the same processor to wait for a lock. It replaces the

busy wait we have used before.

Lecture 16 13

Implementation of YIELD

Apply the principle of least astonishment; deal first with a simpler case: A fixed number of threads (7). All threads run on the same core and in the same address space. When

switching states we do not need to update the PMAR (Page Memory address Register)

When we enter the processor layer change the state of the current thread from RUNNING to RUNNABLE Save the stack pointer in the thread table Invoke the scheduler

The SCHEDULER Searches the thread table for a thread in RUNNABLE state and changes its state to

RUNNING Update the processor table to mark this thread as running on that processor EXITS the Processor layer

Lecture 16 14

Lecture 16 15

shared structure processor_table(7) integer thread_idshared structure thread_table(7) integer topstack integer stateshared lock instance thread_table_lock

procedure GET_THREAD_ID() return processor_table(CPUID).thread_id

procedure YIELD() ACQUIRE (thread_table_lock) ENTER_PROCESSOR_LAYER(GET_THREAD_ID()) RELEASE(thread_table_lock)return

procedure ENTER_PROCESSOR_LAYER(this_thread) thread_table(this_thread).state ß RUNNABLE thread_table(this_thread).topstackß SP SCHEDULER()return

procedure SCHEDULER() jß _GET_THREAD_ID() do jß j+1 (mod 7)

while thread_table(j).state¬= RUNNABLE thread_table(j).state ß RUNNING processor_table(CPUID).thread_idß j EXIT_PROCESSOR_LAYER(j) return

procedure EXIT_PROCESSOR_LAYER(new) SP,-- thread_table(new).topstack return

16Lecture 16