Soft Scheduling - Scalable Continuous Media Streaming Systems - page 79

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

50 100 150 200 250 300 350 400 450 500 550

Media Block Size (KB)

DRS (Quantum Atlas III)
DRS (Quantum Atlas 10K)
DRS (Seagate Barracuda)
DRS (Seagate Cheetah)
DRS (IBM 9ES)
Single-Round Scheduling

Figure 4.10 Per-stream buffer requirement comparison ( R = 150KB/s, ε = 10 − 6 )

4.6.5 Buffer Requirement
Soft scheduling does not modify the way in which buffer is managed and hence has the same
buffer requirement as hard scheduling. For disk scheduling algorithms such as SCAN and
CSCAN, the buffer requirement will be two buffers per stream, one for disk retrieval and
one for transmission. If DRS is employed, then additional buffers will be required to stage
early-retrieved media blocks. Using the formulae in Section 4.3.3, we computed the per-stream
buffer requirement for various media block sizes and summarized the results in Figure 4.10.
Here we have two observations. The first observation is that buffer requirement in all cases
increases with larger media block size as expected. For single-round scheduling (SRS), buffer
requirement is independent of disk models. Differences between disk models in the DRS case
are due to differences in the service round length. The second observation is that the buffer
requirement of DRS is only modestly higher than SRS. This is because additional buffers are
only needed to stage retrievals whereas buffer requirement for transmission is not affected
by DRS. As an illustration, a PC media server with 512MB available memory will have
sufficient buffers for more than 1,000 concurrent streams (Quantum Atlas-10K, Q = 128KB,

150KB/s) using DRS. Hence, whether DRS should be employed would simply become
a cost-effectiveness issue (i.e., memory cost versus disk costs) to be worked out by the system
designer.

R =