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postgresql大部分的内存分配管理都是通过MemoryContext进行操作的#xff0c; 多个相关的MemoryContext构成了一个树型结构#xff0c; 多个树构成了一个森林。
实现了三种MemoryContext:
SlabContextGenerationContextAllocSetContext
使用全局变量CurrentMemo…一、简介
postgresql大部分的内存分配管理都是通过MemoryContext进行操作的 多个相关的MemoryContext构成了一个树型结构 多个树构成了一个森林。
实现了三种MemoryContext:
SlabContextGenerationContextAllocSetContext
使用全局变量CurrentMemoryContext进行MemoryContext切换
二、编程范式
2.1 创建MemoryContext
newcontext AllocSetContextCreate((MemoryContext) NULL,TestMemoryContext,ALLOCSET_DEFAULT_SIZES);2.2 切换需要使用的MemoryContext
MemoryContextSwitchTo(newcontext);2.3 使用相应API进行分配/释放空间
//分配空间
item (text *) palloc(VARHDRSZ len 1);
//分配空间并且清空
plabel palloc0(sizeof(pending_label));
//扩容/缩容
es-steps repalloc(es-steps, sizeof(ExprEvalStep) * es-steps_alloc);
//释放空间
pfree(r);2.4 重置MemoryContext
MemoryContextReset(argContext);2.5 销毁MemoryContext
MemoryContextDelete(tmpCxt);三、实现原理
3.1 数据结构简介
AllocSetContext由三部分构成 MemoryContextData AllocBlock AllocChunk MemoryContextData
由如下字段将各个MemoryContext构成树型结构
MemoryContext parent; /* NULL if no parent (toplevel context) */
MemoryContext firstchild; /* head of linked list of children */
MemoryContext prevchild; /* previous child of same parent */
MemoryContext nextchild; /* next child of same parent */虚函数表实现统一接口不同实现
const MemoryContextMethods *methods; /* virtual function table */AllocBlock
由如下字段将block构成一个双向链表
AllocBlock prev; /* prev block in asets blocks list, if any */
AllocBlock next; /* next block in asets blocks list, if any */标识可用空间范围
char *freeptr; /* start of free space in this block */
char *endptr; /* end of space in this block */AllocChunk
chunk中可用大小
Size size;3.2 初始化
#define AllocSetContextCreate \AllocSetContextCreateInternal#define ALLOCSET_DEFAULT_MINSIZE 0
#define ALLOCSET_DEFAULT_INITSIZE (8 * 1024)
#define ALLOCSET_DEFAULT_MAXSIZE (8 * 1024 * 1024)
#define ALLOCSET_DEFAULT_SIZES \ALLOCSET_DEFAULT_MINSIZE, ALLOCSET_DEFAULT_INITSIZE, ALLOCSET_DEFAULT_MAXSIZETopMemoryContext AllocSetContextCreate((MemoryContext) NULL,TopMemoryContext,ALLOCSET_DEFAULT_SIZES);MemoryContext
AllocSetContextCreateInternal(MemoryContext parent,const char *name,Size minContextSize,Size initBlockSize,Size maxBlockSize)
{int freeListIndex;Size firstBlockSize;AllocSet set;AllocBlock block;
.../* Determine size of initial block */firstBlockSize MAXALIGN(sizeof(AllocSetContext)) ALLOC_BLOCKHDRSZ ALLOC_CHUNKHDRSZ;if (minContextSize ! 0)firstBlockSize Max(firstBlockSize, minContextSize);elsefirstBlockSize Max(firstBlockSize, initBlockSize);/** Allocate the initial block. Unlike other aset.c blocks, it starts with* the context header and its block header follows that.*/set (AllocSet) malloc(firstBlockSize);.../* Fill in the initial blocks block header */block (AllocBlock) (((char *) set) MAXALIGN(sizeof(AllocSetContext)));block-aset set;block-freeptr ((char *) block) ALLOC_BLOCKHDRSZ;block-endptr ((char *) set) firstBlockSize;block-prev NULL;block-next NULL;
