Verilator_gantt creates a visual representation to help analyze Verilator multithreaded simulation performance, by showing when each macro-task starts and ends, and showing when each thread is busy or idle.

For an overview of use of verilator_gantt, see Code Profiling.

Gantt Chart Contents

The generated Gantt chart has time on the X-axis. Times shown are to the scale printed, i.e. a certain about of time for each character width. The Y-axis shows threads, each thread’s execution is shown on one line. That line shows “[” at the position in time when it executes.

Following the “[” is the CPU number the task executed on, followed by zero or more “-” to make the width of the characters match the scaled execution time, followed by a “]”. If the scale is too small, the CPU number and mtask number will not be printed. If the scale is very small, a “&” indicates multiple mtasks started at that time position.

Also creates a value change dump (VCD) format dump file which may be viewed in a waveform viewer (e.g. C<GTKWave>). See below.

Gantt Chart VCD Signals

In waveforms there are the following signals. Most signals the “decimal” format will remove the leading zeros and make the traces easier to read.


The number of mtasks active at this time, for best performance this will match the thread count. You may want to use an “analog step” format to view this signal.


For the given CPU number, the thread number executing.


For the given mtask id, the CPU it is executing on.


For the given thread number, the mtask id executing.

verilator_gantt Arguments


The filename to read data from, defaults to “profile_threads.dat”.


Displays a help summary, the program version, and exits.

--scale <n>

Sets the number of time units per character on the X-axis of the generated Gantt chart. (On x86, time units are rdtsc ticks.) Defaults to 0, which will automatically compute a reasonable scale where no two mtasks need to fit into same character width’s worth of scaled time.


Disables creating a .vcd file.

--vcd <filename>

Sets the output filename for vcd dump. Default is “verilator_gantt.vcd”.