FreeRTOS --（5）內存管理 heap4

FreeRTOS 中的 heap 4 內存管理，可以算是 heap 2 的增強版本，在 《FreeRTOS --（3）內存管理 heap2》中，我們可以看到，每次內存分配后都會產生一個內存塊，多次分配后，會產生很多內存碎片，在較為復雜的場景（需要經常動態分配和釋放場景）下，幾乎是無法勝任；

所以就有了 heap 4，它相比 heap 2 來說，提供了相鄰空閑的內存塊合並的功能，一定程度上減少了內存碎片，使得釋放了的內存能夠再度合並稱為較為大的內存塊，以供有大內存塊的分配場景使用；

 

1、內存大小

和 heap 2 一樣，用於內存管理的內存大小來自於一個大數組，數組的下標就是整個需要被管理的內存的大小，這個是和具體芯片所支持的 RAM 大小相關：

configTOTAL_HEAP_SIZE

被管理的內存定義為：

static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];

 

ucHeap 就是管理的對象；

這些基本的結構都是和之前的 heap2 保持一致，不再多說；

 

2、對齊

對齊的部分也是和 heap 2 一致，不在多說，更多的參考：《FreeRTOS --（3）內存管理 heap2》的對齊章節；

 

3、內存塊

內存塊的定義依然是：

typedef struct A_BLOCK_LINK
{
    struct A_BLOCK_LINK *pxNextFreeBlock;    /*<< The next free block in the list. */
    size_t xBlockSize;                        /*<< The size of the free block. */
} BlockLink_t;

 

沒有任何差別，依然有 xStart 和 xEnd 來描述兩個 marker，和 heap 2 幾乎一樣《FreeRTOS --（3）內存管理 heap2》；這里不再多說；

 

4、內存初始化

和 heap 2 一樣，內存初始化使用 prvHeapInit，在第一次調用內存分配的時候，檢查是否有初始化，否則進行 prvHeapInit 的調用，初始化相關的結構：

static void prvHeapInit( void )
{
BlockLink_t *pxFirstFreeBlock;
uint8_t *pucAlignedHeap;
size_t uxAddress;
size_t xTotalHeapSize = configTOTAL_HEAP_SIZE;
 
    /* Ensure the heap starts on a correctly aligned boundary. */
    uxAddress = ( size_t ) ucHeap;
 
    if( ( uxAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
    {
        uxAddress += ( portBYTE_ALIGNMENT - 1 );
        uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
        xTotalHeapSize -= uxAddress - ( size_t ) ucHeap;
    }
 
    pucAlignedHeap = ( uint8_t * ) uxAddress;
 
    /* xStart is used to hold a pointer to the first item in the list of free
    blocks.  The void cast is used to prevent compiler warnings. */
    xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
    xStart.xBlockSize = ( size_t ) 0;
 
    /* pxEnd is used to mark the end of the list of free blocks and is inserted
    at the end of the heap space. */
    uxAddress = ( ( size_t ) pucAlignedHeap ) + xTotalHeapSize;
    uxAddress -= xHeapStructSize;
    uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
    pxEnd = ( void * ) uxAddress;
    pxEnd->xBlockSize = 0;
    pxEnd->pxNextFreeBlock = NULL;
 
    /* To start with there is a single free block that is sized to take up the
    entire heap space, minus the space taken by pxEnd. */
    pxFirstFreeBlock = ( void * ) pucAlignedHeap;
    pxFirstFreeBlock->xBlockSize = uxAddress - ( size_t ) pxFirstFreeBlock;
    pxFirstFreeBlock->pxNextFreeBlock = pxEnd;
 
    /* Only one block exists - and it covers the entire usable heap space. */
    xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
    xFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
 
    /* Work out the position of the top bit in a size_t variable. */
    xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}

 

從編碼風格的角度上來說，看上去感覺和 heap 2 不是同一個人寫的（同樣的邏輯，不同的表達），不過沒關系，含義都是一樣的；

依然是初始化了 xStart 和 xEnd 兩個 marker；做了一些對齊處理后，將能夠分配的所有的空間一並掛到了 xStart->pxNextFreeBlock 鏈表；

和 heap 2 不一樣的是，heap 4 定義了一個標記，以表示內存是否有被使用，這里定義了 xBlockAllocatedBit；如果是 32bit CPU 的話，這個 xBlockAllocatedBit = 0x01<<31；32 bit 的最高位，這顯然是安全的；

 

5、內存分配

內存分配，還是使用的 pvPortMalloc 接口，正確分配，返回可用的內存地址，出錯返回 NULL：

void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
void *pvReturn = NULL;
 
    vTaskSuspendAll();
    {
        /* If this is the first call to malloc then the heap will require
        initialisation to setup the list of free blocks. */
        if( pxEnd == NULL )
        {
            prvHeapInit();
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
 
