In some cases, a firmware application's available RAM may run low or run out entirely. In these cases, it is necessary to tune the memory usage of the firmware application.
In general, firmware should aim to leave some headroom of free internal RAM to deal with extraordinary situations or changes in RAM usage in future updates.
Before optimizing ESP-IDF RAM usage, it is necessary to understand the basics of {IDF_TARGET_NAME} memory types, the difference between static and dynamic memory usage in C, and the way ESP-IDF uses stack and heap. This information can all be found in :doc:`/api-reference/system/mem_alloc`.
In embedded systems, heap fragmentation can be a significant issue alongside total RAM usage. The heap measurement APIs provide ways to measure the largest free block. Monitoring this value along with the total number of free bytes can give a quick indication of whether heap fragmentation is becoming an issue.
- Generally speaking, minimizing static memory usage requires monitoring the ``.data`` and ``.bss`` sizes. For tools to do this, see :ref:`idf.py-size`.
- Internal ESP-IDF functions do not make heavy use of static RAM in C. In many instances (such as {IDF_TARGET_STATIC_MEANS_HEAP}), static buffers are still allocated from the heap. However, the allocation is performed only once during feature initialization and will be freed if the feature is deinitialized. This approach is adopted to optimize the availability of free memory at various stages of the application's life cycle.
- Constant data can be stored in flash memory instead of RAM, thus it is recommended to declare structures, buffers, or other variables as ``const``. This approach may require modifying firmware functions to accept ``const *`` arguments instead of mutable pointer arguments. These changes can also help reduce the stack usage of certain functions.
:SOC_BT_SUPPORTED:- If using Bluedroid, setting the option :ref:`CONFIG_BT_BLE_DYNAMIC_ENV_MEMORY` will cause Bluedroid to allocate memory on initialization and free it on deinitialization. This does not necessarily reduce the peak memory usage, but changes it from static memory usage to runtime memory usage.
- If using OpenThread, enabling the option :ref:`CONFIG_OPENTHREAD_PLATFORM_MSGPOOL_MANAGEMENT` will cause OpenThread to allocate message pool buffers from PSRAM, which will reduce static memory use.
In FreeRTOS, task stacks are usually allocated from the heap. The stack size for each task is fixed and passed as an argument to :cpp:func:`xTaskCreate`. Each task can use up to its allocated stack size, but using more than this will cause an otherwise valid program to crash, with a stack overflow or heap corruption.
Therefore, determining the optimum sizes of each task stack, minimizing the required size of each task stack, and minimizing the number of task stacks as whole, can all substantially reduce RAM usage.
The Hardware Stack Guard is a reliable method for detecting stack overflow. This method uses the hardware's assist-debug module to monitor the CPU's stack pointer register. A panic is immediately triggered if the stack pointer register goes beyond the bounds of the current stack (see :ref:`Hardware-Stack-Guard` for more details). The Hardware Stack Guard can be enabled via the :ref:`CONFIG_ESP_SYSTEM_HW_STACK_GUARD` option.
End of Stack Watchpoint
~~~~~~~~~~~~~~~~~~~~~~~
The End of Stack Watchpoint feature places a CPU watchpoint at the end of the current stack. If that word is overwritten (such as in a stack overflow), a panic is triggered immediately. End of Stack Watchpoints can be enabled via the :ref:`CONFIG_FREERTOS_WATCHPOINT_END_OF_STACK` option, but can only be used if debugger watchpoints are not already being used.
Stack Canary Bytes
~~~~~~~~~~~~~~~~~~
The Stack Canary Bytes feature adds a set of magic bytes at the end of each task's stack, and checks if those magic bytes have changed on every context switch. If those magic bytes are overwritten, a panic is triggered. Stack Canary Bytes can be enabled via the :ref:`CONFIG_FREERTOS_CHECK_STACKOVERFLOW` option.
There is a less effective alternative, see :ref:`CONFIG_FREERTOS_CHECK_STACKOVERFLOW` docs for details.
..note::
When using the End of Stack Watchpoint or Stack Canary Bytes, it is possible that a stack pointer skips over the watchpoint or canary bytes on a stack overflow and corrupts another region of RAM instead. Thus, these methods cannot detect all stack overflows.
..only:: SOC_ASSIST_DEBUG_SUPPORTED
Recomended and default option is :ref:`CONFIG_ESP_SYSTEM_HW_STACK_GUARD` which avoids this disadvantage.