.../* Remember block as part of block list */set-blocks block;/* Mark block as not to be released at reset time */set-keeper block;...set-initBlockSize initBlockSize;set-maxBlockSize maxBlockSize;set-nextBlockSize initBlockSize;set-freeListIndex freeListIndex;
...set-allocChunkLimit ALLOC_CHUNK_LIMIT;while ((Size) (set-allocChunkLimit ALLOC_CHUNKHDRSZ) (Size) ((maxBlockSize - ALLOC_BLOCKHDRSZ) / ALLOC_CHUNK_FRACTION))set-allocChunkLimit 1;/* Finally, do the type-independent part of context creation */MemoryContextCreate((MemoryContext) set,T_AllocSetContext,AllocSetMethods,parent,name);((MemoryContext) set)-mem_allocated firstBlockSize;return (MemoryContext) set;
}3.3 分配空间
void *
palloc(Size size)
{/* duplicates MemoryContextAlloc to avoid increased overhead */void *ret;MemoryContext context CurrentMemoryContext;
...context-isReset false;ret context-methods-alloc(context, size);
...return ret;
}3.3.1 分配空间大于allocChunkLimit
static void *
AllocSetAlloc(MemoryContext context, Size size)
{AllocSet set (AllocSet) context;AllocBlock block;AllocChunk chunk;int fidx;Size chunk_size;Size blksize;.../** If requested size exceeds maximum for chunks, allocate an entire block* for this request.*/if (size set-allocChunkLimit){chunk_size MAXALIGN(size);blksize chunk_size ALLOC_BLOCKHDRSZ ALLOC_CHUNKHDRSZ;block (AllocBlock) malloc(blksize);if (block NULL)return NULL;context-mem_allocated blksize;block-aset set;block-freeptr block-endptr ((char *) block) blksize;chunk (AllocChunk) (((char *) block) ALLOC_BLOCKHDRSZ);chunk-aset set;chunk-size chunk_size;/** Stick the new block underneath the active allocation block, if any,* so that we dont lose the use of the space remaining therein.*/if (set-blocks ! NULL){block-prev set-blocks;block-next set-blocks-next;if (block-next)block-next-prev block;set-blocks-next block;}else{block-prev NULL;block-next NULL;set-blocks block;}return AllocChunkGetPointer(chunk);}
...
}例如分配16K
char *ptr palloc(16 * 1024);例如分配32K
char *ptr palloc(32 * 1024);3.3.2 分配空间小于等于allocChunkLimit
#define ALLOC_MINBITS 3 /* smallest chunk size is 8 bytes */static inline int
AllocSetFreeIndex(Size size)
{int idx;if (size (1 ALLOC_MINBITS)){
...idx 31 - __builtin_clz((uint32) size - 1) - ALLOC_MINBITS 1;
...}elseidx 0;return idx;
}fidx AllocSetFreeIndex(size);/** Choose the actual chunk size to allocate.*/
chunk_size (1 ALLOC_MINBITS) fidx;分配小于等于8字节 小于等于8的最终将分配8字节 分配大于8字节 通过__builtin_clz计算size-1的前导0的个数然后通过减法计算size的对其位数非常高效的方式
3.3.2.1 当前block有足够空间
if ((block set-blocks) ! NULL)
{
...
}
/** OK, do the allocation*/chunk (AllocChunk) (block-freeptr);
...block-freeptr (chunk_size ALLOC_CHUNKHDRSZ);
...chunk-aset (void *) set;chunk-size chunk_size;
...return AllocChunkGetPointer(chunk);例如分配20byte
char *ptr palloc(20);通过对其实际分配32byte 3.3.2.2 当前block没有足够的空间
随着分配当前block中的可用空间越来越小再次分配时将不够当前分配
Size availspace block-endptr - block-freeptr;
if (availspace (chunk_size ALLOC_CHUNKHDRSZ))
{
...
