Thinking in Parallel – Task Decomposition

New Mexico Supercomputing Challengein partnership with Intel Corp. and NM EPSCoR

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New Mexico EPSCoR Program is funded in part by the National Science Foundation award #0814449 and the State of New Mexico. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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Agenda

Definition review of task decomposition

Steps involved in task decomposition

Conceptual example of task decomposition

Hands-on task decomposition parallel activity

Methodology

Domain decomposition – Used when the same operation is performed on a large number of similar data items.

Task decomposition – Used when some different operations can be performed at the same time.

Pipelining – Used when there are sequential operations on a large amount of data.

Task (Functional) Decomposition

Identify data dependencies between tasks.

Group tasks according to dependencies.

Assign groups to processors.

Task Decomposition – Dependencies

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Edges (arrows) depict data dependencies between tasks.

Is there a logical

way to assign the tasks

to separate cpu’s

?

Task Decomposition – Task Grouping

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Task Decomposition – CPU Assignment

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CPU 2

Task Decomposition – Step 1

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CPU 2

Task Decomposition – Step 2

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CPU 2

Task Decomposition – Step 3

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CPU 2

Task Decomposition – Step 4

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CPU 2

Hands-on Activity 1 –Box Stuffing

Your team will perform a series of tasks.

* See handout for detailed instructions

Activity 1 – Setup

Give all items to each group.

Activity 1 – Execution

1. Select one team member to be the processor.

2. After all of the processors are given the instructions, start timer.

3. The processor should perform these steps as quickly as possible:• Put pen (used for name tag) and pencil of a

different color in box.• Put stapled papers in folder.• Attach sticker to outside of box.• Fill out name tag in pen.• Collate papers (in ROYGBIV or numeric order) and

staple together.• Put folder in box.• Paper clip name tag to folder.

4. Stop timer.

Activity 1 – Debrief

Was there any wasted effort?

• If so, discuss possible reasons.

• If not, discuss possible reasons.

Activity 2 –Discovering Dependencies

Your team will identify dependencies between tasks.

Activity 2 – Setup

Give task list and a pair of scissors to each group.

Activity 2 – Execution

1. Cut task list into strips.

2. Layout all task strips of paper on table.

3. Organize the task strips so that there would be minimal wasted effort if the tasks were performed in that order.

4. Explain how your group organized the list.

Activity 2 – Debrief

Did every group arrive at the same task order?

• If so, what were the key conclusions in common?

• If not, discuss possible reasons for the differences.

Activity 3 – Box Stuffing with Dependencies

Your team will perform tasks in the order dictated by the identified dependencies.

Activity 3 – Setup

Give all items to each group.

Activity 3 – Execution

1. Pick a different team member to be the processor.

2. After all of the processors are given the instructions, start timer.

3. The processor should perform the tasks in the chosen order.

4. Stop timer.

Activity 3 – Debrief

1. Was there less wasted time or effort than in activity 1?

2. Which group was the fastest?

3. Was that group fastest due to the speed of its processor, or the task order?

4. Are there some tasks, or sets of tasks, that could be done simultaneously (in parallel)?

Activity 4 – Identifying Opportunities for Parallelism

Your team will identify opportunities for parallelism, subject to dependencies

Activity 4 – Setup

Give task list to each group.

Activity 4 – Execution

1. Lay out all task strips of paper on table.

2. Separate the tasks into groups that can be performed simultaneously.

3. Organize the tasks within each group according to the dependencies.

4. Explain how your group organized the list.

Activity 4 – Debrief

Did every group arrive at the same task organization?

•If so, what were the key conclusions in common?

•If not, discuss possible reasons for the differences.

Activity 5 – Box Stuffing with Parallel Dependencies

Your team will execute tasks with dependencies and parallelism, assigning sets of tasks to different team members.

Activity 5 – Setup

Give all items to each group.

Activity 5 – Execution

1. After all of the processors are given the instructions, start timer.

2. Perform the tasks in the chosen order, and working in parallel to the extent possible (more than one person in the team can be working at the same time).

3. Stop timer.

Activity 5 – Debrief

1. Was this activity completed faster than activity 3?

2. Were the team members equally occupied?

3. Which group was the fastest?

4. Was that group fastest due to the speed of its members, or the task order?