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MemoryAnalysis.md

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Memory analysis

FreeRTOS heap and task stack

FreeRTOS statically allocate its own heap buffer in a global variable named ucHeap. This is an array of uint8_t. Its size is specified by the definition configTOTAL_HEAP_SIZE in FreeRTOSConfig.h FreeRTOS uses this buffer to allocate memory for tasks stack and all the RTOS object created during runtime (timers, mutexes,...).

The function xPortGetFreeHeapSize() returns the amount of memory available in this ucHeap buffer. If this value reaches 0, FreeRTOS runs out of memory.

NRF_LOG_INFO("Free heap : %d", xPortGetFreeHeapSize());

The function uxTaskGetSystemState() fetches some information about the running tasks like its name and the minimum amount of stack space that has remained for the task since the task was created:

TaskStatus_t tasksStatus[10]
auto nb = uxTaskGetSystemState(tasksStatus, 10, NULL);
for (int i = 0; i < nb; i++) {
  NRF_LOG_INFO("Task [%s] - %d", tasksStatus[i].pcTaskName, tasksStatus[i].usStackHighWaterMark);

Global heap

Heap is used for dynamic memory allocation (malloc() / new). NRF SDK defaults the heap size to 8KB. The size of the heap can be specified by defining __HEAP_SIZE=8192 in src/CMakeLists.txt:

add_definitions(-D__HEAP_SIZE=8192)

You can trace the dynamic memory allocation by using the flag --wrap of the linker. When this flag is enabled, the linker will replace the calls to a specific function by a call to __wrap_the_function(). For example, if you specify -Wl,-wrap,malloc in the linker flags, the linker will replace all calls to void* malloc(size_t) by calls to void* __wrap_malloc(size_t). This is a function you'll have to define in your code. In this function, you can call __real_malloc() to call the actual `malloc()' function.

This technic allows you to wrap all calls to malloc() with you own code.

In src/CMakeLists.txt:

set_target_properties(${EXECUTABLE_NAME} PROPERTIES
        ...
        LINK_FLAGS "-Wl,-wrap,malloc ..."
        ...
        )

In main.cpp:

uint32_t totalMalloc = 0;
extern "C" {
    extern void* __real_malloc(size_t s);
    void *__wrap_malloc(size_t s) {
      totalMalloc += s;
      return __real_malloc(s);
    }
}

This function sums all the memory that is allocated during the runtime. You can monitor or log this value. You can also place breakpoints in this function to determine where the dynamic memory allocation occurs in your code.

Global stack

The stack is used to allocate memory used by functions : parameters and local variables. NRF SDK defaults the heap size to 8KB. The size of the heap can be specified by defining __STACK_SIZE=8192 in src/CMakeLists.txt:

add_definitions(-D__STACK_SIZE=8192)

NOTE: FreeRTOS uses its own stack buffer. Thus, the global stack is only used for main() and IRQ handlers. It should be possible to reduce its size to a much lower value.

NOTE: [?] How to track the global stack usage?

#LittleVGL buffer TODO

#NimBLE buffers TODO

#Tools