Scheduling Policies. Threads Library. Lightweight Processes

Transcription

1 Scheduling Policies User Unbound Threads Bound Threads Threads Library Kernel Lightweight Processes The threads library uses underlying threads of control called lightweight processes that are supported by the kernel. LWP can be thought as virtual CPUs that executes code or system calls.

2 Process Scope-Unbound Threads Unbound threads are created PTHREAD SCOPE PROCESS. They are scheduled in user space to attach or dettach from LWPs. This mode allows threads to float among LWPs and this improves performance. The thread library decides which thread get serviced by the kernel.

3 System Scope-Bound Threads Bound threads are created PTHREAD SYSTEM PROCESS. A bounded thread is permanently attached to an LWP. They are scheduled in user space to attach or dettach from LWPs. This mode allows threads to float among LWPs and this improves performance. All scheduling is done by the operating environment.

4 Attributes Attributes are specified only at thread creation time. They can not be altered while the thread is being used. Attribute object is opaque and can not be modified by assignments. Sets of functions are avalaible to initialize, configure and destroy each objec type. Attribute objects require memory allocation and care to return it to the system.

5 Hints Configure all required state specifications at one time in the early stages of program execution. Once an attribute is initialized it has process-wide scope.

6 Initialize Attributes int pthread attr init (pthread attr t *tattr); pthread attr t tattr; int ret; /* initialize an attribute to the default value */ ret = pthread attr init(&tattr); If successfull returns zero, any other value indicates an error. The value ENOMEM indicates there was not enough memory to initialize the thread attributes.

7 Default Attribute Values Attribute Value Result scope PTHREAD SCOPE PROCESS New thread is unbound detachstate PTHREAD CREATE JOINABLE Exit status and thread are preserved after the thread terminates. stackaddr NULL New thread has system-allocated stack address. stacksize 1 megabyte New thread has system-defined stack size. priority New thread inherits parent priority.

8 Destroy Attributes int pthread attr destroy(pthread attr t *tattr); pthread attr t tattr; int ret; ret=pthread attr destroy(&tattr); Returns zero after completing successfully, any other value indicates an error. The value EINVAL indicates that the value of tattr was not valid.

9 Scheduling Policy pthread attr setschedpolicy(pthread attr t *tattr, int policy); pthread attr t tattr; int policy; int ret; ret = pthread attr setschedpolicy( &tattr, SCHED OTHER); Returns zero when successfull, any other value indicates an error. EINVAL an attempt to set tattr to a value not valid. ENOTSUP an attempt to set to an unsupported value.

10 Scheduling Policies - FIFO SCHED FIFO: First-In-First-Out; threads scheduled to this policy, if not preempted by a higher priority, will proceed until completion. Threads whose contention scope is system (PTHREAD SCOPE SYSTEM) are in real time (RT) scheduling class and the calling process must have an effective user id of 0. For threads that have contention scope of process (PTHREAD SCOPE PROCESS) are in time sharing (TS) scheduling class.

11 Scheduling Policies - Round Robin SCHED RR: Round Robin; threads scheduled to this policy, if not preempted by a higher priority, will execute for a time period determined by the system. Threads whose contention scope is system (PTHREAD SCOPE SYSTEM) are in real time (RT) scheduling class and the calling process must have an effective user id of 0. For threads that have contention scope of process (PTHREAD SCOPE PROCESS) are in time sharing (TS) scheduling class.