Part 1 - Foundation Example

This tutorial teaches you to use the basic concepts of system modeling including traffic, behavior, queues, delays and analysis. This Tutorial also covers the use of the Block Diagram Editor (BDE), construct models out of the blocks contained in the VisualSim libraries, run dynamic simulations, combine different outputs, and interpret the simulation results.

Block Diagram Overview

We use the model to study the performance of a system that has a priority-based FIFO/Queue. The block diagram is shown in Figure 1.

A traffic generator sends data packets or transactions or data structure to the FIFO. The FIFO outputs the next transaction after the previous transaction is processed.

VisualSim Tutorial Model Location

The following location contains the completed model to be built at the end of this tutorial.

VisualSim Library Blocks

Note: Listen-to-Port feature can be selected by right clicking on any of the input/output port that you want to debug. After selecting the Listen-to-Port feature, a blank white window will pop up. Run the model to see the values at the port. For Listen to Block feature, Right-Click on the block to debug and select "Kisten to Block". Listen to Block feature cannot be used for debugging heirarchical blocks as those blocks are used to hold many different blocks.

Construction Steps

Drag and drop the blocks listed below and place them in the BDE as shown previously in Figure 2.

Traffic->Traffic to generate the transactions based on the data structure template.
Behavior->ExpressionList to define the transaction fields and to compute the latency. Note that two ExpressionList blocks are required.
Resources-> Queues to define the FIFO.
Traffic->Delay to emulate the time taken to process a task.
Results->Text_Display to show the contents of the transaction after coming out of the Queue and finished the processing delay.
Results->TimedDataPlotter to display an XY graph of transaction latency vs. simulation time.
Full_Library --> Math Operations->Math and Trig->Const to specify the Queue number to pop. Right-click the Const block and select Appearance -> Flip Ports Horizontally.
Click but not drag the Relation button in the Menu bar. A black diamond will be placed in the BDE. This can be used to define a fan-out connection of the output of the Delay block.

Populate the block parameters as shown in Figures 3 to 7. No change is required for the Const, TimedDataPlotter, and Text_Display blocks. The Data Structure contains 6 "Header" fields and the field content can be used in the rest of the model.

Note that the purpose of using expression input.Priority = irand(1,5) is to assign a random value between 1 and 5 to the Data Structure field Priority. "input" identifies the data structure and is the name of the block port through which the data structure enters. If the block has multiple input ports, then the user must use the name of the respective port to identify the data structure that will be used in the Expression List block. Two new fields called Priority and Execution_Length are added to the Data Structure called input.

The Parameter Output_Ports is a list of all the output ports. The block can have multiple output ports.The Parameters Output_Values and Output_Conditions must match the same number of items as Output_Ports. The corresponding value is sent out on the respective output port.

The parameter Output_Conditions defines whether the data in Output_Values must be sent out on the port listed in Output_Ports. The Condition can be true or false, which is evaluated from an Expression.

Note that the expression TNow - input.TIME returns the end-to-end latency in the system. TNow is the current simulation time stamp and input.TIME is the time at which the transaction was generated by the Traffic block. When a Traffic block generates a data structure, it places the current time in the TIME field. The Expression_List parameter does not contain any expressions. The Output_Values can be an expression containing equations, transaction if-else, parameters and variables.

Connect the blocks as shown in Figure 2. There are two different types of connections in the model, one-to-one and one-to-many.

Port-to-port connections (one-to-one): To connect, click on the right port of the block, hold down the mouse and drag up to the left port of the block to be connected or vice versa. This forms a wire between the source and destination ports. The wire does not add delays to the transfer; it simply displays the connection between the ports. It is not sufficient to move one port over the other port. If the connection is made correctly, the wire is displayed as a thick line. If the connection is incomplete, the line is thin and disappears when other actions are performed.
Many-to-one connections: Use the black diamond or Relation button. You can place the relation on the BDE by using the control button + left-mouse click at the location where the Relation must be placed, or by clicking on the Relation button on the menu. In the latter case, the relation is placed in the middle of the BDE.

Other Connections

About Relation: Click the Relation icon and move it to the appropriate location for aesthetic connectivity. To connect the relation to the block port, hold down the control button and draw a line from the relation to the port. If this operation is performed correctly, the lines are thick. It is not sufficient to simply place the relation over a wire. All the wires must be connected to this diamond.
Block Parameter: Every block contains attributes which can be defined by the user. The latency calculation in ExpressionList2 has been entered directly into the Output_Values field and not in the Expression_List field.
Block Ports: All blocks have polymorphic ports. This means that the block determines the right transaction that flows through any port. There are a few blocks that have preset data types at their ports and the user the must ensure the right data types is sent to such ports. The Queue ‘pop’ port requires either an integer or an array.

Click on the Run button on the VisualSim toolbar to start the simulation.
The results are displayed both at the text window and the plot (refer to Figures 7 and 8).

Results

The value in the latency field is calculated from the time that the transaction was generated to the time it completed the processing. Every transaction is generated with a timestamp field called TIME. TNow is the current simulation time. Notice that the latency increases steadily for the first few transactions and then reduces for the fifth transaction.
Every transaction has the 6 header fields. A transaction can have any number of additional fields. Notice that the Execution_Length and the Priority are different for each transaction. Field ID is a sequence number for transactions generated by the same source. Notice that these transactions arrive out-of-order at the destination. This is because the "Queue" block reorders them in the FIFO, based on priority, before sending them out.