Quality of Service Karrie Karahalios Spring 2007

Announcements Midterm Final Projects

Distributed Multimedia System Disk NetworkCPUNetwork Application ServerClient Process Device Steps: Resources: Guarantees:

Consider End-to-End Behavior NetworkMemoryDiskCPU Apps Operating System NetworkMemoryDiskCPU Operating System MM appsAppsMM apps Meta-scheduler attempts to reserve resources to guarantee end-to-end behavior

Basic Challenges Identification Negotiation Translation Specification Enforcement

Preliminaries Real-time systems Resources Management Reservation Allocation

Real Time (RT) Process Delivers results in predictable time –not necessarily fast –requests deterministic or stochastic Correctness means –errorless computation –meeting deadlines

Deadlines Hard deadlines –cannot be violated (if so, system fault) –cost money or human life Soft deadlines –misses can be tolerated if –not too many and not missed by much Examples

Characteristics of RT Systems Predictable response times to time-critical events Accurate system clocks and timing Able to schedule almost all resources Stability under overload

RT & Multimedia To meet demands of multimedia, use RT techniques along entire data path Relative to RT –relaxes deadline requirements and allows some deadlines to be missed entirely –periodic requests ease scheduling –continuous data allows adaptive allocation

Resource Required by tasks for manipulating data –CPU, disk, memory, network, etc. A resource has a capacity –space, utilization, bandwidth A resource can be: –active or passive –exclusive or shared

Resource Management Maps multimedia requirements onto respective capacities of the system Specified through a QOS model –parameters + relaxation procedures Carried out by a resource manager –ensure adherence to QoS specification

Resource Management Time Audio Mpeg-1 Mpeg-2 Interactive Video Abundant Sufficient, but stressed Insufficient Resource Needs

Resource Reservation Test schedulability –determine if enough remaining capacity Negotiate QOS parameters –If not, determine how close it can come and when –application decides if this is acceptable Reserve resources –allocates resources to meet negotiated QoS Schedule resources –compute appropriate schedule for each resource –algorithm affects previous steps

Resource Allocation Pessimistic –reserve for the worst case –worse utilization, but more guarantees Optimistic –reserve for average or minimum needs –better utilization, but less guarantees

Quality of Service (QoS) Refers to how good provided services are –the more applications demand, the more difficult it is to meet those demands Resource management realizes QoS –better management allow better QoS Examples

Layers of QoS User Application System MM devicesNetwork user QoS application QoS system QoS network QoSdevice QoS

Layers of QoS What are the right metrics? How to specify them? How to negotiate them? User Application System MM devicesNetwork user QoS application QoS system QoS network QoSdevice QoS How to enforce them? How to translate among them?

User and Application QoS Startup time Sample rate Bits per sample Frame rate Resolution Skew relationships Response time for interaction

System QoS Quantitative Bit rate Error rate Processing time Buffer sizes Throughput Qualitative Ordered delivery Error recovery Scheduling options

Network QoS Network load –[min, avg, max] arrival times Packet/cell size Packet loss rate End-to-end delay (latency) Variability in delay (jitter)

Types of Services Guaranteed –threshold or range –deterministic or statistical Predictive –match current to historical performance Best Effort –none or only minimal guarantees

Linear Bounded Arrival Process Divides end-to-end system view into a pipeline of discrete sessions –one session corresponds to a single resource Defines parameterization of workload –arrival of messages at a particular interface A message is one unit of work –typically blocks of CM data (bytes or time) Anderson, D. Metascheduling for Continuous Media, ACM TOCS, 11(3):

LBAP Models message arrival at a resource (I) –M - max message size (bytes) –R - max message rate (messages/second) –W – workload limit (max. messages that may arrive ahead of schedule) Such that for all t 0 < t 1 N I (t 0, t 1 ) < R|t 1 – t 0 | + W

Workload Workload W(t) of an LBAP at time t is w(t) = max{0, N I (t 0, t) - R|t – t 0 | } Property 1: w(t) < W for all t Property 2: for all t 1 < t 2 ; N I (t 1, t 2 ) < w(t 2 ) – w(t 1 ) + R|t 2 – t 1 |

Graph of w(t)

Logical Arrival Time (L) Let m 0 …m n denote sequence of messages and let a 0 …a n be their arrival times L(m 0 ) = a 0 L(m i+1 )=max{a i+1, L(m i ) + 1/R}

Logical Delay Between Interfaces Logical delay d(m) of message m between two interfaces is d(m) = L 2 (m) - L 1 (m), where L i (m) is arrival time of m at interface i. Actual delay of message m may be > L(m), if m arrives ahead of schedule at I 1 < L(m), if m completed ahead of schedule at I 2

Resources and Sessions A resource handles incoming messages –arrive at input interface, delivered to output interface Clients must reserve resource –M (max message size) –R (max message rate) –W in = input max message burst (messages) –W out = output max message burst (messages) –D = max logical delay (seconds), –A = min actual delay (seconds), –U = min unbuffered time (seconds) Arrival process at input interface specified by M, R, Win Arrival process at output interface specified by M, R, Wout

Compound Session S is a sequence of sessions S 1 …S N –input of S is that of S 1 ; output is that of S N –output of S i is input to S i+1

Delay and Buffer Size d(m) = L S N (m) – L S 1 (m) < sum(d i ) Maximum shared buffer size for S W + R(D – U), D = sum(d i ); W = w(t) for S 1 –realized when there is group of W messages followed by one message every 1/R and each resource uses its full delay for each message

QoS Specification Request S with given workload parameters and smallest delay bound If system can accept session –return minimum delay, reserves resources Otherwise –increase delay bound based on cost function –repeat request for S until allocated (or not)

Take Home Exercise Discuss tradeoffs of allowing people to pay for different qualities of service