}3.3.2.2.1 当前block还有一些空间
if (availspace (chunk_size ALLOC_CHUNKHDRSZ)){while (availspace ((1 ALLOC_MINBITS) ALLOC_CHUNKHDRSZ)){Size availchunk availspace - ALLOC_CHUNKHDRSZ;int a_fidx AllocSetFreeIndex(availchunk);...if (availchunk ! ((Size) 1 (a_fidx ALLOC_MINBITS))){a_fidx--;Assert(a_fidx 0);availchunk ((Size) 1 (a_fidx ALLOC_MINBITS));}chunk (AllocChunk) (block-freeptr);
...block-freeptr (availchunk ALLOC_CHUNKHDRSZ);availspace - (availchunk ALLOC_CHUNKHDRSZ);chunk-size availchunk;chunk-aset (void *) set-freelist[a_fidx];set-freelist[a_fidx] chunk;}/* Mark that we need to create a new block */block NULL;}将剩余空间大小按照对其的大小放到freelist中构成一个单链表用于后续分配时直接从这里获取。 可能会有一小块用不了的空间
3.3.2.2.2 创建新block
第一次创建时使用initBlockSize后续将会是上次block的两倍当大于maxBlockSize使用maxBlockSize如果新block大小依然小于需要分配的空间则继续扩大
/** Time to create a new regular (multi-chunk) block?*/if (block NULL){Size required_size;/** The first such block has size initBlockSize, and we double the* space in each succeeding block, but not more than maxBlockSize.*/blksize set-nextBlockSize;set-nextBlockSize 1;if (set-nextBlockSize set-maxBlockSize)set-nextBlockSize set-maxBlockSize;/** If initBlockSize is less than ALLOC_CHUNK_LIMIT, we could need more* space... but try to keep it a power of 2.*/required_size chunk_size ALLOC_BLOCKHDRSZ ALLOC_CHUNKHDRSZ;while (blksize required_size)blksize 1;/* Try to allocate it */block (AllocBlock) malloc(blksize);
...if (block NULL)return NULL;context-mem_allocated blksize;block-aset set;block-freeptr ((char *) block) ALLOC_BLOCKHDRSZ;block-endptr ((char *) block) blksize;/* Mark unallocated space NOACCESS. */VALGRIND_MAKE_MEM_NOACCESS(block-freeptr,blksize - ALLOC_BLOCKHDRSZ);block-prev NULL;block-next set-blocks;if (block-next)block-next-prev block;set-blocks block;}3.3.3 从freelist中分配
fidx AllocSetFreeIndex(size);
chunk set-freelist[fidx];
if (chunk ! NULL)
{set-freelist[fidx] (AllocChunk) chunk-aset;chunk-aset (void *) set;
...return AllocChunkGetPointer(chunk);
}3.4 释放空间
void
pfree(void *pointer)
{MemoryContext context GetMemoryChunkContext(pointer);context-methods-free_p(context, pointer);
...
}3.4.1 直接释放
static void
AllocSetFree(MemoryContext context, void *pointer)
{AllocSet set (AllocSet) context;AllocChunk chunk AllocPointerGetChunk(pointer);
...if (chunk-size set-allocChunkLimit){AllocBlock block (AllocBlock) (((char *) chunk) - ALLOC_BLOCKHDRSZ);.../* OK, remove block from asets list and free it */if (block-prev)block-prev-next block-next;elseset-blocks block-next;if (block-next)block-next-prev block-prev;context-mem_allocated - block-endptr - ((char *) block);
...free(block);}else{...}
}见 3.3.1, 大于chunklimit的直接malloc的空间而不是从block中分配的因此直接释放并且将此block从双向链表中删除。
3.4.2 加入freelist中
static void
AllocSetFree(MemoryContext context, void *pointer)
{AllocSet set (AllocSet) context;AllocChunk chunk AllocPointerGetChunk(pointer);/* Allow access to private part of chunk header. */VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);#ifdef MEMORY_CONTEXT_CHECKING/* Test for someone scribbling on unused space in chunk */if (chunk-requested_size chunk-size)if (!sentinel_ok(pointer, chunk-requested_size))elog(WARNING, detected write past chunk end in %s %p,set-header.name, chunk);
#endifif (chunk-size set-allocChunkLimit){...}else{/* Normal case, put the chunk into appropriate freelist */int fidx AllocSetFreeIndex(chunk-size);chunk-aset (void *) set-freelist[fidx];
...set-freelist[fidx] chunk;}
}
