PostgreSQL中RecordAndGetPageWithFreeSpace有什么作用

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一、数据结构

FSMAddress
内部的FSM处理过程以逻辑地址scheme的方式工作,树的每一个层次都可以认为是一个独立的地址文件.

/* * The internal FSM routines work on a logical addressing scheme. Each * level of the tree can be thought of as a separately addressable file. * 内部的FSM处理过程工作在一个逻辑地址scheme上. * 树的每一个层次都可以认为是一个独立的地址文件. */typedef struct{    //层次    int         level;          /* level */    //该层次内的页编号    int         logpageno;      /* page number within the level */} FSMAddress;/* Address of the root page. *///根页地址static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};

FSMPage
FSM page数据结构.详细可参看src/backend/storage/freespace/README.

/* * Structure of a FSM page. See src/backend/storage/freespace/README for * details. * FSM page数据结构.详细可参看src/backend/storage/freespace/README. */typedef struct{    /*     * fsm_search_avail() tries to spread the load of multiple backends by     * returning different pages to different backends in a round-robin     * fashion. fp_next_slot points to the next slot to be returned (assuming     * there's enough space on it for the request). It's defined as an int,     * because it's updated without an exclusive lock. uint16 would be more     * appropriate, but int is more likely to be atomically     * fetchable/storable.     * fsm_search_avail()函数尝试通过在一轮循环中返回不同的页面到不同的后台进程,     *   从而分散在后台进程上分散负载.     * 该字段因为无需独占锁,因此定义为整型.     * unit16可能会更合适,但整型看起来更适合于原子提取和存储.     */    int         fp_next_slot;    /*     * fp_nodes contains the binary tree, stored in array. The first     * NonLeafNodesPerPage elements are upper nodes, and the following     * LeafNodesPerPage elements are leaf nodes. Unused nodes are zero.     * fp_nodes以数组的形式存储二叉树.     * 第一个NonLeafNodesPerPage元素是上一层的节点,接下来的LeafNodesPerPage元素是叶子节点.     * 未使用的节点为0.     */    uint8       fp_nodes[FLEXIBLE_ARRAY_MEMBER];} FSMPageData;typedef FSMPageData *FSMPage;

FSMLocalMap
对于小表,不需要创建FSM来存储空间信息,使用本地的内存映射信息.

/* Either already tried, or beyond the end of the relation *///已尝试或者已在表的末尾之后#define FSM_LOCAL_NOT_AVAIL 0x00/* Available to try *///可用于尝试#define FSM_LOCAL_AVAIL     0x01/* * For small relations, we don't create FSM to save space, instead we use * local in-memory map of pages to try.  To locate free space, we simply try * pages directly without knowing ahead of time how much free space they have. * 对于小表,不需要创建FSM来存储空间信息,使用本地的内存映射信息. * 为了定位空闲空间,我们不需要知道他们有多少空闲空间而是直接简单的对page进行尝试. * * Note that this map is used to the find the block with required free space * for any given relation.  We clear this map when we have found a block with * enough free space, when we extend the relation, or on transaction abort. * See src/backend/storage/freespace/README for further details. * 注意这个map用于搜索给定表的请求空闲空间. * 在找到有足够空闲空间的block/扩展了relation/在事务回滚时,则清除这个map的信息. * 详细可查看src/backend/storage/freespace/README. */typedef struct{    BlockNumber nblocks;//块数    uint8       map[HEAP_FSM_CREATION_THRESHOLD];//数组}           FSMLocalMap;static FSMLocalMap fsm_local_map ={    0,    {        FSM_LOCAL_NOT_AVAIL    }};#define FSM_LOCAL_MAP_EXISTS (fsm_local_map.nblocks > 0)

二、源码解读

RecordAndGetPageWithFreeSpace返回满足条件的block,其主要逻辑如下:
1.初始化相关变量
2.如存在本地map,则首先使用该文件,调用fsm_local_search
3.如果没有本地map也没有FSM,创建本地map,然后调用fsm_local_search
4.使用FSM搜索
4.1获取FSM中原page可用空间对应的catalog
4.2根据所需空间大小,获取FSM中相应的catalog
4.3根据原页面,获取heap block所在的位置(FSMAddress)
4.4检索获取目标slot
4.5如目标slot合法,则获取相应的block,否则使用fsm_search搜索合适的block

