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Chapter 5: CPU Scheduling part II. Chapter 5: CPU Scheduling. Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling Operating Systems Examples Java Thread Scheduling Algorithm Evaluation. Multiple-Processor Scheduling.

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chapter 5 cpu scheduling
Chapter 5: CPU Scheduling
  • Multiple-Processor Scheduling
  • Real-Time Scheduling
  • Thread Scheduling
  • Operating Systems Examples
  • Java Thread Scheduling
  • Algorithm Evaluation
multiple processor scheduling
Multiple-Processor Scheduling
  • CPU scheduling more complex when multiple CPUs are available
  • Homogeneous processors within a multiprocessor
    • Makes it easy to share processes/threads
    • Any processor can run any process
    • Limitations: one processor may have unique resources (disk drive, etc.)
  • Load sharing
    • Goal is to make each processor work equally
multiple processor scheduling1
Multiple-Processor Scheduling
  • Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing
    • Uses a “master” server
    • Simple to implement
    • No coherency problems
  • Symmetric multiprocessing (SMP) – each processor is self scheduling
    • Processes may be in single queue
    • Or each processor may have its own queue
    • Regardless, each processor runs own scheduler
    • Mutual exclusion problems
    • SMP is supported by all modern operating sytstems: XP, 2000, Solaris, Linux, Mac OS X
multiple processor scheduling2
Multiple-Processor Scheduling
  • Processor Affinity – process is associated with a particular processor during its entire lifetime
    • Reason: cache problem. If switch processors must flush one cache and repopulate another
    • Soft affinity: SMP system tries to keep process on same processor but doesn’t guarantee it.
    • Hard affinity: SMP system guarantees that process will remain on a single procesor
    • Linux allows both. Special system call to get hard affinity.
multiple processor scheduling3
Multiple-Processor Scheduling
  • Load Balancing –
    • On SMP systems must keep the workload balanced across processors
    • Only necessary in systems where each processor has own queue
    • In most contemporary systems each processor does have its own queue
multiple processor scheduling4
Multiple-Processor Scheduling
  • Two general approaches
    • Push migration A specific task periodically checks the load on each processor
      • if it finds an imbalance, it moves (pushes) processes to idle processors
    • Pull migration. An idle processor pulls a waiting task from a busy processor.
    • Hybrid. Uses both push and pull.
      • Example: Linux scheduler and the ULE scheduler (FreeBSD) implement both
      • Linux runs balancing algorithm every 200 milliseconds (push)
      • Or whenever the run queue for a processor is empty (pull)
multiple processor scheduling5
Multiple-Processor Scheduling
  • Problem: load balancing often counteracts the benefits of processor affinity
    • If use push or pull migration, take a process from its processor
    • This violates processor affinity
    • No absolute rule governing which policy is best
      • In some systems an idle processor always pulls a process from a non-idle process
      • In some systems process are moved only if the imbalance exceeds a threshold.
symmetric multithreading
Symmetric Multithreading
  • Alternative to SMP
  • Provides multiple logical (not physical) processor
  • Also called hyperthreading technology (on Intel processors)
  • Idea: create multiple logical processors on the same physical processor.
    • Each logical processor has its won architecture state
    • Includes general-purpose and machine-state registers
    • Each logical processor is responsible for its own interrupt handling
    • Interrupts are delivered to and handled by logical processors rather than physical processors
    • Each logical processor shares the resources of its physical processor
      • Including cache, memory, buses
    • See next slide
symmetric multithreading1
Symmetric Multithreading

Two physical processors each with two logical processors

OS sees 4 processors available for work.