        /* Check the requested block size is not so large that the top bit is
        set.  The top bit of the block size member of the BlockLink_t structure
        is used to determine who owns the block - the application or the
        kernel, so it must be free. */
        if( ( xWantedSize & xBlockAllocatedBit ) == 0 )
        {
            /* The wanted size is increased so it can contain a BlockLink_t
            structure in addition to the requested amount of bytes. */
            if( xWantedSize > 0 )
            {
                xWantedSize += xHeapStructSize;
 
                /* Ensure that blocks are always aligned to the required number
                of bytes. */
                if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0x00 )
                {
                    /* Byte alignment required. */
                    xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
                    configASSERT( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) == 0 );
                }
                else
                {
                    mtCOVERAGE_TEST_MARKER();
                }
            }
            else
            {
                mtCOVERAGE_TEST_MARKER();
            }
 
            if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
            {
                /* Traverse the list from the start    (lowest address) block until
                one    of adequate size is found. */
                pxPreviousBlock = &xStart;
                pxBlock = xStart.pxNextFreeBlock;
                while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
                {
                    pxPreviousBlock = pxBlock;
                    pxBlock = pxBlock->pxNextFreeBlock;
                }
 
                /* If the end marker was reached then a block of adequate size
                was    not found. */
                if( pxBlock != pxEnd )
                {
                    /* Return the memory space pointed to - jumping over the
                    BlockLink_t structure at its start. */
                    pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );
 
                    /* This block is being returned for use so must be taken out
                    of the list of free blocks. */
                    pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;
 
                    /* If the block is larger than required it can be split into
                    two. */
                    if( ( pxBlock->xBlockSize - xWantedSize ) > heapMINIMUM_BLOCK_SIZE )
                    {
                        /* This block is to be split into two.  Create a new
                        block following the number of bytes requested. The void
                        cast is used to prevent byte alignment warnings from the
                        compiler. */
                        pxNewBlockLink = ( void * ) ( ( ( uint8_t * ) pxBlock ) + xWantedSize );
                        configASSERT( ( ( ( size_t ) pxNewBlockLink ) & portBYTE_ALIGNMENT_MASK ) == 0 );
 
                        /* Calculate the sizes of two blocks split from the
                        single block. */
                        pxNewBlockLink->xBlockSize = pxBlock->xBlockSize - xWantedSize;
                        pxBlock->xBlockSize = xWantedSize;
 
                        /* Insert the new block into the list of free blocks. */
                        prvInsertBlockIntoFreeList( pxNewBlockLink );
                    }
                    else
                    {
                        mtCOVERAGE_TEST_MARKER();
                    }
 
                    xFreeBytesRemaining -= pxBlock->xBlockSize;
 
                    if( xFreeBytesRemaining < xMinimumEverFreeBytesRemaining )
                    {
                        xMinimumEverFreeBytesRemaining = xFreeBytesRemaining;
                    }
                    else
                    {
                        mtCOVERAGE_TEST_MARKER();
                    }
 
                    /* The block is being returned - it is allocated and owned
                    by the application and has no "next" block. */
                    pxBlock->xBlockSize |= xBlockAllocatedBit;
                    pxBlock->pxNextFreeBlock = NULL;
                }
                else
                {
                    mtCOVERAGE_TEST_MARKER();
                }
            }
            else
            {
                mtCOVERAGE_TEST_MARKER();
            }
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
 
        traceMALLOC( pvReturn, xWantedSize );
    }
    ( void ) xTaskResumeAll();
 
    #if( configUSE_MALLOC_FAILED_HOOK == 1 )
    {
        if( pvReturn == NULL )
        {
            extern void vApplicationMallocFailedHook( void );
            vApplicationMallocFailedHook();
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
    }
    #endif
 
    configASSERT( ( ( ( size_t ) pvReturn ) & ( size_t ) portBYTE_ALIGNMENT_MASK ) == 0 );
    return pvReturn;
}

 

內存分配部分的代碼和之前的 heap 2 的代碼幾乎完全一樣，這里不做過多的解釋；

 

6、內存釋放

內存釋放是 vPortFree 接口：

void vPortFree( void *pv )
{
uint8_t *puc = ( uint8_t * ) pv;
BlockLink_t *pxLink;
 
    if( pv != NULL )
    {
        /* The memory being freed will have an BlockLink_t structure immediately
        before it. */
        puc -= xHeapStructSize;
 
        /* This casting is to keep the compiler from issuing warnings. */
        pxLink = ( void * ) puc;
 
