Multimedia/Real-Time
Prashanth Reddy
Multimedia and Real-Time
• Audio and Video – Continuous Media (CM)
• Graphics – Discrete media
• CM
– High data rates
– Timing requirements
• Soft real-time applications
Problems
• Traditional OSs do not provide good support
– Schedulers
• Round Robin, Priority scheduling
• No sense of urgency
– Large number of user-kernel level interactions
• Efficient usage of resources
• Varying demands
• Delays, jitter
Papers
• Scheduling and IPC Mechanisms for Continuous
Media
– New scheduling and IPC mechanisms
• Split-level scheduling
– User-level and kernel-level scheduler
– Use shared memory
• Memory mapped streams
– Shared memory FIFO
Papers
• User-specified Adaptive Scheduling in a Streaming
Media Network
– Deliver data in resource-limited, small-scale
environments
– Meet the preferences and demands of the users
– MediaNet – a distributed stream processing system
Deadline/Workahead Scheduling
• Each message has a logical arrival time l(m)
• Each real-time process has a fixed logical
delay bound
• A real-time process is critical at time ‘t’ if
l(m) <= t
• Workahead processes - real-time processes
that have pending messages but not critical
DWS contd…
Split-level CPU scheduling
• Combines the advantages of thread and
LWPs
• Minimizes user/kernel interactions
• Also prioritizes LWPs in different VASs
correctly
• Kernel level scheduler and user level
scheduler communicate through shared
memory
Split-level scheduling contd…
Split-level scheduling contd…
Memory Mapped Streams
• CM data must be moved to/from kernel
• Components
– Control and Synchronization
– Data location transfer
– Data transfer
• Requires user/kernel interaction in one or
more of the above components
MMS contd…
• A shared memory FIFO
• Uses shared memory for control and
synchronization
• Uses shared memory to hold data location
and the data
• Mechanisms
– Statically shared pages of physical memory
– Fixed range of virtual pages, mapped dynamically
– Array of “message descriptors”
Performance Evaluation
Performance Evaluation contd…
Summary
• Proper scheduling through Split-level
scheduling and usage of shared memory
• MMS – low-overhead IPC mechanism
• Reduced user-kernel interactions
User-specified Adaptive Scheduling
in a Streaming Media Network
• MediaNet – a distributed stream processing
system
• Users can specify how the system should
adapt
• Uses local and global resource scheduling
• Adapts without resource reservation
MediaNet Architecture
Video source
User’s desired
stream &
adaptation prefs
subscribe
schedules
Global
scheduling
service
Video description,
location, &
resource info
publish
from MediaNet’s website
feedback
Video player
Specifications
• Continuous Media Network (CMN)
– Directed acyclic graph of operations
• frame dropping, transcoding, compression and
decompression etc.
• User specification
– One or more CMNs, each with associated
utility value
• Goal: Maximize users’ utility while
utilizing the network efficiently
Operations
frm1
…
Op
…
frmn
Other attributes
• Fixed location?
• Transitional?
from MediaNet’s website
Frame size Interval
Example User Specification
Utility
CMN
1.0
Vid
Prio*
pcS
0.3
pcD
Vid
Prio*
pcS
0.1
Vid
pcS
User
Prio*
Drop
B
Prio*
Drop
PB
Prio*
User
pcD
User
pcD
Scheduling
Scheduling contd…
Implementation
• Global scheduling service
– implemented on a single node
– eventually hierarchical
• Local, per-node schedulers
– monitor and report available bandwidth; eventually
CPU + memory usage
– implement local CMNs
• Global scheduler reconfigures schedules online
Local Scheduler
• Implement the CMN given by the GS
– Must correctly reconfigure on-line
• Report monitoring info back to GS
• Implemented in Cyclone
– Type-safe, C-like language
– One component per operation, dynamically
reconfigurable
• Uses TCP for send/receive
Monitoring
• Monitor available bandwidth
– Keep track of TCP throughput, attempted vs.
actual bandwidth
• Send regular reports to global scheduler
• Too pessimistic
– “creep” bandwidth estimate additively to
optimistically attempt higher utility configs
Reconfiguration
• New configuration is applied in parallel with the
current configuration
– Old operations are “flushed” along the dataflow path
– New operations are enabled when all old ones are
flushed on a particular node
• Challenges
– Rapid reconfiguration
– Low disruption to stream
Experiments
• Conducted on 8 850 MHz PIII’s, 512 MB RAM, 100 Mb/s
Ethernet, Red Hat Linux 7.1 using a “bowtie” topology:
MPEG clip
Bandwidth Requirements
Frame rate
I+P+B
I+P
I
30 fps
145 KB/s
88 KB/s
27 KB/s
Results
No adaptation
Local, proactive frame dropping
Priority-based frame dropping
MediaNet
Multi-User Performance
Multi-User Performance contd…
Multi-User Performance contd…
B frame dropping op
Multi-User Performance contd…
B frame dropping op
Summary
• Application-specific QoS via user specs
• High network utilization and per-user utility
via global scheduling:
– Share resources between flows in a multicast-like
manner, but generalized to CMNs
– Utilize multiple, redundant paths
– Intelligently place operations to reduce network
utilization
• Adapts to resource availabilities on-line
Conclusion
• Split-level scheduling
– Tight coupling between user-level and kernellevel schedulers
– Difficult to employ multiple scheduling policies
• Scalability
– Hierarchical global scheduling service
• Automate setting utility values
Acknowledgements
• Some of the slides have been taken from
MediaNet’s website:
http://www.cs.umd.edu/projects/medianet/
Thank You