symmetric multithreading2
Symmetric Multithreading
  • Note that SMT is provided in hardware not software
  • Hardware provides the representation of the architecture state for each logical processor
  • Also provides for interrupt handling
  • OS do not have to recognize the difference between physical and logical processors
    • Can gain performance if OS is aware of SMT
    • Better to keep two physical processors busy than two logical processors on a single physical processor in previous slide.
symmetric multithreading3
Symmetric Multithreading
  • Why does hyperthreading work?
    • Superscalar architectures: many different hardware components exist
    • Example: mulitple integer arithmetic units.
    • To take advantage of these units, a process must be able to execute multiple instructions in parallel
    • Often not possible.
    • Idea: if run two processes simultaneously, can keep more of the architecture units busy.
    • The processor coordinates the simultaneous execution of multiple processes.
real time scheduling
Real-Time Scheduling
  • Hard real-time systems – required to complete a critical task within a guaranteed amount of time
  • Soft real-time computing – requires that critical processes receive priority over less fortunate ones
thread scheduling
Thread Scheduling
  • Scheduling is different for user-level threads and kernel-level threads
    • Kernel does not know about user-level threads thus does not schedule them
    • Thread library cannot schedule kernel level threads or processes
thread scheduling1
Thread Scheduling
  • Local Scheduling – on many-to-one and many-to-many systems
    • threads library decides which thread to put onto an available LWP
    • Called process-contention scope (PCS) since competition takes place among threads in the same process
    • The thread library schedules the thread onto a LWP
    • but the kernel must schedule the LWP; the thread library cannot do this.
thread scheduling2
Thread Scheduling
  • PCS scheduling
    • Done according to priority
    • User-level thread priorities set by the programmer
    • Priorities are not adjusted by the thread library
    • Some thread libraries may allow programmer to change priority of threads dynamically
    • PCS typically preempt current thread in favor of a higher-priority thread
thread scheduling3
Thread Scheduling
  • Global Scheduling – on one-to-one systems (XP, Solaris 9, Linux)
    • How the kernel decides which kernel thread to run next
    • Kernel uses system-contention scope (SCS)
    • Competition for the CPU with SCS scheduling takes place among all threads in the system.
pthread scheduling api
Pthread Scheduling API
  • POSIX Pthread API
  • Allows specification of PCS or SCS during thread creation.
  • PTHREAD_SCOPE_PROCESS
    • Schedules threads using PCS scheduling
    • Thread library will map threads onto available LWPs
    • May use scheduler activations
  • PTHREAD_SCOPE_SYSTEM
    • Schedules threads using SCS scheduling
    • Will create and bind an LWP for each user-level thread on many-to-many systems
    • This creates a one-to-one mapping
pthread scheduling api1
Pthread Scheduling API
  • POSIX Pthread API
    • To set/get the scheduling policy:

pthread_attr_setscope(pthread_attr_t *attr, int scope)

pthread_attr_getscope(pthread_attr_t *attr, int *scope)

    • First parameter is a pointer to the attribute set for the thread
    • Second parameter for setscope function is either
      • PTHREAD_SCOPE_SYSTEM or
      • PTHREAD_SCOPE_PROCESS
    • Second parameter for getscope function is a pointer to an int
      • On return, will contain the integer representing the policy
    • Both functions return non-zero values on error
    • On some systems only certain contention scope values are allowed
      • Linux and Mac OS X only allow PTHREAD_SCOPE_SYSTEM
pthread scheduling api2
Pthread Scheduling API
  • POSIX Pthread API example: next slide
  • First determines the existing contention scope
  • Then sets it to PTHREAD_SCOPE_PROCESS
  • Then creates 5 separate threads that run using the SCS policy
pthread scheduling api3
Pthread Scheduling API

#include <pthread.h>

#include <stdio.h>

#define NUM THREADS 5

int main(int argc, char *argv[])

{

int i;

pthread t tid[NUM THREADS];

pthread attr t attr;

/* get the default attributes */

pthread attr init(&attr);

/* set the scheduling algorithm to PROCESS or SYSTEM */

pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM);

/* set the scheduling policy - FIFO, RT, or OTHER */

pthread attr setschedpolicy(&attr, SCHED OTHER);

/* create the threads */

for (i = 0; i < NUM THREADS; i++)

pthread create(&tid[i],&attr,runner,NULL);

pthread scheduling api4
Pthread Scheduling API

/* now join on each thread */

for (i = 0; i < NUM THREADS; i++)

pthread join(tid[i], NULL);

}

/* Each thread will begin control in this function */

void *runner(void *param)

{

printf("I am a thread\n");

pthread exit(0);

}

operating system examples
Operating System Examples
  • Solaris scheduling
  • Windows XP scheduling
  • Linux scheduling
slide24

