The Windows Scheduler Peter Shirley-Quirk

The Windows Scheduler In this presentation we will look at: What the scheduler is How it works Tips for developers

In this presentation we will look at:

What the scheduler is

How it works

Tips for developers

What is a scheduler? A scheduler shares out the computer system’s processing resource fairly between all the processes it has to run Types of scheduler Long-term decides whether the system can run any additional jobs/processes Mid-term used to swap out a process Short-term decides which thread is to be executed

A scheduler shares out the computer system’s processing resource fairly between all the processes it has to run

Types of scheduler

Long-term decides whether the system can run any additional jobs/processes

Mid-term used to swap out a process

Short-term decides which thread is to be executed

How does Windows Scheduler work? Scheduling algorithm Managing a queue of threads that are ready to execute Performing context switches and allocating time slices to respond to interrupts that affect which threads are able to run Ensuring that threads get a chance to run

Scheduling algorithm

Managing a queue of threads that are ready to execute

Performing context switches and allocating time slices to respond to interrupts that affect which threads are able to run

Ensuring that threads get a chance to run

When is a thread Ready to run? Figure 3-19, Inside Microsoft Windows 2000, Third Edition by David A. Solomon and Mark E. Russinovich

Context switching (1) Context switching happens: At the end of a time slice When a higher priority thread becomes ready e.g. garbage collection thread starts When the currently running thread goes into a wait state To run a thread the processor needs the full context registers stack (user and kernel) environment

Context switching happens:

At the end of a time slice

When a higher priority thread becomes ready e.g. garbage collection thread starts

When the currently running thread goes into a wait state

To run a thread the processor needs the full context

registers

stack (user and kernel)

environment

Context switching (2) To switch context the scheduler: Saves the context for the executing thread Places that thread at the end of the queue for its priority Finds the highest priority queue that contains ready threads Removes the thread at the head of the queue, loads its context, and executes it

To switch context the scheduler:

Saves the context for the executing thread

Places that thread at the end of the queue for its priority

Finds the highest priority queue that contains ready threads

Removes the thread at the head of the queue, loads its context, and executes it

Process Priority Classes The Process Priority Class is under the designer’s control Can be read or set using System.Diagnostics.Process.PriorityClass AboveNormal Time-critical tasks that must be executed immediately e.g. the Task List dialog High BelowNormal E.g. multimedia applications RealTime E.g. screen saver, indexer etc. Idle No special scheduling needs. Normal

Where does the priority come from? The thread’s base priority is based on several factors Process Priority Class Thread Priority Level Foreground / background These combine to give the Thread’s Base Priority The dynamic priority can be boosted by the scheduler

The thread’s base priority is based on several factors

Process Priority Class

Thread Priority Level

Foreground / background

These combine to give the Thread’s Base Priority

The dynamic priority can be boosted by the scheduler

When does the scheduler boost the priority? Avoiding starvation To prevent a high priority task from completely blocking a lower priority task the latter will be given a priority boost Avoiding choking CPU bound vs. IO bound – burst characteristics A CPU bound thread can choke resources from an IO bound thread – perceived as poor responsiveness from mouse or keyboard

Avoiding starvation

To prevent a high priority task from completely blocking a lower priority task the latter will be given a priority boost

Avoiding choking

CPU bound vs. IO bound – burst characteristics

A CPU bound thread can choke resources from an IO bound thread – perceived as poor responsiveness from mouse or keyboard

Resolving priority inversion Thread A, a low-priority thread, is executing code in a critical section (lock). Thread B, the high-priority thread, begins waiting for a shared resource (e.g. the lock) from thread A. Thread C has medium priority. Thread C receives all the processor time Scheduler boosts priority of A so that it can exit the lock A B C Wait Ready Time Priority A

Thread A, a low-priority thread, is executing code in a critical section (lock).

Thread B, the high-priority thread, begins waiting for a shared resource (e.g. the lock) from thread A.

Thread C has medium priority. Thread C receives all the processor time

Scheduler boosts priority of A so that it can exit the lock

What if there’s more than one processor? The system uses a SMP model to schedule threads on multiple processors Any thread can be assigned to any processor The system schedules threads to run concurrently You can influence the scheduler Thread Affinity Thread Ideal Processor

The system uses a SMP model to schedule threads on multiple processors

Any thread can be assigned to any processor

The system schedules threads to run concurrently

You can influence the scheduler

Thread Affinity

Thread Ideal Processor

Multi-Processor Architecture Hyperthreading 2 virtual processors but sharing some processor resources – consider using thread affinity Intel Dual-core Sharing onboard cache – thread affinity AMD dual core Treat as independent processors The future: 4-way, 8-way Design for parallel processing

Hyperthreading

2 virtual processors but sharing some processor resources – consider using thread affinity

Intel Dual-core

Sharing onboard cache – thread affinity

AMD dual core

Treat as independent processors

The future: 4-way, 8-way

Design for parallel processing

Some final thoughts Designing for concurrency Take advantage of multiple processors Aim to blend CPU and IO work Know your synchronization mechanisms Design to avoid deadlock Keep locks private and short Be aware of Garbage Collection in .NET Usually quick, occasionally quite slow

Designing for concurrency

Take advantage of multiple processors

Aim to blend CPU and IO work

Know your synchronization mechanisms

Design to avoid deadlock

Keep locks private and short

Be aware of Garbage Collection in .NET

Usually quick, occasionally quite slow

More information http://msdnwiki.microsoft.com Managing Processes Sample System.Diagnostics namespace Process ProcessThread System.Threading namespace Thread

http://msdnwiki.microsoft.com

Managing Processes Sample

System.Diagnostics namespace

Process

ProcessThread

System.Threading namespace

Thread