分析PostgreSQL中的数据结构HTAB
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一、数据结构
/* * Top control structure for a hashtable --- in a shared table, each backend * has its own copy (OK since no fields change at runtime) * 哈希表的顶层控制结构. * 在这个共享哈希表中,每一个后台进程都有自己的拷贝 * (之所以没有问题是因为fork出来后,在运行期没有字段会变化) */ struct HTAB { //指向共享的控制信息 HASHHDR *hctl; /* => shared control information */ //段目录 HASHSEGMENT *dir; /* directory of segment starts */ //哈希函数 HashValueFunc hash; /* hash function */ //哈希键比较函数 HashCompareFunc match; /* key comparison function */ //哈希键拷贝函数 HashCopyFunc keycopy; /* key copying function */ //内存分配器 HashAllocFunc alloc; /* memory allocator */ //内存上下文 MemoryContext hcxt; /* memory context if default allocator used */ //表名(用于错误信息) char *tabname; /* table name (for error messages) */ //如在共享内存中,则为T bool isshared; /* true if table is in shared memory */ //如为T,则固定大小不能扩展 bool isfixed; /* if true, don't enlarge */ /* freezing a shared table isn't allowed, so we can keep state here */ //不允许冻结共享表,因此这里会保存相关状态 bool frozen; /* true = no more inserts allowed */ /* We keep local copies of these fixed values to reduce contention */ //保存这些固定值的本地拷贝,以减少冲突 //哈希键长度(以字节为单位) Size keysize; /* hash key length in bytes */ //段大小,必须为2的幂 long ssize; /* segment size --- must be power of 2 */ //段偏移,ssize的对数 int sshift; /* segment shift = log2(ssize) */ }; /* * Header structure for a hash table --- contains all changeable info * 哈希表的头部结构 -- 存储所有可变信息 * * In a shared-memory hash table, the HASHHDR is in shared memory, while * each backend has a local HTAB struct. For a non-shared table, there isn't * any functional difference between HASHHDR and HTAB, but we separate them * anyway to share code between shared and non-shared tables. * 在共享内存哈希表中,HASHHDR位于共享内存中,每一个后台进程都有一个本地HTAB结构. * 对于非共享哈希表,HASHHDR和HTAB没有任何功能性的不同, * 但无论如何,我们还是把它们区分为共享和非共享表. */ struct HASHHDR { /* * The freelist can become a point of contention in high-concurrency hash * tables, so we use an array of freelists, each with its own mutex and * nentries count, instead of just a single one. Although the freelists * normally operate independently, we will scavenge entries from freelists * other than a hashcode's default freelist when necessary. * 在高并发的哈希表中,空闲链表会成为竞争热点,因此我们使用空闲链表数组, * 数组中的每一个元素都有自己的mutex和条目统计,而不是使用一个. * * If the hash table is not partitioned, only freeList[0] is used and its * spinlock is not used at all; callers' locking is assumed sufficient. * 如果哈希表没有分区,那么只有freelist[0]元素是有用的,自旋锁没有任何用处; * 调用者锁定被认为已足够OK. */ /* Number of freelists to be used for a partitioned hash table. */ //#define NUM_FREELISTS 32 FreeListData freeList[NUM_FREELISTS]; /* These fields can change, but not in a partitioned table */ //这些域字段可以改变,但不适用于分区表 /* Also, dsize can't change in a shared table, even if unpartitioned */ //同时,就算是非分区表,共享表的dsize也不能改变 //目录大小 long dsize; /* directory size */ //已分配的段大小(<= dsize) long nsegs; /* number of allocated segments (<= dsize) */ //正在使用的最大桶ID uint32 max_bucket; /* ID of maximum bucket in use */ //进入整个哈希表的模掩码 uint32 high_mask; /* mask to modulo into entire table */ //进入低位哈希表的模掩码 uint32 low_mask; /* mask to modulo into lower half of table */ /* These fields are fixed at hashtable creation */ //下面这些字段在哈希表创建时已固定 //哈希键大小(以字节为单位) Size keysize; /* hash key length in bytes */ //所有用户元素大小(以字节为单位) Size entrysize; /* total user element size in bytes */ //分区个数(2的幂),或者为0 long num_partitions; /* # partitions (must be power of 2), or 0 */ //目标的填充因子 long ffactor; /* target fill factor */ //如目录是固定大小,则该值为dsize的上限值 long max_dsize; /* 'dsize' limit if directory is fixed size */ //段大小,必须是2的幂 long ssize; /* segment size --- must be power of 2 */ //段偏移,ssize的对数 int sshift; /* segment shift = log2(ssize) */ //一次性分配的条目个数 int nelem_alloc; /* number of entries to allocate at once */ #ifdef