sample-7 - SEC. 7.4 475 7.5 MULTIMEDIA FILE SYSTEM...

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Unformatted text preview: SEC. 7.4 475 7.5 MULTIMEDIA FILE SYSTEM PARADIGMS Now that we have covered process scheduling in multimedia systems, let us continue our study by looking at multimedia file systems. These file systems use a different paradigm than traditional file systems. We will first review traditional file I/O, then turn our attention to how multimedia file servers are organized. To access a file, a process first issues an open system call. If this succeeds, the caller is given some kind of token, called a file descriptor in UNIX or a handle in Win- dows to be used in future calls. At that point the process can issue a read system call, providing the token, buffer address, and byte count as parameters. The operating system then returns the requested data in the buffer. Additional read calls can then be made until the process is finished, at which time it calls close to close the file and return its resources. This model does not work well for multimedia on account of the need for real-time behavior. It works especially poorly for displaying multimedia files coming off a remote video server. One problem is that the user must make the read calls fairly precisely spaced in time. A second problem is that the video server must be able to supply the data blocks without delay, something that is dif- ficult for it to do when the requests come in unplanned and no resources have been reserved in advance. To solve these problems, a completely different paradigm is used by mul- timedia file servers: they act like VCRs (Video Cassette Recorders). To read a multimedia file, a user process issues a start system call, specifying the file to be read and various other parameters, for example, which audio and subtitle tracks to use. The video server then begins sending out frames at the required rate. It is up to the user to handle them at the rate they come in. If the user gets bored with the movie, the stop system call terminates the stream. File servers with this stream- ing model are often called push servers (because they push data at the user) and are contrasted with traditional pull servers where the user has to pull the data in one block at a time by repeatedly calling read to get one block after another. The difference between these two models is illustrated in Fig. 7-1. 7.5.1 VCR Control Functions Most video servers also implement standard VCR control functions, including pause, fast forward, and rewind. Pause is fairly straightforward. The user sends a message back to the video server that tells it to stop. All it has to do at that point is remember which frame goes out next. When the user tells the server to resume, it just continues from where it left off. However, there is one complication here. To achieve acceptable perfor- mance, the server may reserve resources such as disk bandwidth and memory buffers for each outgoing stream. Continuing to tie these up while a movie is paused wastes resources, especially if the user is planning a trip to the kitchen to 476 MULTIMEDIA OPERATING SYSTEMS...
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This note was uploaded on 05/20/2011 for the course COP 4600 taught by Professor Yavuz-kahveci during the Spring '07 term at University of Florida.

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sample-7 - SEC. 7.4 475 7.5 MULTIMEDIA FILE SYSTEM...

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