1. A Dynamic Critical Path Algorithm for Scheduling Scientific Workflow Applications on Global Grids
e-Science IEEE 2007
Report: Wei-Cheng Lee

2. Abstract
Dynamic Critical Path (DCP) based workflow scheduling algorithm that determines efficient mapping of tasks by calculating the critical path in the workflow task graph at every step.
It assigns priority to a task in the critical path which is estimated to complete earlier.
Using simulation, we have compared the performance of our proposed approach with other existing heuristic and meta-heuristic based scheduling strategies for different type and size of workflows.

3. Outline 3 1 Introduction 2 3 4 Performance Evaluation 3 5
The Proposed DCP-G Algorithm 3
Calculation of AEST and ALST in DCP-G 4
Performance Evaluation 3 5
Results and Observations 4
Conclusions

4. Dynamic Critical Path (DCP)
Introduction
Dynamic Critical Path (DCP)
Design for mapping tasks on to homogeneous processors.
Dynamic Critical Path for Grids (DCP-G)
Map and schedule tasks in a workflow on to heterogeneous resources in a dynamic Grid environment.
Heuristic
Myopic, Min-Min, Max-Min, HEFT.
Meta-heuristic
GRASP, Genetic Algorithm.

5. The Proposed DCP-G Algorithm
For a task graph, the lower and upper bounds of starting time for a task are denoted:
Absolute Earliest Start Time (AEST)
Absolute Latest Start Time (ALST)
DCP algorithm
DCP-G
The initial AEST and ALST values are calculated for the resource which
provides the minimum execution time for the task.
For mapping a task on the critical path, all available resources are
considered by DCP-G
When a task is mapped to a resource, its execution time and data
transfer time from the parent node are updated accordingly.

6. Calculation of AEST and ALST in DCP-G
In DCP-G, the start time of a task is not finalized until it is mapped to a resource.
Absolute Execution Time
Absolute Data Transfer Time

7. Calculation of AEST and ALST in DCP-G
Here, the communication time between two tasks is considered to be zero if they are mapped to the same resource, and equal to the ADTT of parent task, if the child is not mapped yet.
Dynamic Critical Path Length
ALST of a task t in resource R, denoted as ALST(t, R)

8. Resource Selection
To select the resource that provides the minimum execution time for that task.
This is discovered by checking all the available resources for one that minimizes the potential start time of the critical child task on the same resource, where the critical child task is the one with the least difference of AEST and ALST among all the child tasks of the critical task.

9. DCP-G Example

10. DCP-G Example T0`s AET is: 150000MI / 1500MIPS = 100/PerSecond
T0`s ADTT is: 1GByte / 200Mbps = 8Gb/200Mbps
= 40/PerSecond
T2`s AEST is: = 140
T2`s ALST is: 420 – 200 – 80 = 140
DCPL is : = 890

11. DCP-G Example T2`s AEST is: 0 + 100 + 0 = 100
T2`s ALST is: 380 – 200 – 80 = 100
If T4 was mapped to R2, the DCPL should equal to 770.
Because the AEST(T4) is: =720
So, DCPL is: = 770.
Whereas, mapping to R1, AEST(T4) is:
= 700,
and DCPL is: = 750

12. Performance Evaluation
1. Workflow model.

13. Performance Evaluation
2. Resource model.

14. Results and Observations
1. Execution time in static environment.

15. Results and Observations

16. Results and Observations
2. Execution time in dynamic environment.

17. Results and Observations

18. Results and Observations
3. Scheduling time.
GRASP creates Restricted Candidate List (RCL) for each unmapped ready task and then selects resource for the task in random.

19. Conclusion
It is evident that among the heuristics- based scheduling techniques, DCP-G can generate better schedule by up to 20% in static environment, especially for random and parallel workflow, irrespective or workflow size.
GA and GRASP can generate more effective schedule than DCP-G for random and fork-join workflow, but they suffer from the problem of higher scheduling time.
In dynamic environment, heuristics based techniques adapt to the dynamic nature of resources and can avoid performance degradation. But metaheuristics based techniques perform worse in this situation due to the unavailability of mapped resources at certain intervals.

20. Thank You For Your Attention!
Wei-Cheng Lee
Thank You
For Your Attention!