/* * RecordAndGetPageWithFreeSpace - update info about a page and try again. * RecordAndGetPageWithFreeSpace - 更新page info并再次尝试. * * We provide this combo form to save some locking overhead, compared to * separate RecordPageWithFreeSpace + GetPageWithFreeSpace calls. There's * also some effort to return a page close to the old page; if there's a * page with enough free space on the same FSM page where the old one page * is located, it is preferred. * 相对于单独的RecordPageWithFreeSpace + GetPageWithFreeSpace调用, *   我们提供这个组合形式用于节省一些锁的负载. * 这里同样存储一些努力用于返回接近旧page的page. * 如果与旧的page在同一个FSM page上有足够空闲空间的page存在,那这个page会被选中. * * For very small heap relations that don't have a FSM, we update the local * map to indicate we have tried a page, and return the next page to try. * 对于非常小的堆表,是不需要FSM的,直接更新本地map来提示进程需要尝试获得一个page,并返回下一个page. */BlockNumberRecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,                              Size oldSpaceAvail, Size spaceNeeded){    int         old_cat;    int         search_cat;    FSMAddress  addr;//FSM地址    uint16      slot;//槽号    int         search_slot;    BlockNumber nblocks = InvalidBlockNumber;    /* First try the local map, if it exists. */    //如存在本地map,则首先使用该文件.    //#define FSM_LOCAL_MAP_EXISTS (fsm_local_map.nblocks > 0)    if (FSM_LOCAL_MAP_EXISTS)    {        Assert((rel->rd_rel->relkind == RELKIND_RELATION ||                rel->rd_rel->relkind == RELKIND_TOASTVALUE) &&               fsm_local_map.map[oldPage] == FSM_LOCAL_AVAIL);        //设置oldPage为不可用        fsm_local_map.map[oldPage] = FSM_LOCAL_NOT_AVAIL;        //搜索并返回结果        return fsm_local_search();    }    if (!fsm_allow_writes(rel, oldPage, InvalidBlockNumber, &nblocks))    {        //---- 如果FSM不允许写        /*         * If we have neither a local map nor a FSM, we probably just tried         * the target block in the smgr relation entry and failed, so we'll         * need to create the local map.         * 如果没有本地map也没有FSM,         *   那么我们只是尝试了smgr relation中的目标block而且失败了,那么需要创建本地map.         */        //设置本地map        fsm_local_set(rel, nblocks);        //搜索本地map        return fsm_local_search();    }    /* Normal FSM logic follows */    //------ 使用FSM的逻辑    //oldSpaceAvail/32,最大255/254    old_cat = fsm_space_avail_to_cat(oldSpaceAvail);    //(needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP    //#define FSM_CAT_STEP   (BLCKSZ / FSM_CATEGORIES)    //#define FSM_CATEGORIES   256    search_cat = fsm_space_needed_to_cat(spaceNeeded);    /* Get the location of the FSM byte representing the heap block */    //获得对应heap block的位置    addr = fsm_get_location(oldPage, &slot);    //在给定的FSM page和slot中设置值,并返回slot    search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);    /*     * If fsm_set_and_search found a suitable new block, return that.     * Otherwise, search as usual.     * 如fsm_set_and_search成功找到合适的block,则返回;否则,执行常规的检索.     */    if (search_slot != -1)        return fsm_get_heap_blk(addr, search_slot);    else        return fsm_search(rel, search_cat);} /* * Search the local map for an available block to try, in descending order. * As such, there is no heuristic available to decide which order will be * better to try, but the probability of having space in the last block in the * map is higher because that is the most recent block added to the heap. * 以倒序的方式检索本地map找可用的block. * 在这种情况下,没有特别好的办法用于确定那种排序方法更好, *   但在map中最后一个block中存在空闲空间的可能性更高,因为这是最近添加到堆中的block. * * This function is used when there is no FSM. * 如无FSM则使用该函数. */static BlockNumberfsm_local_search(void){    BlockNumber target_block;    /* Local map must be set by now. */    //现在本地map必须已设置    Assert(FSM_LOCAL_MAP_EXISTS);    //目标block    target_block = fsm_local_map.nblocks;    do    {        //循环        target_block--;//从最后一个block开始        if (fsm_local_map.map[target_block] == FSM_LOCAL_AVAIL)            return target_block;//最后一个block可用,则返回    } while (target_block > 0);    //target_block == 0    /*     * If we didn't find any available block to try in the local map, then     * clear it.  