Using Dhrystone to Evaluate CPU Integer Performance

I. Overview

Dhrystone is an industry-standard benchmark program specifically designed to evaluate a processor's integer and logical operation performance. It simulates typical program behavior by executing a predefined series of computationally intensive operations that do not involve floating-point calculations.

The test results are typically measured using two key metrics:

Dhrystones per Second: Represents the number of times the processor can complete the main Dhrystone loop in one second. A higher value indicates better performance.
DMIPS (Dhrystone Million Instructions Per Second): A standardized performance metric derived by comparing the Dhrystones per Second value against a baseline (the performance of a VAX 11/780 computer). It provides a relative performance reference across different platforms and architectures.

II. Enabling Dhrystone

To use the Dhrystone benchmark tool, you must set the following Kconfig option in your openvela board-level configuration file.

# Enable the Dhrystone benchmark
CONFIG_BENCHMARK_DHRYSTONE=y

III. Executing the Test

After configuring and compiling the firmware, you can run the test in the openvela NSH (NuttShell) terminal.

Command

In the terminal, execute the following command to start the test:

dhrystone

Sample Output

The program first attempts a small number of runs. If the execution time is too short, it automatically increases the number of runs to obtain a more accurate measurement. After the test completes, it prints the final performance data.

nsh> dhrystone

Dhrystone Benchmark, Version C, Version 2.2
Program compiled without 'register' attribute
Using MSC clock(), HZ=100

Trying 50000 runs through Dhrystone:
Measured time too small to obtain meaningful results

Trying 500000 runs through Dhrystone:
Final values of the variables used in the benchmark:

Int_Glob:            5
        should be:   5
Bool_Glob:           1
        should be:   1
... (Detailed intermediate variable verification output omitted) ...
Str_2_Loc:           DHRYSTONE PROGRAM, 2'ND STRING
        should be:   DHRYSTONE PROGRAM, 2'ND STRING

Microseconds for one run through Dhrystone:        8.5 
Dhrystones per Second:                          117647

IV. Interpreting the Results

After the test is complete, the program outputs detailed verification data and two key performance metrics.

Key Performance Metrics

Metric	Description
`Microseconds for one run through Dhrystone`	The average time required to execute one main Dhrystone loop, measured in microseconds (µs).
`Dhrystones per Second`	The number of main Dhrystone loops the processor can execute per second.

Calculating DMIPS

DMIPS is a more valuable standardized metric. It compares the test results with the performance of a VAX 11/780 computer (defined as 1 MIPS), which has a Dhrystone score of 1757 Dhrystones/sec.

You can use the following formula to convert the test result to DMIPS:

DMIPS = Dhrystones per Second / 1757

Based on the sample output above:

Dhrystones per Second = 117647
DMIPS = 117647 / 1757 ≈ 66.96

Normalized Performance (DMIPS/MHz)

To make a fair comparison between processors with different clock frequencies, DMIPS/MHz is often used as a normalized metric.

DMIPS/MHz = DMIPS / Processor Clock Frequency (MHz)

For example, if the processor runs at 100 MHz, its normalized performance is:

DMIPS/MHz = 66.96 / 100 ≈ 0.67