204 lines
6.6 KiB
ReStructuredText
204 lines
6.6 KiB
ReStructuredText
.. _microkernel_memory_pools:
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Memory Pools
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############
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Concepts
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********
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The microkernel's :dfn:`memory pool` objects provide dynamic allocation and
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release of variable-size memory blocks.
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Unlike :ref:`memory map <microkernel_memory_maps>` objects, which support
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memory blocks of only a *single* size, a memory pool can support memory blocks
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of *various* sizes. The memory pool does this by subdividing blocks into smaller
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chunks, where possible, to more closely match the actual needs of a requesting
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task.
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Any number of memory pools can be defined in a microkernel system. Each memory
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pool has:
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* A **name** that uniquely identifies it.
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* A **minimum** and **maximum** block size, in bytes, of memory blocks
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within the pool.
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* The **number of maximum-size memory blocks** initially available.
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A task that needs to use a memory block simply allocates it from a memory
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pool. When a block of the desired size is unavailable, the task can wait
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for one to become available. Following a successful allocation, the
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:c:data:`pointer_to_data` field of the block descriptor supplied by the
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task indicates the starting address of the memory block. When the task is
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finished with a memory block, it must release the block back to the memory
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pool that allocated it so that the block can be reused.
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Any number of tasks can wait on a memory pool simultaneously; when a
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memory block becomes available, it is given to the highest-priority task
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that has waited the longest.
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When a request for memory is sufficiently smaller than an available
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memory pool block, the memory pool will automatically split the block into
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4 smaller blocks. The resulting smaller blocks can also be split repeatedly,
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until a block just larger than the needed size is available, or the minimum
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block size, as specified in the MDEF, is reached.
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If the memory pool cannot find an available block that is at least
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the requested size, it will attempt to create one by merging adjacent
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free blocks. If a suitable block can't be created, the request fails.
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Although a memory pool uses efficient algorithms to manage its blocks,
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the splitting of available blocks and merging of free blocks takes time
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and increases overhead block allocation. The larger the allowable
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number of splits, the larger the overhead. However, the minimum and maximum
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block-size parameters specified for a pool can be used to control the amount
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of splitting, and thus the amount of overhead.
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Unlike a heap, more than one memory pool can be defined, if needed. For
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example, different applications can utilize different memory pools; this
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can help prevent one application from hijacking resources to allocate all
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of the available blocks.
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Purpose
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*******
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Use memory pools to allocate memory in variable-size blocks.
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Use memory pool blocks when sending data to a mailbox asynchronously.
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Usage
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*****
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Defining a Memory Pool
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======================
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The following parameters must be defined:
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*name*
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This specifies a unique name for the memory pool.
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*min_block_size*
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This specifies the minimum memory block size in bytes.
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It should be a multiple of the processor's word size.
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*max_block_size*
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This specifies the maximum memory block size in bytes.
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It should be a power of 4 times larger than *minBlockSize*;
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therefore, maxBlockSize = minBlockSize * 4^n, where n>=0.
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*num_max*
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This specifies the number of maximum size memory blocks
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available at startup.
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Public Memory Pool
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------------------
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Define the memory pool in the application's MDEF with the following
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syntax:
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.. code-block:: console
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POOL name min_block_size max_block_size num_max
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For example, the file :file:`projName.mdef` defines two memory pools
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as follows:
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.. code-block:: console
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% POOL NAME MIN MAX NMAX
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% =======================================
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POOL MY_POOL 32 8192 1
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POOL SECOND_POOL_ID 64 1024 5
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A public memory pool can be referenced by name from any source file that
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includes the file :file:`zephyr.h`.
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.. note::
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Private memory pools are not supported by the Zephyr kernel.
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Example: Requesting a Memory Block from a Pool with No Conditions
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=================================================================
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This code waits indefinitely for an 80 byte memory block to become
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available, then fills it with zeroes.
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.. code-block:: c
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struct k_block block;
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task_mem_pool_alloc(&block, MYPOOL, 80, TICKS_UNLIMITED);
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memset(block.pointer_to_data, 0, 80);
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Example: Requesting a Memory Block from a Pool with a Conditional Time-out
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==========================================================================
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This code waits up to 5 ticks for an 80 byte memory block to become
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available and gives a warning if a suitable memory block is not obtained
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in that time.
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.. code-block:: none
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struct k_block block;
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if (task_mem_pool_alloc(:&block, MYPOOL, 80, 5) == RC_OK) {
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/* use memory block */
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} else {
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printf('Memory allocation timeout');
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}
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Example: Requesting a Memory Block from a Pool with a No-Blocking Condition
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===========================================================================
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This code gives an immediate warning when it can not satisfy the request for
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a memory block of 80 bytes.
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.. code-block:: none
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struct k_block block;
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if (task_mem_pool_alloc (&block, MYPOOL, 80, TICKS_NONE) == RC_OK) {
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/* use memory block */
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} else {
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printf('Memory allocation timeout');
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}
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Example: Freeing a Memory Block Back to a Pool
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==============================================
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This code releases a memory block back to a pool when it is no longer needed.
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.. code-block:: c
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struct k_block block;
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task_mem_pool_alloc(&block, MYPOOL, size, TICKS_NONE);
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/* use memory block */
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task_mem_pool_free(&block);
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Example: Manually Defragmenting a Memory Pool
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=============================================
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This code instructs the memory pool to concatenate any unused memory blocks
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that can be merged. Doing a full defragmentation of the entire memory pool
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before allocating a number of memory blocks may be more efficient than doing
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an implicit partial defragmentation of the memory pool each time a memory
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block allocation occurs.
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.. code-block:: c
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task_mem_pool_defragment(MYPOOL);
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APIs
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****
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Memory Pools APIs provided by :file:`microkernel.h`
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===================================================
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:cpp:func:`task_mem_pool_alloc()`
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Wait for a block of memory; wait the period of time defined by the time-out
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parameter.
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:cpp:func:`task_mem_pool_free()`
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Return a block of memory to a memory pool.
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:cpp:func:`task_mem_pool_defragment()`
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Defragment a memory pool.
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