This prevents us from using the map again without setting it     * first, which would otherwise lead to the same conclusion again and     * again.     * 在本地map中没有发现可用的block,则清除相关信息.     * 这可以防止我们在没有正确设置map的情况下使用该map,     * 这会导致重复的相同结论(没有可用的block).     */    FSMClearLocalMap();    //返回InvalidBlockNumber    return InvalidBlockNumber;}/* * Initialize or update the local map of blocks to try, for when there is * no FSM. * 如无FSM,则初始化并更新本地map * * When we initialize the map, the whole heap is potentially available to * try.  Testing revealed that trying every block can cause a small * performance dip compared to when we use a FSM, so we try every other * block instead. * 在我们初始化map的时候,整个堆可能已可用. * 测试表名,与使用FSM相比,尝试每个块会导致小幅的性能下降,因此尝试每一个块. */static voidfsm_local_set(Relation rel, BlockNumber cur_nblocks){    BlockNumber blkno,                cached_target_block;    /* The local map must not be set already. */    //验证    Assert(!FSM_LOCAL_MAP_EXISTS);    /*     * Starting at the current last block in the relation and working     * backwards, mark alternating blocks as available.     * 在关系的当前最后一个块开始往后减少,标记可更新的块可用.     */    blkno = cur_nblocks - 1;//最后一个块    while (true)    {        //更新为可用        fsm_local_map.map[blkno] = FSM_LOCAL_AVAIL;        if (blkno >= 2)            blkno -= 2;        else            break;    }    /* Cache the number of blocks. */    //缓存块数    fsm_local_map.nblocks = cur_nblocks;    /* Set the status of the cached target block to 'unavailable'. */    //设置缓存的目标块状态为未可用    cached_target_block = RelationGetTargetBlock(rel);    if (cached_target_block != InvalidBlockNumber &&        cached_target_block < cur_nblocks)        fsm_local_map.map[cached_target_block] = FSM_LOCAL_NOT_AVAIL;}/* * Return category corresponding x bytes of free space * 返回相应有x字节空间空间的目录 */static uint8fsm_space_avail_to_cat(Size avail){    int         cat;    //确保请求的小于块大小    Assert(avail < BLCKSZ);    //如大于最大请求大小,返回255    //#define MaxFSMRequestSize   MaxHeapTupleSize    //#define MaxHeapTupleSize   (BLCKSZ - MAXALIGN(SizeOfPageHeaderData + sizeof(ItemIdData)))    if (avail >= MaxFSMRequestSize)        return 255;    //#define FSM_CAT_STEP   (BLCKSZ / FSM_CATEGORIES)    //#define FSM_CATEGORIES   256    //块大小为8K则FSM_CAT_STEP = 32    cat = avail / FSM_CAT_STEP;    /*     * The highest category, 255, is reserved for MaxFSMRequestSize bytes or     * more.     * 最高层的目录,255,保留用于MaxFSMRequestSize或者更大的大小.     */    if (cat > 254)        cat = 254;//返回254    return (uint8) cat;}/* * Which category does a page need to have, to accommodate x bytes of data? * While fsm_size_to_avail_cat() rounds down, this needs to round up. * 哪一个目录有需要的page,可满足x bytes大小的数据. * 因为fsm_size_to_avail_cat()往下取整,因此这里需要往上取整. */static uint8fsm_space_needed_to_cat(Size needed){    int         cat;    /* Can't ask for more space than the highest category represents */    //不能要求最大目录可能表示的空间大小    if (needed > MaxFSMRequestSize)        elog(ERROR, "invalid FSM request size %zu", needed);    if (needed == 0)        return 1;    cat = (needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP;    if (cat > 255)        cat = 255;    return (uint8) cat;}/* * Return the FSM location corresponding to given heap block. * 返回给定堆block的FSM位置. *///addr = fsm_get_location(oldPage, &slot);static FSMAddressfsm_get_location(BlockNumber heapblk, uint16 *slot){    FSMAddress  addr;    addr.level = FSM_BOTTOM_LEVEL;    //#define SlotsPerFSMPage   LeafNodesPerPage    //#define LeafNodesPerPage   (NodesPerPage - NonLeafNodesPerPage)    //#define NodesPerPage (BLCKSZ - MAXALIGN(SizeOfPageHeaderData) - \                       offsetof(FSMPageData, fp_nodes))    //#define NonLeafNodesPerPage (BLCKSZ / 2 - 1)    addr.logpageno = heapblk / SlotsPerFSMPage;    *slot = heapblk % SlotsPerFSMPage;    return addr;}

