Empirical Workload Measurements and Analysis

  
Figure 2: VOSAIC Data

Our study is based on an understanding of real video workload environments, user behavioral patterns and system responses to changes in workload. We start by identifying a baseline class of workload components and a simplified characterization of the workload. For instance, we expect the video workloads to be largely I/O bound and not CPU bound .

Our metrics and measurements are derived from different Video-on-Demand (VOD) systems implemented at the University of Illinois, Urbana-Champaign. The first VOD system considered is the VOSAIC web system and we will observe the user access log characteristics obtained over a 20 day observation period of the VOSAIC (video-over-the-internet) system [4]. The user access logs allow us to analyze the user access statistics and general user information important for modeling of user satisfaction (see section 4.1). The second system considered is a hierarchical VOD system with a hierarchical set of video servers [15]. In this system, we observe performance characteristics such as jitter and frame loss rate. The third system is a simple remote VCR system with a single VOD server and client that we use to observe synchronization skew performance characteristics. The jitter, loss rate and skew characteristics allow us to analyze the system access and resource dependencies important for modeling of resource consumption (see section 4.2). Before we discuss performance analysis results, we briefly present the main architectural features of the three VOD systems.

• VOSAIC is essentially a framework for the integration of audio-visual information into a standard hypertext document and Web environment. It transmits information on-demand in real-time over the current Internet. While traditional VOD systems are designed to operate in a constrained environment, VOSAIC is designed to operate in unconstrained environments. VOSAIC is currently designed as a client-server video playback system. The session protocol used in VOSAIC is VDP (Video Datagram Protocol), which uses a combination of TCP and UDP: the playback command and request from client to server are sent over TCP connections and actual video and audio data transmission from server to client are carried out over UDP connections. VDP uses a feedback scheme to detect network congestion and automatically delete frames in response to the congestion. The HTTP protocol is used for text/data communications between the client and server while MPEG video/audio uses VDP.
• The hierarchical client-server VOD system utilizes a distributed server approach. The VOD system consists of a set of Cache Servers(CS) being on the same LAN with VOD Clients, a Cache Agent running on the CSs, and a Primary Server (PS) that is on an external network. The CS downloads the requested video streams from PS in non-realtime (using a best effort service) and provides real-time services to VOD clients. Although there is an initial startup delay while CS downloads the data for the first time, the VOD client does not have to cache the whole video as is the case with WWW applications. Once the video is cached on the CS, streaming for display and real-time services such as fast forward and rewind are available. The CA controls resources of the set of CSs on the same LAN to achieve good load balancing and better performance. The hierarchical VOD system consists of two protocols: (1) Hierarchical Media Management Protocol(HMMP) used by the CA, and (2) Multimedia Stream Session Protocol(MSSP). HMMP uses TCP/IP for management communication among the CSs. MSSP uses TCP/IP for (1) command control communication during connection setup and transmission, (2) downloading of a new video from PS to CS and (3) audio streaming from CS to VOD client. MSSP uses UDP/IP for video streaming from the CS to the VOD client. The CA with HMMP provides resource reservation during the connection setup, hence there is no adaptation activity during transmission over an admitted connection. In contrast, VOSAIC does not employ resource reservation during the connection setup, but supports feedback-oriented adaptation during video transmission.
• The remote VCR system consists of a single VOD client and VOD server utilizing an adaptive synchronization protocol [19]. The adaptive synchronization protocol synchronizes MPEG compressed videos. The connection setup and command control are performed over TCP/IP. The transmission component is built on top of the UDP/IP protocol stack. This protocol does not have any resource reservation, but it enforces adaptive synchronization to keep synchronization skews within acceptable bounds.

• User Access Analysis
• System Overhead Analysis