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1/31/2006 Lecture9 gac1 1

ASAP and ALAP scheduling

Were now entering the final portion of the course

Scheduling and retiming

Resource sharing algorithms

Floorplanning

Function Approximation

Perspectives for the future

This lecture covers

The ASAP scheduling algorithm

The ALAP scheduling algorithm and operation slack Introducing timing constraints into schedules

1/31/2006 Lecture9 gac1 2

ASAP Scheduling

The simplest type of scheduling occurs when wewish to optimize the overall latency of the

computation and do not care about the number ofresources required

This can be achieved by simply starting each

operation in a CDFG as soon as its predecessorshave completed

This strategy gives rise to the name ASAP for As

Soon As Possible

1/31/2006 Lecture9 gac1 3

ASAP Scheduling

Lets label each edge in the CDFG with the latencyof the node producing that edge

Then scheduling under ASAP is equivalent to

finding the longest path between each operationand the source node

Since a CDFG is a DAG, we can use the DAGlongest path algorithm presented in Lecture 8

Consider the original example from Lecture 1, andassume that multiplication takes two cycles,

whereas addition and comparison take one cycle

1/31/2006 Lecture9 gac1 4

ASAP Scheduling

Applying the DFG algorithm to finding the longest path

between the start and end nodes leads to the scheduledstart times on the right-hand diagram

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