Schedule Viewer - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

The Schedule Viewer provides a detailed view of the synthesized RTL, showing each operation and control step of the function, and the clock cycle that it executes in. It helps you to identify any loop dependencies that are preventing parallelism, timing violations, and data dependencies.

The Schedule Viewer is displayed by default in the Analysis perspective. You can open it from the Module Hierarchy window by right-clicking a module and selecting Open Schedule Viewer from the menu.

In the Schedule Viewer,

  • The left vertical axis shows the names of operations and loops in the RTL hierarchy. Operations are in topological order, implying that an operation on line n can only be driven by operations from a previous line, and will only drive an operation in a later line. Depending upon the type of violations found the Schedule Viewer shows additional information for each operation:
    • Resource limitation: displays the type of operation(read/write), type of memory used( RAM_1p or RAM_2p). In the image below the vecIn is a memory which is a dual port ram and trying to perform 3 reads in a single iteration. This causes an II violation because of a resource limitation and the tool is highlighting the operation which is scheduled in the next cycle of the load operation.
    • Dependency: displays information related to iterations which have a loop carried dependency. For example, a read transaction could have a dependency on a prior write value.
  • The top horizontal axis shows the clock cycles in consecutive order.
  • The vertical dashed line in each clock cycle shows the reserved portion of the clock period due to clock uncertainty. This time is left by the tool for the Vivado back-end processes, like place and route.
  • Each operation is shown as a gray box in the table. The box is horizontally sized according to the delay of the operation as percentage of the total clock cycle. In case of function calls, the provided cycle information is equivalent to the operation latency.
  • Multi-cycle operations are shown as gray boxes with a horizontal line through the center of the box.
  • The Schedule Viewer also displays general operator data dependencies as solid blue lines. As shown in the figure below, when selecting an operation you can see solid blue arrows highlighting the specific operator dependencies. This gives you the ability to perform detailed analysis of data dependencies. The green dotted line indicates an inter-iteration data dependency.
  • Memory dependencies are displayed using golden lines.
  • In addition, lines of source code are associated with each operation in the Schedule Viewer report. Right-click the operation to use the Goto Source command to open the input source code associated with the operation.

In the figure below, the loop called RD_Loop_Row is selected. This is a pipelined loop and the initiation interval (II) is explicitly stated in the loop bar. Any pipelined loop is visualized unfolded, meaning one full iteration is shown in the schedule viewer. Overlap, as defined by II, is marked by a thick clock boundary on the loop marker.

The total latency of a single iteration is equivalent to the number of cycles covered by the loop marker. In this case, it is three cycles.

Figure 1. Schedule Viewer

The Schedule Viewer displays a menu bar at the top right of the report that includes the following features:

  • A drop-down menu, initially labeled Focus Off, that lets you specify operations or events in the report to select.
  • A text search field to search for specific operations or steps (), and commands to Scroll Up or Scroll Down through the list of objects that match your search text
  • Zoom In, Zoom Out, and Zoom Fit commands ().
  • The Filter command () lets you dynamically filter the operations that are displayed in the viewer. You can filter operations by type, or by clustered operations.
    • Filtering by type allows you to limit what operations get presented based on their functionality. For example, visualizing only adders, multipliers, and function calls will remove all of the small operations such as “and” and “or”s.
    • Filtering by clusters exploits the fact that the scheduler is able to group basic operations and then schedule them as one component. The cluster filter setting can be enabled to color the clusters or even collapse them into one large operation in the viewer. This allows a more concise view of the schedule.
Figure 2. Operation Causing Violation

You can quickly locate II violations using the drop-down menu in the Schedule Viewer, as shown in the figure above. You can also select it through the context menu in the Module Hierarchy view.

To locate the operations causing the violation in the source code, right-click the operation and use the Goto Source command, or double-click the operation and the source viewer will appear and identify the root of the object in the source.

Timing violations can also be quickly found from the Module Hierarchy view context menu, or by using the drop-down menu in the Schedule Viewer menu. A timing violation is a path of operations requiring more time than the available clock cycle. To visualize this, the problematic operation is represented in the Schedule Viewer in a red box.

By default all dependencies (blue lines) are shown between each operation in the critical timing path.

Properties View

At the bottom of the Schedule Viewer, as shown in the top figure, is the Properties view that displays the properties of a currently selected object in the Schedule Viewer. This lets you see details of the specific function, loop, or operation that is selected in the Schedule Viewer. The types of elements that can be selected, and the properties displayed include:

  • Functions or Loops
    Initiation Interval (II)
    The number of clock cycles before the function or loop can accept new input data.
    Loop Iteration Latency
    The number of clock cycles it takes to complete one iteration of the loop.
    The number of clock cycles required for the function to compute all output values, or for the loop to complete all iterations.
    Indicates that the function or loop are pipelined in the RTL design.
    The timing slack for the function or loop.
    The number of iterations a loop completes.
    Resource Utilization
    Displays the number of BRAM, DSP, LUT, or FF used to implement the function or loop.
  • Operation and Storage Mapping
    Location which contains the code.
    Op Code
    Operation which has been scheduled, for example, add, sub, and mult. For more information, refer to the BIND_OP or BIND_STORAGE pragmas or directives.
    Op Latency
    Displays the default or specified latency for the binding of the operation or storage.
    Bitwidth of the Operation.
    Defines the implementation used for the specified operation or storage.