HASH_STATISTICS /* * Count statistics here. NB: stats code doesn't bother with mutex, so * counts could be corrupted a bit in a partitioned table. * 统计信息. * 注意:统计相关的代码不会影响mutex,因此对于分区表,统计可能有一点点问题 */ long accesses; long collisions; #endif }; /* * Per-freelist data. * 空闲链表数据. * * In a partitioned hash table, each freelist is associated with a specific * set of hashcodes, as determined by the FREELIST_IDX() macro below. * nentries tracks the number of live hashtable entries having those hashcodes * (NOT the number of entries in the freelist, as you might expect). * 在一个分区哈希表中,每一个空闲链表与特定的hashcodes集合相关,通过下面的FREELIST_IDX()宏进行定义. * nentries跟踪有这些hashcodes的仍存活的hashtable条目个数. * (注意不要搞错,不是空闲的条目个数) * * The coverage of a freelist might be more or less than one partition, so it * needs its own lock rather than relying on caller locking. Relying on that * wouldn't work even if the coverage was the same, because of the occasional * need to "borrow" entries from another freelist; see get_hash_entry(). * 空闲链表的覆盖范围可能比一个分区多或少,因此需要自己的锁而不能仅仅依赖调用者的锁. * 依赖调用者锁在覆盖面一样的情况下也不会起效,因为偶尔需要从另一个自由列表“借用”条目,详细参见get_hash_entry() * * Using an array of FreeListData instead of separate arrays of mutexes, * nentries and freeLists helps to reduce sharing of cache lines between * different mutexes. * 使用FreeListData数组而不是一个独立的mutexes,nentries和freelists数组有助于减少不同mutexes之间的缓存线共享. */ typedef struct { //该空闲链表的自旋锁 slock_t mutex; /* spinlock for this freelist */ //相关桶中的条目个数 long nentries; /* number of entries in associated buckets */ //空闲元素链 HASHELEMENT *freeList; /* chain of free elements */ } FreeListData; /* * HASHELEMENT is the private part of a hashtable entry. The caller's data * follows the HASHELEMENT structure (on a MAXALIGN'd boundary). The hash key * is expected to be at the start of the caller's hash entry data structure. * HASHELEMENT是哈希表条目的私有部分. * 调用者的数据按照HASHELEMENT结构组织(位于MAXALIGN的边界). * 哈希键应位于调用者hash条目数据结构的开始位置. */ typedef struct HASHELEMENT { //链接到相同桶中的下一个条目 struct HASHELEMENT *link; /* link to next entry in same bucket */ //该条目的哈希函数结果 uint32 hashvalue; /* hash function result for this entry */ } HASHELEMENT; /* Hash table header struct is an opaque type known only within dynahash.c */ //哈希表头部结构,非透明类型,用于dynahash.c typedef struct HASHHDR HASHHDR; /* Hash table control struct is an opaque type known only within dynahash.c */ //哈希表控制结构,非透明类型,用于dynahash.c typedef struct HTAB HTAB; /* Parameter data structure for hash_create */ //hash_create使用的参数数据结构 /* Only those fields indicated by hash_flags need be set */ //根据hash_flags标记设置相应的字段 typedef struct HASHCTL { //分区个数(必须是2的幂) long num_partitions; /* # partitions (must be power of 2) */ //段大小 long ssize; /* segment size */ //初始化目录大小 long dsize; /* (initial) directory size */ //dsize上限 long max_dsize; /* limit to dsize if dir size is limited */ //填充因子 long ffactor; /* fill factor */ //哈希键大小(字节为单位) Size keysize; /* hash key length in bytes */ //参见上述数据结构注释 Size entrysize; /* total user element size in bytes */ // HashValueFunc hash; /* hash function */ HashCompareFunc match; /* key comparison function */ HashCopyFunc keycopy; /* key copying function */ HashAllocFunc alloc; /* memory allocator */ MemoryContext hcxt; /* memory context to use for allocations */ //共享内存中的哈希头部结构地址 HASHHDR *hctl; /* location of header in shared mem */ } HASHCTL; /* A hash bucket is a linked list of HASHELEMENTs */ //哈希桶是HASHELEMENTs链表 typedef HASHELEMENT *HASHBUCKET; /* A hash segment is an array of bucket headers */ //hash segment是桶数组 typedef HASHBUCKET *HASHSEGMENT; /* * Hash functions must have this signature. * Hash函数必须有它自己的标识 */ typedef uint32 (*HashValueFunc) (const void *key, Size keysize); /* * Key comparison functions must have this signature. Comparison functions * return zero for match, nonzero for no match. (The