Context switching time

embOS is designed to perform fast context switches. This section describes two different methods to calculate the execution time of a context switch from a task with lower priority to a task with a higher priority.
The first method uses port pins and requires an oscilloscope. The second method uses the high-resolution measurement functions. Example programs for both methods are supplied in the Sample directory of your embOS shipment.
Segger uses these programs to benchmark the embOS performance. You can use these examples to evaluate the benchmark results. Note, that the actual performance depends on many factors (CPU, clock speed, toolchain, memory model, optimization, configuration, etc.).
The following table gives an overview about the variations of the context switch time depending on the memory type and the CPU mode:

Target OS Version Memory CPU Mode Time
ATMEL AT91SAM7S256 3.50b Flash Thumb 7.562us
ATMEL AT91SAM7S256 3.50b Flash ARM 7.875us
ATMEL AT91SAM7S256 3.50b RAM ARM 5.896us
ATMEL AT91SAM7S256 3.50b RAM Thumb 6.187us

All named example performance values in the following section are determined with the following system configuration: ATMEL AT91SAM7S256 running with 48 MHz clock speed. All sources are compiled with IAR Embedded Workbench version 4.40A using thumb or arm mode with high optimization level.

Measurement with port pins and oscilloscope

The context switching time is the time between switching the LED on and off. If the LED is switched on with an active high signal, the context switching time is the time between rising and falling edge of the signal. If the LED is switched on with an active low signal, the signal polarity is reversed.
The real context switching time is shorter, because the signal also contains the overhead of switching the LED on and off. The time of this overhead is also displayed on the oscilloscope as a small peak right before the task switching time display and has to be subtracted from the displayed context switching time. The picture below shows a simplified oscilloscope signal with an active-low LED signal (low means LED is illuminated). There are switching points to determine:

  • A = LED is switched on for overhead measurement
  • B = LED is switched off for overhead measurement
  • C = LED is switched on right before context switch in low-prio task
  • D = LED is switched off right after context switch in high-prio task

The time needed to switch the LED on and off in subroutines is marked as time tAB. The time needed for a complete context switch including the time needed to switch the LED on and off in subroutines is marked as time tCD.
The context switching time tCS is calculated as follows:
tCS = tCD - tAB

Example measurements AT91SAM7S, ARM code in RAM

Task switching time has been measured with the parameters listed below:
embOS Version V3.50b
Application program: MeasureCST_Scope.c
Hardware: AT91SAM7S256 processor with 48MHz
Program is executing in RAM
ARM mode is used
Compiler used: IAR V4.40A
CPU frequency (fCPU): 47.9232MHz
CPU clock cycle (tCycle): tCycle = 1 / fCPU = 1 / 47.9232MHz = 20,866ns

Measuring tAB and tCD

tAB is measured as 312ns.
The number of cycles calculates
as follows:
CyclesAB = tAB / tCycle
=312ns / 20.866ns
= 14.952Cycles
=> 15Cycles
tCD is measured as 6217.6ns.
The number of cycles calculates
as follows:
CyclesCD = tCD / tCycle
= 6217.6ns / 20.866ns
= 297.977Cycles
=> 298Cycles

 

Resulting context switching time and number of cycles
The time which is required for the pure context switch is:
tContextSwitch = tCD - tAB = 298Cycles - 15Cycles => 283Cycles (5.9us @48MHz).

 

Example measurements AT91SAM7S, Thumb code in FLASH

Task switching time has been measured with the parameters listed below:
embOS Version V3.50b
Application program: MeasureCST_Scope.c
Hardware: AT91SAM7S256 processor with 48MHz
Program is executing in FLASH
Thumb mode is used
Compiler used: IAR V4.40A
CPU frequency (fCPU): 47.9232MHz
CPU clock cycle (tCycle): tCycle = 1 / fCPU = 1 / 47.9232MHz = 20,866ns

Measuring tAB and tCD

tAB is measured as 436.8ns.
The number of cycles calculates
as follows:
CyclesAB = tAB / tCycle
=436.8ns / 20.866ns
= 20.933Cycles
=> 21Cycles
tCD is measured as 8012ns.
The number of cycles calculates
as follows:
CyclesCD = tCD / tCycle
= 8012ns / 20.866ns
= 383.973Cycles
=> 384Cycles

 

Resulting context switching time and number of cycles
The time which is required for the pure context switch is:
tContextSwitch = tCD - tAB = 384Cycles - 21Cycles => 363Cycles (7.56us @48MHz).