        /* Check the block is actually allocated. */
        configASSERT( ( pxLink->xBlockSize & xBlockAllocatedBit ) != 0 );
        configASSERT( pxLink->pxNextFreeBlock == NULL );
 
        if( ( pxLink->xBlockSize & xBlockAllocatedBit ) != 0 )
        {
            if( pxLink->pxNextFreeBlock == NULL )
            {
                /* The block is being returned to the heap - it is no longer
                allocated. */
                pxLink->xBlockSize &= ~xBlockAllocatedBit;
 
                vTaskSuspendAll();
                {
                    /* Add this block to the list of free blocks. */
                    xFreeBytesRemaining += pxLink->xBlockSize;
                    traceFREE( pv, pxLink->xBlockSize );
                    prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
                }
                ( void ) xTaskResumeAll();
            }
            else
            {
                mtCOVERAGE_TEST_MARKER();
            }
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
    }
}

 

釋放的時候，將之前的內存通過調用 prvInsertBlockIntoFreeList 插入到 Free List 中，內存塊的合並就體現在這個函數；

 

6.1、合並

當釋放內存的時候調用到了 prvInsertBlockIntoFreeList，它實現了相鄰內存塊的合並工作，這也是 heap 4 與 heap 2 最大不同的地方：

static void prvInsertBlockIntoFreeList( BlockLink_t *pxBlockToInsert )
{
BlockLink_t *pxIterator;
uint8_t *puc;
 
    /* Iterate through the list until a block is found that has a higher address
    than the block being inserted. */
    for( pxIterator = &xStart; pxIterator->pxNextFreeBlock < pxBlockToInsert; pxIterator = pxIterator->pxNextFreeBlock )
    {
        /* Nothing to do here, just iterate to the right position. */
    }
 
    /* Do the block being inserted, and the block it is being inserted after
    make a contiguous block of memory? */
    puc = ( uint8_t * ) pxIterator;
    if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )
    {
        pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;
        pxBlockToInsert = pxIterator;
    }
    else
    {
        mtCOVERAGE_TEST_MARKER();
    }
 
    /* Do the block being inserted, and the block it is being inserted before
    make a contiguous block of memory? */
    puc = ( uint8_t * ) pxBlockToInsert;
    if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) pxIterator->pxNextFreeBlock )
    {
        if( pxIterator->pxNextFreeBlock != pxEnd )
        {
            /* Form one big block from the two blocks. */
            pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
            pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
        }
        else
        {
            pxBlockToInsert->pxNextFreeBlock = pxEnd;
        }
    }
    else
    {
        pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;
    }
 
    /* If the block being inserted plugged a gab, so was merged with the block
    before and the block after, then it's pxNextFreeBlock pointer will have
    already been set, and should not be set here as that would make it point
    to itself. */
    if( pxIterator != pxBlockToInsert )
    {
        pxIterator->pxNextFreeBlock = pxBlockToInsert;
    }
    else
    {
        mtCOVERAGE_TEST_MARKER();
    }
}

 

在 heap 2中，也有這個函數，但是它是按照內存塊的小到大進行排序，由於有相鄰內存塊合並的要求，所以在 heap 4 中，內存塊鏈表的組織形式，是按照他們的地址順序由小到大組織的；

首先通過 xStart 開始，獲取第一個空閑塊的地址，並比較它的下一個空閑塊地址和待插入空閑塊地址的大小，以便獲得合適的插入位置；

獲得合適的位置了以后，判斷待插入的這個空閑塊是否和前一個空閑塊相鄰，如果相鄰的話，就將兩個塊合並到一起：

/* Do the block being inserted, and the block it is being inserted after
    make a contiguous block of memory? */
    puc = ( uint8_t * ) pxIterator;
    if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )
    {
        pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;
        pxBlockToInsert = pxIterator;
    }
    else
    {
        mtCOVERAGE_TEST_MARKER();
    }

 

然后在判斷是否和后一個空閑塊也相鄰，如果相鄰，也將他們合並，當然這里需要判斷是否是緊挨着 xEnd：

/* Do the block being inserted, and the block it is being inserted before
    make a contiguous block of memory? */
    puc = ( uint8_t * ) pxBlockToInsert;
    if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) pxIterator->pxNextFreeBlock )
    {
        if( pxIterator->pxNextFreeBlock != pxEnd )
        {
            /* Form one big block from the two blocks. */
            pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
            pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
        }
        else
        {
            pxBlockToInsert->pxNextFreeBlock = pxEnd;
        }
    }
    else
    {
        pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;
    }

 

打個比方：

多次分配和釋放后，內存布局如下所示：

如果要釋放中間那個內存，那么就會觸發向上和向下的合並：

官方針對這個 heap 4 的官圖為：