Contemporary Scheduling

  • Involuntary CPU sharing -- timer interrupts
    • Time quantum determined by interval timer -- usually fixed for every process using the system
    • Sometimes called the time slice length
  • Priority-based process (job) selection
    • Select the highest priority process
    • Priority reflects policy
  • With preemption
  • Usually a variant of Multi-Level Queues using RR within a queue
solaris scheduling
Solaris Scheduling
  • Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, symmetric multiprocessing, and real-time scheduling.
  • Scheduling: priority-based thread scheduling with 4 classes of priority:
    • Real time (highest priority)
      • Run before a process in any other class
    • System
      • (only kernel processes; user process running in kernel mode are not given this priority)
    • Time sharing
    • Interactive (lowest priority)
  • Within each class there are different priorities and different scheduling algorithms
solaris scheduling1
Solaris Scheduling
  • Scheduler converts class-specific priorities into global priorities and then selects the thread with the highest global priority to run.
  • Selected threads run until
    • It blocks
    • It uses its time slice
    • It is preempted by a higher-priority thread
    • If there are multiple threads with the same priority, scheduler uses round-robin queue.
solaris scheduling2
Solaris Scheduling
  • Default class is time sharing
  • Policy for Time sharing:
    • Uses a mulitlevel feedback queue
    • Different levels have different time slice lengths
    • Dynamically alters priorities
    • Inverse relationship between priority and time slice
      • The higher the priority, the smaller the time slice
      • The lower the priority, the larger the time slice
      • I/O bound typically have higher priority
      • CPU-bound typically have lower priority
      • Get good response time for I/O-bound
      • Get good throughput for CPU-bound
solaris scheduling3
Solaris Scheduling
  • Policy for Interactive processes:
    • Same policy as time-sharing
    • Gives windowing applications a higher priority for better performance
solaris 2 scheduling
Solaris 2 Scheduling
  • Before Solaris 9 used a many-to-many model
    • Solaris 9 switched to a one-to-one model
  • Solaris 2 resource needs of thread types:
    • Kernel thread: small data structure and a stack; thread switching does not require changing memory access information – relatively fast.
    • LWP: PCB with register data, accounting and memory information; switching between LWPs is relatively slow.
    • User-level thread: only need stack and program counter; no kernel involvement means fast switching. Kernel only sees the LWPs that support user-level threads.
solaris 9 scheduling
Solaris 9 Scheduling
  • Dispatch table for scheduling interactive and time-sharing threads
    • See next slide
    • These two classes include 60 priority levels (only a few are shown)
    • Dispatch table fields:
      • Priority The class-dependent priority. Higher number means higher priority.
      • Time quantum. The time quantum for the associated priority. Notice the inverse relationship.
      • Time quantum expired. The new priority of a thread that has used its entire time quantum without blocking. The thread is now considered CPU-bound. Priority is lowered.
      • Return from sleep. The new priority of a thread that is returning from sleeping. Its priority is boosted to between 50 and 59. Assures good response time for interactive processes.
solaris 9 scheduling1
Solaris 9 Scheduling
  • Solaris 9 scheduling
    • Introduced two new scheduling classes:
      • Fixed priority. These have the same priority range as those in time-sharing class
      • Their priorities are not dynamically adjusted.
      • Fair share. Uses CPU shares instead of priorities to make scheduling decisions.
      • CPU shares are allocated to a set of processes (a project)
linux scheduling
Linux Scheduling
  • Kernel v. 1.0.9 (very old)
  • Dispatcher is a kernel function, schedule( )
  • Function is called from
    • Other system functions
    • After every system call
    • After every interrupt
  • Dispatcher jobs:
    • Performs periodic work (e.g., processes pending signals)
    • Inspects set of tasks in the TASK_RUNNING state (the ready list)
    • Chooses a task to execute
    • Dispatches the task to CPU
linux scheduling1
Linux Scheduling
  • Policy: variant of RR
    • Uses conventional timeslicing mechanism
    • Dynamic priority computed based on value assigned to task by nice( ) or setpriority( )
    • and by amount of time process has been waiting
  • Count field in the task descriptor is adjusted on each timer interrupt
    • Interrupt handler adjusts each timer field for task
  • Dispatcher selects the ready task with max counter value.
slide38

/*

* …

* NOTE!! Task 0 is the ‘idle’ task, which gets called when no

* other tasks can run. It cannot be killed, and it cannot

* sleep. The ‘state’ information in task[0] is never used.

* …

*/

Asmlinkage void schedule(void)

{

int c;

struct task_struct * p;

// Pointer to the process descriptor currently being inspected

struct task_struct * next;

unsigned long ticks;

/* check alarm, wake up any interruptible tasks that have got a signal */

… // This code is elided from the description

/* this is the scheduler proper: */

#if 0

/* give the process that go to sleep a bit higher priority … */

/* This depends on the values for TASK_XXX */

/* This gives smoother scheduling for some things, but */

/* can be very unfair under some circumstances, so .. */

if (TASK_UNINTERRUPTIBLE >= (unsigned) current->state &&

current->counter < current->priority*2){

++ current->counter;

}

#endif

slide39

c = -1; // Choose the task with the highest c == p->counter value

next = p = &init_task;

for(;;) {

if ((p = p->next_task) == &init_task)

goto confuse_gcc; // this is the loop exit

if (p->state == TASK_RUNNING && p->counter > c)

c = p->counter, next = p;

// this task has the highest p->count so far

// but keep looking

}

Confuse_gcc:

if (!c){

for_each_task(p)

p->counter = (p->counter >> 1) + p->priority;

}

if (current != next)

kstat.context_switch++;

switch_to(next); // this is the context switch

… // more code

};