Download the sample project below to test, compare and verify our context switching time measurements:

embOS context switching time sample

Description Compiler Download
embOS Measurement for the AT91SAM7S-EK IAR Embedded Workbench 4.41A embOS_Measurement

Measurement with port pins and oscilloscope

The example file MeasureCST_Scope.c uses the LED.c module to set and clear a port pin. This allows measuring the context switch time with an oscilloscope. The following source code is excerpt from MeasureCST_Scope.c:


#include "RTOS.h"
#include "LED.h"

static OS_STACKPTR int StackHP[128], StackLP[128]; // Task stacks
static OS_TASK TCBHP, TCBLP; // Task-control-blocks

/*********************************************************************
*
* HPTask
*/
static void HPTask(void) {
  while (1) {
    OS_Suspend(NULL); // Suspend high priority task
    LED_ClrLED0(); // Stop measurement
  }
}

/*********************************************************************
*
* LPTask
*/
static void LPTask(void) {
  while (1) {
    OS_Delay(100); // Synchronize to tick to avoid jitter
    //
    // Display measurement overhead
    //
    LED_SetLED0();
    LED_ClrLED0();
    //
    // Perform measurement
    //
    LED_SetLED0(); // Start measurement
    OS_Resume(&TCBHP); // Resume high priority task to force task switch
  }
}

/*********************************************************************
*
* main
*/
int main(void) {
  OS_IncDI(); // Initially disable interrupts
  OS_InitKern(); // Initialize OS
  OS_InitHW(); // Initialize Hardware for OS
  LED_Init(); // Initialize LED ports
  OS_CREATETASK(&TCBHP, "HP Task", HPTask, 100, StackHP);
  OS_CREATETASK(&TCBLP, "LP Task", LPTask, 99, StackLP);
  OS_Start(); // Start multitasking
  return 0;
}

 

Oscilloscope analysis

Measurement with high-resolution timer

The context switch time may be measured with the high-resolution timer.
The example MeasureCST_HRTimer_embOSView.c uses a high-resolution timer to measure the context switch time from a low priority task to a high priority task and displays the results on embOSView.


#include "RTOS.h"
#include "stdio.h"

static OS_STACKPTR int StackHP[128], StackLP[128]; // Task stacks
static OS_TASK TCBHP, TCBLP; // Task-control-blocks
static OS_U32 _Time; // Timer values

/*********************************************************************
*
* HPTask
*/
static void HPTask(void) {
  while (1) {
    OS_Suspend(NULL); // Suspend high priority task
    OS_Timing_End(&_Time); // Stop measurement
  }
}

/*********************************************************************
*
* LPTask
*/
static void LPTask(void) {
  char acBuffer[100]; // Output buffer

  OS_U32 MeasureOverhead; // Time for Measure Overhead
  OS_U32 v;
  //
  // Measure Overhead for time measurement so we can take
  // this into account by subtracting it
  //
  OS_Timing_Start(&MeasureOverhead);
  OS_Timing_End(&MeasureOverhead);
  //
  // Perform measurements in endless loop
  //
  while (1) {
    OS_Delay(100); // Sync. to tick to avoid jitter
    OS_Timing_Start(&_Time); // Start measurement
    OS_Resume(&TCBHP); // Resume high priority task 
    to force task switch
    v = OS_Timing_GetCycles(&_Time) - OS_Timing_GetCycles(&MeasureOverhead);
    v = OS_ConvertCycles2us(1000 * v); // Convert 
    cycles to nano-seconds
    sprintf(acBuffer, "Context switch time: %u.%.3u 
    usecr", v / 1000, v % 1000);
    OS_SendString(acBuffer);
  }
}

The example program calculates and subtracts the measurement overhead itself, so there is no need to do this. The results will be transmitted to embOSView, so the example runs on every target that supports UART communication to embOSView. The example program MeasureCST_HRTimer_Printf.c is equal to the example program MeasureCST_HRTimer_embOSView.c but displays the results with the printf() function for those debuggers which support terminal output emulation.