Run-time Methods to Determine Stack Size
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- The :cpp:func:`uxTaskGetStackHighWaterMark` returns the minimum free stack memory of a task throughout the task's lifetime, which gives a good indication of how much stack memory is left unused by a task.
- The easiest time to call :cpp:func:`uxTaskGetStackHighWaterMark` is from the task itself: call ``uxTaskGetStackHighWaterMark(NULL)`` to get the current task's high water mark after the time that the task has achieved its peak stack usage, i.e., if there is a main loop, execute the main loop a number of times with all possible states, and then call :cpp:func:`uxTaskGetStackHighWaterMark`.
- Often, it is possible to subtract almost the entire value returned here from the total stack size of a task, but allow some safety margin to account for unexpected small increases in stack usage at runtime.
- Avoid stack heavy functions. String formatting functions (like ``printf()``) are particularly heavy users of the stack, so any task which does not ever call these can usually have its stack size reduced.
- Avoid allocating large variables on the stack. In C, any large structures or arrays allocated as an automatic variable (i.e., default scope of a C declaration) uses space on the stack. To minimize the sizes of these, allocate them statically and/or see if you can save memory by dynamically allocating them from the heap only when they are needed.
- Avoid deep recursive function calls. Individual recursive function calls do not always add a lot of stack usage each time they are called, but if each function includes large stack-based variables then the overhead can get quite high.
Combine tasks. If a particular task is never created, the task's stack is never allocated, thus reducing RAM usage significantly. Unnecessary tasks can typically be removed if those tasks can be combined with another task. In an application, tasks can typically be combined or removed if:
ESP-IDF allocates a number of internal tasks for housekeeping purposes or operating system functions. Some are created during the startup process, and some are created at runtime when particular features are initialized.
The default stack sizes for these tasks are usually set conservatively high to allow all common usage patterns. Many of the stack sizes are configurable, and it may be possible to reduce them to match the real runtime stack usage of the task.
If internal task stack sizes are set too small, ESP-IDF will crash unpredictably. Even if the root cause is task stack overflow, this is not always clear when debugging. It is recommended that internal stack sizes are only reduced carefully (if at all), with close attention to high water mark free space under load. If reporting an issue that occurs when internal task stack sizes have been reduced, please always include the following information and the specific configuration that is being used.
-:doc:`/api-reference/system/esp_event` system task to execute callbacks for the default system event loop has stack size :ref:`CONFIG_ESP_SYSTEM_EVENT_TASK_STACK_SIZE`.
- The Ethernet driver creates a task for the MAC to receive Ethernet frames. If using the default config ``ETH_MAC_DEFAULT_CONFIG`` then the task stack size is 4 KB. This setting can be changed by passing a custom :cpp:class:`eth_mac_config_t` struct when initializing the Ethernet MAC.
- FreeRTOS idle task stack size is configured by :ref:`CONFIG_FREERTOS_IDLE_TASK_STACKSIZE`.
- If using the :doc:`/api-reference/protocols/mqtt` component, it creates a task with stack size configured by :ref:`CONFIG_MQTT_TASK_STACK_SIZE`. MQTT stack size can also be configured using ``task_stack`` field of :cpp:class:`esp_mqtt_client_config_t`.
- To see how to optimize RAM usage when using ``mDNS``, please check `Minimizing RAM Usage <https://docs.espressif.com/projects/esp-protocols/mdns/docs/latest/en/index.html#minimizing-ram-usage>`__.
Aside from built-in system features such as ESP-timer, if an ESP-IDF feature is not initialized by the firmware, then no associated task is created. In those cases, the stack usage is zero, and the stack-size configuration for the task is not relevant.
Normally, optimizing heap usage consists of analyzing the usage and removing calls to ``malloc()`` that are not being used, reducing the corresponding sizes, or freeing previously allocated buffers earlier.
:SOC_WIFI_SUPPORTED:- :ref:`wifi-buffer-usage` describes options to either reduce the number of static buffers or reduce the maximum number of dynamic buffers in use, so as to minimize memory usage at a possible cost of performance. Note that static Wi-Fi buffers are still allocated from the heap when Wi-Fi is initialized, and will be freed if Wi-Fi is deinitialized.
