Soft Scheduling - Scalable Continuous Media Streaming Systems - page 67

Transmission for new stream
starts at round i+2.

U+1 U+1

u u u u

2 2 2 2

Transmission

1 1 1 1

. . . . . . . . .

Disk Retrieval

New stream arrives
during round i.

Retrieval for new stream
starts at round i+1.

Figure 4.1 Admission scheduling in conventional round-based scheduler

two rounds and the average admission delay is 1.5 rounds (Figure 4.1). This admission delay
not only affects the start-up latency, but also affects the system response time when interactive
playback controls (e.g., seeking to a new playback point) are performed.

4.4.1 Admission Algorithm
Soft scheduling allows a new way of shortening this admission delay. The principle is to try
to admit a new stream into the current round rather than waiting for the next round to begin. If
admission to the current round is successful, then we can reduce the admission delay by one
service round as shown in Figure 4.2.
Let s i be the start time for round i . Consider a new stream arriving at time t a when service
round i is in progress. At that instant, the disk head is either moving to the next target track
(Case 1) or stationed in a track reading data (Case 2). Let n be the track number for the target
track (Case 1) or current track (Case 2). Assuming Poisson arrival and randomized placement,
the disk head would be equally probable to be located at any one of the N tracks for non-zoned
disks when a new request arrives. For zoned disks, the probability for the disk head to be
located at track n when a new stream arrives, denoted by p n , would be skewed by the track
size:

z n
N −

p n = (4.12)

j = 0

1

z j

where z j is the size of track j .
We consider the case where the disk head is scanning in the forward direction with increasing
track number. The case for reverse direction scanning is similar and thus is omitted here. To