Dhrystone Benchmarking on MCUs

Posted on 3 mins read

Getting the cache to work The dhrystone benchmark application for Stratify OS is up and running. Dhrystone is a simple benchmark application that is designed to mimic a typical program. It characterizes the performance of both the compiler and the processor (and in this case the operating system). The result of the program is a number called DMIPS which is the amount of time to run one “dhrystone” divided by 1757 which is the number of dhrystones some old 1 MIPS processor could output.

I ran the benchmark on a variety of processors. The benchmark was running inside of Stratify OS. So there are a couple of factors that reduce the maximum possible DMIPS score that is advertised by chip manufacturers.

  • The application was compiled using mlong-calls which has a 5% to 10% penalty.
  • The kernel (including libc) was compiled with -Os rather than being purely speed optimized.
  • In some cases, I may have just made an error on the cache/flash settings that is causing degraded performance. I will update this post if I find any issues.
  • The application is running with the OS. Some processor time is being used by context switching and providing metrics over USB. This takes away another 10% or so as shown by the following task analysis from sl app.run:path=dhrystone,terminal task.analyze. The output was taken from a run with the STM32H743 but is rougly applicable to all tests.
summary: 
dhrystone-1.2: 
info: 
    name: dhrystone
    id: 2
    pid: 1
    memorySize: 32768
    stack: 0x2407FCF0
    heap: 0x24078000
activity: 
    cpuTime: 492453363
    cpuUsage: 92.787010%
    maximumStack: 808
    maximumHeap: 12696
    maximumMemoryUsage: 41.21%

The Results

Processor CPU DMIPS DMIPS/MHz Notes
RT1052 525MHz 675 DMIPS 1.29 DMIPS/MHz Running from external SDRAM with cache on
RT1052 525MHz 26 DMIPS 0.05 DMIPS/MHz Running from external SDRAM with cache off
STM32H743ZI 400MHz 457 DMIPS 1.14 DMIPS/MHz Running from internal flash/RAM with cache on
STM32H743ZI 400MHz 79 DMIPS 0.20 DMIPS/MHz Running from internal flash/RAM with cache off
STM32F746ZG 216MHz 231 DMIPS 1.07 DMIPS/MHz Running from internal flash/RAM with cache and flash accelerator on
STM32F746ZG 216MHz 120 DMIPS 0.55 DMIPS/MHz Running from internal flash/RAM with cache off and flash accelerator on
STM32F746ZG 216MHz 53 DMIPS 0.25 DMIPS/MHz Running from internal flash/RAM with cache and flash accelerator off
STM32F723ZG 216MHz 246 DMIPS 1.14 DMIPS/MHz Running from internal flash/RAM with cache on and flash accelerator off
STM32F723ZG 216MHz 53 DMIPS 0.25 DMIPS/MHz Running from internal flash/RAM with cache off and flash accelerator off
STM32F446ZE 168MHz 106 DMIPS 0.63 DMIPS/MHz Running from internal flash with data/instruction cache on
STM32F446ZE 168MHz 52 DMIPS 0.31 DMIPS/MHz Running from internal flash/RAM with data/instruction cache off
LPC4078 120MHz 85 DMIPS 0.71 DMIPS/MHz Running from internal ram
LPC4078 120MHz 63 DMIPS 0.53 DMIPS/MHz Running from internal flash
LPC1768 96MHz 51 DMIPS 0.53 DMIPS/MHz Running from internal flash
STM32L475VG 80MHz 52 DMIPS 0.65 DMIPS/MHz Running from internal ram
STM32L475VG 80MHz 50 DMIPS 0.62 DMIPS/MHz Running from internal flash
STM32F411VE 96MHz 62 DMIPS 0.65 DMIPS/MHz Running from internal flash with data/instruction cache on
STM32F411VE 96MHz 47 DMIPS 0.49 DMIPS/MHz Running from internal ram with with data/instruction cache off
STM32F411VE 96MHz 39 DMIPS 0.41 DMIPS/MHz Running from internal flash with with data/instruction cache off

Things to Notice

  • Running code on external SDRAM with cache on vs cache off makes a huge difference (obviously) on the RT1052.
  • The cache makes a big difference on the STM32H7/F7 chips even when running from internal flash and RAM. This is mainly due to the flash running at speeds lower than the CPU.
  • The LPC4078 has better performance executing from RAM than from flash.

Last Thing

The image at the top of the page shows the benchmark dashboard for when I finally got the cache working on the STM32H743. To see all the data, you can login to the Stratify Dashboard and browse to things.

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