:esp32:- The Ethernet driver allocates DMA buffers for the internal Ethernet MAC when it is initialized - configuration options are :ref:`CONFIG_ETH_DMA_BUFFER_SIZE`, :ref:`CONFIG_ETH_DMA_RX_BUFFER_NUM`, :ref:`CONFIG_ETH_DMA_TX_BUFFER_NUM`.
- Several Mbed TLS configuration options can be used to reduce heap memory usage. See the :ref:`reducing_ram_usage_mbedtls` docs for details.
:esp32:- In single-core mode only, it is possible to use IRAM as byte-accessible memory added to the regular heap by enabling :ref:`CONFIG_ESP32_IRAM_AS_8BIT_ACCESSIBLE_MEMORY`. Note that this option carries a performance penalty, and the risk of security issues caused by executable data. If this option is enabled, then it is possible to set other options to prefer certain buffers allocated from this memory: :ref:`CONFIG_MBEDTLS_MEM_ALLOC_MODE`, :ref:`NimBLE <CONFIG_BT_NIMBLE_MEM_ALLOC_MODE>`.
:esp32:- Reduce :ref:`CONFIG_BTDM_CTRL_BLE_MAX_CONN` if using Bluetooth LE.
There are other configuration options that increases heap usage at runtime if changed from the defaults. These options are not listed above, but the help text for the configuration item will mention if there is some memory impact.
The available DRAM at runtime for heap usage is also reduced by the static IRAM usage. Therefore, one way to increase available DRAM is to reduce IRAM usage.
If the app allocates more static IRAM than available, then the app will fail to build, and linker errors such as ``section '.iram0.text' will not fit in region 'iram0_0_seg'``, ``IRAM0 segment data does not fit``, and ``region 'iram0_0_seg' overflowed by 84-bytes`` will be seen. If this happens, it is necessary to find ways to reduce static IRAM usage in order to link the application.
To analyze the IRAM usage in the firmware binary, use :ref:`idf.py-size`. If the firmware failed to link, steps to analyze are shown at :ref:`idf-size-linker-failed`.
The following options will reduce IRAM usage of some ESP-IDF features:
- Enable :ref:`CONFIG_FREERTOS_PLACE_FUNCTIONS_INTO_FLASH`. Provided these functions are not incorrectly used from ISRs, this option is safe to enable in all configurations.
- Enable :ref:`CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH`. Provided these functions are not incorrectly used from ISRs, this option is safe to enable in all configurations.
- Enable :ref:`CONFIG_RINGBUF_PLACE_ISR_FUNCTIONS_INTO_FLASH`. This option is not safe to use if the ISR ringbuf functions are used from an IRAM interrupt context, e.g., if :ref:`CONFIG_UART_ISR_IN_IRAM` is enabled. For the ESP-IDF drivers where this is the case, you can get an error at run-time when installing the driver in question.
:SOC_WIFI_SUPPORTED:- Disabling Wi-Fi options :ref:`CONFIG_ESP_WIFI_IRAM_OPT` and/or :ref:`CONFIG_ESP_WIFI_RX_IRAM_OPT` options frees available IRAM at the cost of Wi-Fi performance.
:CONFIG_ESP_ROM_HAS_SPI_FLASH:- Enabling :ref:`CONFIG_SPI_FLASH_ROM_IMPL` frees some IRAM but means that esp_flash bugfixes and new flash chip support are not available, see :doc:`/api-reference/peripherals/spi_flash/spi_flash_idf_vs_rom` for details.
:esp32:- Disabling :ref:`CONFIG_SPI_FLASH_ROM_DRIVER_PATCH` frees some IRAM but is only available in some flash configurations, see the configuration item help text.
:esp32:- If the application uses PSRAM and is based on ESP32 rev. 3 (ECO3), setting :ref:`CONFIG_ESP32_REV_MIN` to ``3`` disables PSRAM bug workarounds, saving 10 KB or more of IRAM.
- Disabling :ref:`CONFIG_ESP_EVENT_POST_FROM_IRAM_ISR` prevents posting ``esp_event`` events from :ref:`iram-safe-interrupt-handlers` but saves some IRAM.
:SOC_GPSPI_SUPPORTED:- Disabling :ref:`CONFIG_SPI_MASTER_ISR_IN_IRAM` prevents spi_master interrupts from being serviced while writing to flash, and may otherwise reduce spi_master performance, but saves some IRAM.
