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江苏营销型网站建设公司,上杭网站,wordpress搬家后重新安装,wordpress 去掉页脚一、简介 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®;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-freelistfidx 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;} }