RESOURCE MANAGEMENT AND QOS

RESOURCE MANAGEMENT AND QOS

A computer system has many resources, which may be

required to solve a problem: CPU, memory at different levels,

bandwidth of I/O devices, e.g., disk and host networkinterface,

and bandwidth of the system bus. In Figure 3, the

basic resource types CPU, memory, and bandwidth are

partitioned into concrete system resources.

One of the primary functions of an OS is to multiplex

these resources among the users of the system. In the advent

of conflicting resource requests, the traditional OS

must decide which requests are allocated resources to operate

the computer system fairly and efficiently [Peterson et

al. 85]. Fairness and efficiency are still the most important

goals for resource management in today’s commodity OSs.

However, with respect to multimedia applications, other

goals that are related to timeliness become of central importance.

For example, user interactions and synchronization

require short response times with upper bounds, and

multimedia streams require a minimum throughput for a

certain period of time. These application requirements are

specified as QoS requirements. Typical application-level

QoS specifications include parameter types like frame rate,

resolution, jitter, end-to-end delay, and synchronization

skew [Nahrstedt et al. 99]. These high-level parameters

have to be broken down (or mapped) into low-level parameters

and resources that are necessary to support the

requested QoS, like CPU time per period, amount of memory,

and average and peak network bandwidth. A discussion

of this mapping process is beyond the scope of this

paper, but we want to emphasize at this point that such a

specification of resource requirements is difficult to

achieve. A constant frame rate does not necessarily require

constant throughput and constant execution time per period.

Furthermore, user interactions can generate unpredictable

resource requirements.

In order to meet QoS requirements from applications

and users, it is necessary to manage the system resources in

such a manner that sufficient resources are available at the

right time to perform the task with the requested QoS. In

particular, resource management in OS for QoS comprises

the following basic tasks:

Specification and allocation request for resources that

are required to perform the task with the requested

QoS.

Admission control includes a test whether enough resources

are available to satisfy the request without interfering

with previously granted requests. The way

this test is performed depends on requirement specification

and allocation mechanism used for this resource.

Allocation and scheduling mechanisms assure that a

sufficient share of the resource is available at the right

time. The type of mechanism depends on the resource

type. Resources that can only exclusively be used by a

single process at a time have to be multiplexed in the

temporal domain. In other words, exclusive resources,

like CPU or disk I/O, have to be scheduled. Basically,

we can differentiate between fair scheduling, real-time

scheduling, and work and non-work conserving scheduling

mechanisms. So-called shared resources, like

memory, basically require multiplexing in the spatial

domain, which can be achieved, e.g., with the help of a

table.

Accounting tracks down the actual amount of resources

that is consumed in order to perform the task.

Accounting information is often used in scheduling

mechanisms to determine the order of waiting requests.

Accounting information is also necessary to

make sure that no task consumes more resources than

negotiated and steals them (in overload situations)

from other tasks. Furthermore, accounting information

might trigger system-initiated adaptation.

Adaptation might be initiated by the user/application

or the system and can mean to downgrade QoS and

corresponding resource requirements, or to upgrade

them. Adaptation leads in any case to new allocation

parameters. Accounting information about the actual

resource consumption might be used to optimize

resource utilization.

Deallocation frees the resources.

Specification, admission control, and allocation and

scheduling strongly depend on the particular resource type,

while adaptation and resource accounting represent more

resource type independent principles. Thus, the following

two subsections introduce adaptation and resource accounting,

before we discuss the different system resources

in more detail.

 ADAPTATION

There are two motivations for adaptation in multimedia

systems: (1) resource requirements are hard to predict, e.g.,

VBR video and interactivity; and (2) resource availability

cannot be guaranteed if the system includes best-effort sub-

Memory

CPU

Cache

Main memory

Disk

Disk I/O

Network

I/O

Resources Bandwidth Bus

Figure 3: Operating system resources