:SOC_GPSPI_SUPPORTED:- Disabling :ref:`CONFIG_SPI_SLAVE_ISR_IN_IRAM` prevents spi_slave interrupts from being serviced while writing to flash, which saves some IRAM.
- Setting :ref:`CONFIG_HAL_DEFAULT_ASSERTION_LEVEL` to disable assertion for HAL component saves some IRAM, especially for HAL code who calls ``HAL_ASSERT`` a lot and resides in IRAM.
- Refer to the sdkconfig menu ``Auto-detect Flash chips``, and you can disable flash drivers which you do not need to save some IRAM.
:SOC_GPSPI_SUPPORTED:- Enable :ref:`CONFIG_HEAP_PLACE_FUNCTION_INTO_FLASH`. Provided that :ref:`CONFIG_SPI_MASTER_ISR_IN_IRAM` is not enabled and the heap functions are not incorrectly used from ISRs, this option is safe to enable in all configurations.
:esp32c2:- Enable :ref:`CONFIG_BT_RELEASE_IRAM`. Release BT text section and merge BT data, bss & text into a large free heap region when ``esp_bt_mem_release`` is called. This makes Bluetooth unavailable until the next restart, but saving ~22 KB or more of IRAM.
The SRAM1 memory area is normally used for DRAM, but it is possible to use parts of it for IRAM with :ref:`CONFIG_ESP_SYSTEM_ESP32_SRAM1_REGION_AS_IRAM`. This memory would previously be reserved for DRAM data usage (e.g., ``.bss``) by the software bootloader and later added to the heap. After this option was introduced, the bootloader DRAM size was reduced to a value closer to what it normally actually needs.
To use this option, ESP-IDF should be able to recognize that the new SRAM1 area is also a valid load address for an image segment. If the software bootloader was compiled before this option existed, then the bootloader will not be able to load the app that has code placed in this new extended IRAM area. This would typically happen if you are doing an OTA update, where only the app would be updated.
Apps compiled with :ref:`CONFIG_ESP_SYSTEM_ESP32_SRAM1_REGION_AS_IRAM` may fail to boot, if used together with a software bootloader that was compiled before this config option was introduced. If you are using an older bootloader and updating over OTA, please test carefully before pushing any updates.
When using SPI flash driver API and other APIs based on the former (NVS, Partition APIs, etc.), the Cache will be disabled. During this period, any code executed must reside in internal RAM, see :ref:`concurrency-constraints-flash`. Hence, interrupt handlers that are not in internal RAM will not be executed.
To achieve this, ESP-IDF drivers usually have the following two options:
Placing additional code into IRAM will exacerbate IRAM usage. For this reason, there is :ref:`CONFIG_SPI_FLASH_AUTO_SUSPEND`, which can alleviate the aforementioned kinds of IRAM usage. By enabling this feature, the Cache will not be disabled when SPI flash driver APIs and SPI flash driver-based APIs are used. Therefore, code and data in flash can be executed or accessed normally, but with some minor delay. See :ref:`auto-suspend` for more details about this feature.
When compiling for ESP32 revisions older than ECO3 (:ref:`CONFIG_ESP32_REV_MIN`), the PSRAM Cache bug workaround (:ref:`CONFIG_SPIRAM_CACHE_WORKAROUND`) option is enabled, and the C library functions normally located in ROM are recompiled with the workaround and placed into IRAM instead. For most applications, it is safe to move many of the C library functions into flash, reclaiming some IRAM. Corresponding options include:
The exact amount of IRAM saved will depend on how much C library code is actually used by the application. In addition, the following options may be used to move more of the C library code into flash, however note that this may result in reduced performance. Be careful not to use the C library function allocated with :c:macro:`ESP_INTR_FLAG_IRAM` flag from interrupts when cache is disabled, refer to :ref:`iram-safe-interrupt-handlers` for more details. For these reasons, the functions ``itoa``, ``memcmp``, ``memcpy``, ``memset``, ``strcat``, ``strcmp``, and ``strlen`` are always put in IRAM.
Moving frequently-called functions from IRAM to flash may increase their execution time.
..note::
Other configuration options exist that will increase IRAM usage by moving some functionality into IRAM, usually for performance, but the default option is not to do this. These are not listed here. The IRAM size impact of enabling these options is usually noted in the configuration item help text.