}

contemporary linux scheduling
Contemporary Linux Scheduling
  • Prior to version 2.5 Linux kernel ran a variable of the traditional UNIX scheduling algorithm.
    • Poor support for SMP
    • Does not scale well as the number of tasks on the system grows
  • New kernel
    • Scheduling algorithm runs in constant O(1) time regardless of the number of tasks
    • Includes support for SMP: processor affinity, load balancing, interactive tasks, etc.
contemporary linux scheduling1
Contemporary Linux Scheduling
  • Linux scheduler is preemptive, priority-based algorithm
  • Two algorithms: time-sharing and real-time
    • Real time priorities range from 0-99
    • Time-sharing priorities range from 100-140
    • These two ranges are mapped into a global priority scheme (lower numbers have higher priority)
    • Higher-priority tasks get longer time-quanta
    • Lower-priority tasks get shorter time-quanta
contemporary linux scheduling2
Contemporary Linux Scheduling
  • Time-sharing
    • Prioritized credit-based – process with most credits is scheduled next
    • Credit subtracted when timer interrupt occurs
    • When credit = 0, another process chosen
    • When all processes have credit = 0, recrediting occurs
      • Based on factors including priority and history
      • Use a tasks nice value plus or minus 5
      • The interactivity of a task determines whether 5 is added to or subtracted from the nice value.
      • Interactivity determined by how long task has been sleeping while waiting for I/O
      • Tasks that are more interactive have longer sleep times, thus get adjustments closer to –5
      • Scheduler favors interactive taks
      • Tasks that have shorter sleep times are CPU-bound and thus get adjustments closer to +5
contemporary linux scheduling3
Contemporary Linux Scheduling
  • Time-sharing
    • Kernel maintains all runable tasks in a runqueue data structure
      • Each processor has own runqueue (on SMP systems)
      • Each runqueue contains two priority arrays: active and expired
      • The active array contains all tasks with time remaining in their time slices
      • Expired array contains all expired tasks
      • Each of these arrays are priority arrays: list is indexed according to priority (see next slide)
      • When all tasks have exhausted their time slices (active array is empty) the two priority arrays are exchanged.
contemporary linux scheduling4
Contemporary Linux Scheduling
  • Real-time
    • Soft real-time
    • Real-time tasks have static priorities
    • Posix.1b compliant – two classes
      • FCFS and RR
      • Highest priority process always runs first
bsd 4 4 scheduling
BSD 4.4 Scheduling
  • Involuntary CPU Sharing
  • Preemptive algorithms
  • Dispatcher selects a process from highest priority queue:
    • only processes in highest priority, non-empty queue can run
  • Within a queue uses RR
  • 32 Multi-Level Queues
    • Queues 0-7 are reserved for system functions
    • Queues 8-31 are for user space functions
    • nice influences (but does not dictate) queue level
    • Once per time quantum scheduler recomputes each processes priority
    • Priority function of nice and recent demand on CPU (more utilization means lower priority)
java thread scheduling
Java Thread Scheduling
  • JVM Uses a Preemptive, Priority-Based Scheduling Algorithm.
  • FIFO Queue is Used if There Are Multiple Threads With the Same Priority.
java thread scheduling cont
Java Thread Scheduling (cont)

JVM Schedules a Thread to Run When:

  • The Currently Running Thread Exits the Runnable State.
  • A Higher Priority Thread Enters the Runnable State

* Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not.

time slicing
Time-Slicing
  • Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used:

while (true) {

// perform CPU-intensive task

. . .

Thread.yield();

}

This Yields Control to Another Thread of Equal Priority.

thread priorities
Thread Priorities
  • Thread Priorities:PriorityComment

Thread.MIN_PRIORITY Minimum Thread Priority

Thread.MAX_PRIORITY Maximum Thread Priority

Thread.NORM_PRIORITY Default Thread Priority

Priorities May Be Set Using setPriority() method:

setPriority(Thread.NORM_PRIORITY + 2);

algorithm evaluation
Algorithm Evaluation
  • Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload
  • Queueing models
  • Implementation
java thread scheduling1
Java Thread Scheduling
  • JVM Uses a Preemptive, Priority-Based Scheduling Algorithm
  • FIFO Queue is Used if There Are Multiple Threads With the Same Priority
java thread scheduling cont1
Java Thread Scheduling (cont)

JVM Schedules a Thread to Run When:

  • The Currently Running Thread Exits the Runnable State
  • A Higher Priority Thread Enters the Runnable State

* Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not

time slicing1
Time-Slicing

Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method

May Be Used:

while (true) {

// perform CPU-intensive task

. . .

Thread.yield();

}

This Yields Control to Another Thread of Equal Priority

thread priorities1
Thread Priorities

PriorityComment

Thread.MIN_PRIORITY Minimum Thread Priority

Thread.MAX_PRIORITY Maximum Thread Priority

Thread.NORM_PRIORITY Default Thread Priority

Priorities May Be Set Using setPriority() method:

setPriority(Thread.NORM_PRIORITY + 2);

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