Understanding stack/heap system stats

ladislas · May 11, 2022, 9:28pm

Hi all –

To get a better understanding of our system, we recently turned on Mbed statistics for heap, stack and cpu.

https://os.mbed.com/docs/mbed-os/v6.15/apis/mbed-statistics.html

Looking at the code, I’m not sure I understand the difference between current, max, total, reserved as seen here:

github.com

ARMmbed/mbed-os/blob/e2e927e2ee0742e6d360870b05f6d31bd0517582/platform/include/platform/mbed_stats.h#L61-L69


      
          typedef struct {
              uint32_t current_size;      /**< Bytes currently allocated on the heap */
              uint32_t max_size;          /**< Maximum bytes allocated on the heap at one time since reset */
              uint32_t total_size;        /**< Cumulative sum of bytes allocated on the heap that have not been freed */
              uint32_t reserved_size;     /**< Current number of bytes reserved for the heap */
              uint32_t alloc_cnt;         /**< Current number of allocations that have not been freed since reset */
              uint32_t alloc_fail_cnt;    /**< Number of failed allocations since reset */
              uint32_t overhead_size;     /**< Number of bytes used to store heap statistics. This overhead takes up space on the heap, reducing the available heap space */
          } mbed_stats_heap_t;

and here:

github.com

ARMmbed/mbed-os/blob/e2e927e2ee0742e6d360870b05f6d31bd0517582/platform/include/platform/mbed_stats.h#L81-L86


      
          typedef struct {
              uint32_t thread_id;         /**< Identifier for the thread that owns the stack or 0 if representing accumulated statistics */
              uint32_t max_size;          /**< Maximum number of bytes used on the stack since the thread was started */
              uint32_t reserved_size;     /**< Current number of bytes reserved for the stack */
              uint32_t stack_cnt;         /**< The number of stacks represented in the accumulated statistics or 1 if representing a single stack */
          } mbed_stats_stack_t;

What does reserved mean in case of stack and heap? What about max?

Thanks a lof for your kind help

Best,
– Ladislas

ladislas · May 12, 2022, 2:24pm

Okay, I think I’ve got the solutions.

Digging deeper into:

I’ve come to the conclusion that the naming is a bit misleading.

Regarding stack:

reserved_size - → Current number of bytes reserved for the stack
max_size - → Could be named used_size as it is equal to total size - free space (see mbed-os/mbed_stats.c at e2e927e2ee0742e6d360870b05f6d31bd0517582 · ARMmbed/mbed-os · GitHub)

Regarding heap:

reserved_size - → Current number of bytes reserved for the heap. In practice == reserved_size == mbed_heap_size, see mbed-os/mbed_alloc_wrappers.cpp at e2e927e2ee0742e6d360870b05f6d31bd0517582 · ARMmbed/mbed-os · GitHub
total_size - → Cumulative sum of bytes allocated on the heap ~~that have not been freed~~ (There’s not decrement, so freeing heap will not change this value (unless I’m missing something), see mbed-os/mbed_alloc_wrappers.cpp at e2e927e2ee0742e6d360870b05f6d31bd0517582 · ARMmbed/mbed-os · GitHub
max_size - → Maximum bytes allocated on the heap at one time since reset
current_size - → Bytes currently allocated on the heap

ladislas · May 12, 2022, 8:11pm

Well actually, stack::max_size seems to really be max_size.

I wonder if it’s possible to have the current stack size.

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Understanding stack/heap system stats

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