A brief discussion of PHP source code 32: emalloc/efree layer and heap layer in PHP memory pool

This article mainly introduces about PHP source code 32: emalloc/efree layer and heap layer in the PHP memory pool. It has certain reference value. Now I share it with you. Friends in need can Refer to

Brief Talk on PHP Source Code 32: emalloc/efree layer and heap layer in PHP memory pool
emalloc/efree layer is the top-level structure in the entire memory system. It is passed through Heap layer exchange uses PHP's own memory management mechanism. If USE_ZEND_ALLOC is set to 0, malloc/free and other functions can be used to directly operate the memory.
Here we will analyze the interaction between the emalloc/efree layer and the heap layer, and the memory management mechanism of the heap layer from the implementation of the emalloc and efree functions.

[emalloc]
The emalloc function starts from line 70 of zend_alloc.h.
emalloc is a macro, which corresponds to the _emalloc function.
In the _emalloc function, if zend's memory management mechanism is not used, the malloc function is called directly, otherwise it is called

_zend_mm_alloc_int
[emalloc() -> _emalloc() -> _zend_mm_alloc_int() ]

In the _zend_mm_alloc_int function, the program will process whether the actual required memory is less than or greater than or equal to In the two cases of ZEND_MM_MAX_SMALL_SIZE (272), if it is less than ZEND_MM_MAX_SMALL_SIZE, free_buckets will be searched to see if there is a suitable memory block. If a suitable block can be found in free_buckets, jump directly to zend_mm_finished_searching_for_block, otherwise zend_mm_search_large_block()

[emalloc() -> _emalloc() -> _zend_mm_alloc_int() -> zend_mm_search_large_block()]
zend_mm_search_large_block function is used to find suitable memory blocks in large_free_buckets. When the ZEND_MM_LARGE_BUCKET_INDEX (true_size) size is not found, you need to find the smallest block in the larger block list.

If it is not found in both the large block list and the small block list, you need to search from the remaining list blocks. If it is found, it will also jump to zend_mm_finished_searching_for_block
If it is not found in the three lists, then Memory allocation needs to be increased again. At this time, the allocation function of the storage layer is called for allocation. The size of the memory. If the memory to be allocated is larger than block_size, it needs to be recalculated based on the size. Otherwise, memory of block_size size is allocated directly.
After the memory is allocated, the heap needs to be rearranged. At this time, the memory size in the heap needs to be recalculated and the newly allocated memory is added to the front of segments_list.

If you jump directly to zend_mm_finished_searching_for_block in the above operation, you need to remove the used memory block from the corresponding list (this should be a marking process, pseudo removal)

Next, depending on how much memory is left, move it to the free list or the remaining list.

Finally returns the allocated block.

During the entire process of emalloc, there are the following points to note.
ZEND_MM_BUCKET_INDEX(true_size) is located at the position in the bucket. This value is greater than or equal to 0 and less than 32.
The implementation is as follows:

#define ZEND_MM_BUCKET_INDEX(true_size) ((true_size>>ZEND_MM_ALIGNMENT_LOG2)-(ZEND_MM_ALIGNED_MIN_HEADER_SIZE>>ZEND_MM_ALIGNMENT_LOG2))

The values ​​of free_bitmap and large_free_bitmap are both 0 to 31.

[efree]
The efree function starts from line 72 of zend_alloc.h.
efree is a macro, which corresponds to the _efree function.
In the _efree function, if the memory management mechanism of zend is not used, the free function is called directly, otherwise the _

zend_mm_free_int
[efree() -> _efree() -> _zend_mm_free_int() ]

Heap first reduces the size of the entire heap. If the next block of the current block is a free block, then delete the next free block from the free block list and merge it with the current block. If the previous block of the current block is a free block, delete the previous free block from the free block list and merge it with the current block. , the pointer points to the previous free block. If the current block is the starting block at this time, call zend_mm_del_segment to clear the entire memory segment. If it is not the starting block, the merged block is added to the free block list.

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