cpre 458/558: real-time systems
DESCRIPTION
CprE 458/558: Real-Time Systems. Distributed Real-Time Systems (contd.). Layered architecture of a node in a distributed real-time system. SENDER. RECEIVER. Local scheduling. Resource Reclaiming. Resource Reclaiming. Task Scheduling. Task Scheduling. Information Exchange. TP. SP. - PowerPoint PPT PresentationTRANSCRIPT
CprE 458/558: Real-Time Systems (G. Manimaran) 1
CprE 458/558: Real-Time Systems
Distributed Real-Time Systems (contd.)
CprE 458/558: Real-Time Systems (G. Manimaran) 2
Layered architecture of a node in a distributed real-time system
Location policy
Task Scheduling
Resource
Reclaiming
TP SP IP
Routi
ng
Sch
ed.Call
Admission
Control
Location policy
Task Scheduling
IP SP TP
Routi
ng
Sch
ed.Call
Admission
Control
Resource
Reclaiming
Routi
ng
Sch
ed.Call
Admission
Control
Message Flow
Real-Time Channel Establishment
Information Exchange
SENDER RECEIVER
INTERMEDIATE NODE
Local scheduling
Global scheduling
CprE 458/558: Real-Time Systems (G. Manimaran) 3
Algo 1: Focused Addressing with Bidding (FAB)
• Information policy– Periodic– Fraction of CPU available
• Transfer policy– Local scheduler admission decision
• Selection policy– The task that fails the admission test
• Location policy– Focused node with bidding
CprE 458/558: Real-Time Systems (G. Manimaran) 4
Focused addressing with bidding
• Focused node (Ns) may not have enough surplus for accommodating the migrated task, due to stale state info.
• In parallel, the sender sends request for bid (RFB) message to other lightly loaded nodes (based on estimate various times), asking them to send bid to Ns for receiving the migrated task. The RFB contains info about the task.
• Bid specifies how quickly the node can process the task, etc.
• If Ns cannot guarantee the task, it evaluates the bids and (re)migrates the task to the best bidder.
• In all theses calculations, the decision time, scheduling time, migration time are estimated and added with comp. time to see if the task can meet the deadline. If not, no bid is sent.
CprE 458/558: Real-Time Systems (G. Manimaran) 5
Algo 2: Buddy set algorithm
• Transfer policy– Threshold-based – three thresholds are maintained based on
which the node’s state is identified as Underload (U), Normal, Overload
• Selection policy– The tasks that fail admission test at the local node
• Information policy– Based on buddy set.– When a node makes transition into or out of U state, it informs
its buddies of its state
• Location policy– One of the buddies is chosen as the receiver, based on the
load info provided by the info policy
CprE 458/558: Real-Time Systems (G. Manimaran) 6
Buddy set algorithm (contd.) -- Issues
• Choosing buddy set– Too large set: high communication overhead– Too small set: may not be able to find a suitable receiver within a
buddy set– Topology needs to be taken into account while choosing the buddy
nodes
• Choice of thresholds– Larger Upper threshold: lower the rate at which tasks will be
migrated– The choices of thresholds depend on size of buddy set, topology,
network bandwidth
• Thrashing– A node, X, could be a buddy for several nodes. When many of these
several nodes become overloaded, they migrate their tasks to the node X and making it overloaded. This results in further migration of task.
– Therefore, the buddy set should be carefully constructed.
CprE 458/558: Real-Time Systems (G. Manimaran) 7
Algo 3: Integrated scheme
• Information policy– Based on Maekawa set concept
• Transfer policy and Info policy– Load estimation based on tasks in the queue
• Location policy– Chooses receiver node not only based on node
state but also the link/path state so as to achieve feasible (bounded) task migration
• Promotes interaction among schedulers– Message scheduler and Transfer policy– Message scheduler and Location policy
CprE 458/558: Real-Time Systems (G. Manimaran) 8
Maekawa set based information policy
• Based on symmetric set concept• Fully decentralized algorithm• Each node assumes equal responsibility in obtaining global state
• Each node maintains three sets
– Request set (Ri): Set of nodes to whom request the state info– Information set (Si): Set of nodes to whom sent your state information– Status set (Si): Set of nodes whose state is maintained by the given node
– For all i, j: Intersection(Ri,Rj) is not null; Keep the size of the set minimum.
• Message complexity for obtaining global info is K = O(Sqrt(N)) as opposed to O(N), where N is the number of nodes
– Optimal set size exists for: N = K * (K-1) + 1. Other values of N: degenerate case.
• Construction method: Finite projective plane, Grid method
CprE 458/558: Real-Time Systems (G. Manimaran) 9
R1 = I1 = {1,2,4} S1 = {1,5,7}
R2 = I2 = {2,3,5} S2 = {1,2,6}
R3 = I3 = {3,4,6} S3 = {2,3,7}
R4 = I4 = {4,5,7} S4 = {1,3,4}
R5 = I5 = {5,6,1} S5 = {2,4,5}
R6 = I6 = {6,7,2} S6 = {3,5,6}
R7 = I7 = {7,1,3} S7 = {4,6,7}
Maekawa sets – example for 7 nodes
•Request set (Ri): Set of nodes to which it sends requests for state information
•Information set (Ii): Set of nodes to which it sends information about its state
•Status set (Si): Set of nodes whose state information it maintains