4.1 Advanced Operating Systems Desktop Scheduling You are running some long simulations. In the mean time, why not watch an illegally downloaded Simpsons.

4.1 Advanced Operating Systems

Desktop Scheduling

You are running some long simulations.

In the mean time, why not watch an illegally downloaded Simpsons episode?

How can the operating system know that the movie player slowdown is much more perceivable than the batch process slowdown?


Scheduling Metric

Operating systems’ schedulers need a new metric to identify and prioritize interactive and multimedia jobs.


Real-Time Priority

Most of UNIX and Windows versions support a real-time scheduling.A real-time process is served before the other processes will be served.In some OSes the real-time definition is set on coding time, while in some other OSes the definition can be set on running time.Both shift the burden to the user/programmer


What can be done automatically?

Desktop operating systems prioritize processes based on CPU consumption.– The less CPU consumed by a process, the higher

its priority.

It’s good for fairness:– Each process gets a fair share of the CPU.– Processes that wake up get a higher priority.

This scheme originates from UNIX (1970s).

And… operating systems still work that way.


Linux Non-Realtime Scheduling

The Linux scheduler partitions time into epochs.In each epoch, every process has a time slice of how long it may run.When there are no ready processes with a time slice left, a new epoch is started.The order within the epoch is set according to the dynamic priority.– The dynamic priority is a number between 100 to

140 that is set according to the nice value plus/minus bonus of 5. The bonus reflects the CPU consuming of the processes.


Linux Scheduling (cont.)

The calculation of the time slice is based on:– The "nice" value.– In Linux 2.4 Processes that did not use up all their previous

time slice transfer half of it to the new epoch.• In Linux 2.6 the half of the unused time is not added.

Linux 2.6 tries to recognize interactive processes. Essentially, tasks get an interactive credit when they sleep for a longtime, and lose an interactive credit when they run for a long time.– Linux 2.6 will give another time slice in the same epoch to an

interactive process, if the interactive process uses up its initial time slice.

In Linux 2.4 the time slices were calculated in the beginning of the epoch. In Linux 2.6 the time slice is calculated when a process leaves the CPU.


Windows 2000 Scheduling

The "nice" value of Windows 2000 is called "thread's type".

There are also some dynamic rules:– Threads associated with the focus window get a triple

quantum.• Multi-Media threads are not always in the focus window.

– After an I/O wait, the priority is boosted by a factor that is inversely proportional to the speed of the I/O device. This is then decremented by one at the end of each quantum, until the original priority is reached again.

• Keyboard and mouse are very slow; hence contribute a big boost.


What About Interactivity?

In its early days, interactive computing was text based.– Drawing text does not require much CPU.– User input devices are very slow.

Interactive jobs waited a lot, and consumed little CPU.

Result: the little-CPU-long-waits-high-priority scheme favored interactive jobs.

Unfortunately, this is no longer the case.


Contemporary Interactive Applications

Nowadays, applications use a lot of visual and audible means for user interaction e.g.:– Window based systems and the graphical user

interface widgets.– Movie players with CPU intensive decompression.– Games based on CPU massive graphical engines.

Modern Interactive and multimedia applications consume substantial amounts of CPU cycles.Let us denote such processes as Human-Centered, or HuC for short.


CPU Consumption

CPU consumption can no longer separate the HuC processes from the rest– Mplayer and Xine are movie players and Quake is a computer game.


Voluntary Context Switches

Switches that are induced by the process itself.

Does a HuC process relinquish the CPU voluntarily more often?– Reactive nature.– Block on device.– Wait for a timer.


Effective Quantum Length

Effective Quantum Length is the interval from when a process is allocated a CPU, until the CPU is relinquished.Does a HuC process consume CPU time in small portions?


Huc Characteristic

Because CPU consumption patterns cannot automatically identify HuC processes, the correct way is to directly quantify the amount of user interaction for the various processes.Devices that mediate between the user and a process will be called HuC devices e.g. keyboard, mouse, screen, joystick, speaker.– For now, we will focus on the first three…


Quantifying User Interactions

In UNIX, all user I/O is mediated by the X-Windows system - specifically by the X-Server.The X-Server has been modified a little:– X saves user I/O statistics for each window.– These statistics are periodically reported to the

operating system kernel.

Quantifying I/O with the user raises cognitive questions about relative importance of events.– Is a keyboard press more important than a mouse drag?– Is drawing a character more important than drawing an

image?


The Simplest Solution

Input:– simply accumulate input events.– Dragging the mouse will generate multiple events, while

clicking the mouse or a keyboard press are just one event.

• The user is very slow!

Output:– quantify the changes produced by each graphical request.

• In graphical shapes, count the pixels that have been changed.• In text, assume all of the pixels in the character's box have been

changed.

– Do not count changes in hidden portions of the window.


Scheduling by I/O Quantification

Windows with input are the most important ones.The next priority is widows with output.Windows without I/O are less important.Applications like Emacs are unaffected.– Emacs requires less than 1% of the CPU cycles and

gets that in the old and the modified scheduler.

The responsiveness of the windows manager is also improved. – On heavy loaded systems the windows manager

(using the old scheduler) finds difficult in moving windows around.


Experimental Results

Desktop Scheduling autonomously identifies HuC processes, and prioritizes them.


Open Questions

Where does sound fit in?

Is it practical to measure network bandwidth in order to improve streaming?

How can other devices integrated into this model?