三、跟踪分析

测试脚本

15:54:13 (xdb@[local]:5432)testdb=# insert into t1 values (1,'1','1');

启动gdb,设置断点

(gdb) b RecordAndGetPageWithFreeSpaceBreakpoint 1 at 0x8879e4: file freespace.c, line 152.(gdb) cContinuing.Breakpoint 1, RecordAndGetPageWithFreeSpace (rel=0x7fad0df13788, oldPage=1, oldSpaceAvail=16, spaceNeeded=32)    at freespace.c:152152     int         old_cat = fsm_space_avail_to_cat(oldSpaceAvail);(gdb)

输入参数

(gdb) p *rel$5 = {rd_node = {spcNode = 1663, dbNode = 16402, relNode = 50820}, rd_smgr = 0x2084b00, rd_refcnt = 1, rd_backend = -1,   rd_islocaltemp = false, rd_isnailed = false, rd_isvalid = true, rd_indexvalid = 1 '\001', rd_statvalid = false,   rd_createSubid = 0, rd_newRelfilenodeSubid = 0, rd_rel = 0x7fad0df139a0, rd_att = 0x7fad0df13ab8, rd_id = 50820,   rd_lockInfo = {lockRelId = {relId = 50820, dbId = 16402}}, rd_rules = 0x0, rd_rulescxt = 0x0, trigdesc = 0x0,   rd_rsdesc = 0x0, rd_fkeylist = 0x0, rd_fkeyvalid = false, rd_partkeycxt = 0x0, rd_partkey = 0x0, rd_pdcxt = 0x0,   rd_partdesc = 0x0, rd_partcheck = 0x0, rd_indexlist = 0x7fad0df12820, rd_oidindex = 0, rd_pkindex = 0,   rd_replidindex = 0, rd_statlist = 0x0, rd_indexattr = 0x0, rd_projindexattr = 0x0, rd_keyattr = 0x0, rd_pkattr = 0x0,   rd_idattr = 0x0, rd_projidx = 0x0, rd_pubactions = 0x0, rd_options = 0x0, rd_index = 0x0, rd_indextuple = 0x0,   rd_amhandler = 0, rd_indexcxt = 0x0, rd_amroutine = 0x0, rd_opfamily = 0x0, rd_opcintype = 0x0, rd_support = 0x0,   rd_supportinfo = 0x0, rd_indoption = 0x0, rd_indexprs = 0x0, rd_indpred = 0x0, rd_exclops = 0x0, rd_exclprocs = 0x0,   rd_exclstrats = 0x0, rd_amcache = 0x0, rd_indcollation = 0x0, rd_fdwroutine = 0x0, rd_toastoid = 0,   pgstat_info = 0x20785f0}(gdb)

1.初始化相关变量
2.如存在本地map,则首先使用该文件,调用fsm_local_search
3.如果没有本地map也没有FSM,创建本地map,然后调用fsm_local_search
4.使用FSM搜索
4.1获取FSM中原page可用空间对应的catalog -> 0
4.2根据所需空间大小,获取FSM中相应的catalog -> 1

(gdb) n153     int         search_cat = fsm_space_needed_to_cat(spaceNeeded);(gdb) 159     addr = fsm_get_location(oldPage, &slot);(gdb) p old_cat$1 = 0(gdb) p search_cat$2 = 1(gdb)

4.3根据原页面,获取heap block所在的位置(FSMAddress)

(gdb) n161     search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);(gdb) p addr$3 = {level = 0, logpageno = 0}(gdb)

4.4检索获取目标slot

(gdb) n167     if (search_slot != -1)(gdb) p search_slot$4 = 4(gdb)

4.5如目标slot合法,则获取相应的block,否则使用fsm_search搜索合适的block

(gdb) n168         return fsm_get_heap_blk(addr, search_slot);(gdb) 171 }(gdb) RelationGetBufferForTuple (relation=0x7fad0df13788, len=32, otherBuffer=0, options=0, bistate=0x0, vmbuffer=0x7ffe1b797dcc,     vmbuffer_other=0x0) at hio.c:397397     while (targetBlock != InvalidBlockNumber)(gdb) p targetBlock$6 = 4(gdb)

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