comparison function * definition is designed to allow memcmp() and strncmp() to be used directly * as key comparison functions.) * 哈希键对比函数必须有自己的标识. * 如匹配则对比函数返回0,不匹配返回非0. * (对比函数定义被设计为允许在对比键值时可直接使用memcmp()和strncmp()) */ typedef int (*HashCompareFunc) (const void *key1, const void *key2, Size keysize); /* * Key copying functions must have this signature. The return value is not * used. (The definition is set up to allow memcpy() and strlcpy() to be * used directly.) * 键拷贝函数必须有自己的标识. * 返回值无用. */ typedef void *(*HashCopyFunc) (void *dest, const void *src, Size keysize); /* * Space allocation function for a hashtable --- designed to match malloc(). * Note: there is no free function API; can't destroy a hashtable unless you * use the default allocator. * 哈希表的恐惧分配函数 -- 被设计为与malloc()函数匹配. * 注意:这里没有释放函数API;不能销毁哈希表,除非使用默认的分配器. */ typedef void *(*HashAllocFunc) (Size request);
其结构如下图所示:
二、跟踪分析
测试脚本
\pset footer off \pset tuples_only on \o /tmp/drop.sql SELECT 'drop table if exists tbl' || id || ' ;' as "--" FROM generate_series(1, 20000) AS id; \i /tmp/drop.sql \pset footer off \pset tuples_only on \o /tmp/create.sql SELECT 'CREATE TABLE tbl' || id || ' (id int);' as "--" FROM generate_series(1, 10000) AS id; begin; \o /tmp/ret.txt \i /tmp/create.sql
跟踪分析
... HASHSEGMENT *dir --> HASHELEMENT ***dir; dir --> HASHELEMENT *** (gdb) p *hctl $1 = {freeList = {{mutex = 0 '\000', nentries = 312, freeList = 0x7fd906ab84c0}, {mutex = 0 '\000', nentries = 298, freeList = 0x7fd907097c40}, {mutex = 0 '\000', nentries = 292, freeList = 0x7fd906ac2520}, {mutex = 0 '\000', nentries = 321, freeList = 0x7fd906ac8120}, { mutex = 0 '\000', nentries = 341, freeList = 0x7fd907229980}, {mutex = 0 '\000', nentries = 334, freeList = 0x7fd906ad3f08}, {mutex = 0 '\000', nentries = 316, freeList = 0x7fd906ad6fb8}, { mutex = 0 '\000', nentries = 299, freeList = 0x7fd906ade550}, {mutex = 0 '\000', nentries = 328, freeList = 0x7fd906ae1600}, {mutex = 0 '\000', nentries = 328, freeList = 0x7fd906ae62e8}, { mutex = 0 '\000', nentries = 308, freeList = 0x7fd906aeb660}, {mutex = 0 '\000', nentries = 327, freeList = 0x7fd90706f338}, {mutex = 0 '\000', nentries = 346, freeList = 0x7fd906af6bc0}, { mutex = 0 '\000', nentries = 323, freeList = 0x7fd907237bc0}, {mutex = 0 '\000', nentries = 304, freeList = 0x7fd9071ddb40}, {mutex = 0 '\000', nentries = 311, freeList = 0x7fd906b06238}, { mutex = 0 '\000', nentries = 292, freeList = 0x7fd90707b620}, {mutex = 0 '\000', nentries = 303, freeList = 0x7fd90723dd20}, {mutex = 0 '\000', nentries = 302, freeList = 0x7fd906b137e0}, { mutex = 0 '\000', nentries = 307, freeList = 0x7fd9070873c8}, {mutex = 0 '\000', nentries = 314, freeList = 0x7fd90723bb68}, {mutex = 0 '\000', nentries = 279, freeList = 0x7fd906b22678}, { mutex = 0 '\000', nentries = 297, freeList = 0x7fd907073e08}, {mutex = 0 '\000', nentries = 309, freeList = 0x7fd90721f888}, {mutex = 0 '\000', nentries = 317, freeList = 0x7fd906b33880}, { mutex = 0 '\000', nentries = 283, freeList = 0x7fd907086168}, {mutex = 0 '\000', nentries = 331, freeList = 0x7fd906b3d838}, {mutex = 0 '\000', nentries = 330, freeList = 0x7fd906b41f38}, { mutex = 0 '\000', nentries = 313, freeList = 0x7fd906b46440}, {mutex = 0 '\000', nentries = 304, freeList = 0x7fd906b4b5c0}, {mutex = 0 '\000', nentries = 310, freeList = 0x7fd90720ed80}, { mutex = 0 '\000', nentries = 323, freeList = 0x7fd906b575a0}}, dsize = 256, nsegs = 16, max_bucket = 4095, high_mask = 8191, low_mask = 4095, keysize = 16, entrysize = 152, num_partitions = 16, ffactor = 1, max_dsize = 256, ssize = 256, sshift = 8, nelem_alloc = 48} (gdb) p *hashp $2 = {hctl = 0x7fd906aae980, dir = 0x7fd906aaecd8, hash = 0xa79ac6, match = 0x47cb70 , keycopy = 0x47d0a0 , alloc = 0x8c3419 , hcxt = 0x0, tabname = 0x160f1d0 "LOCK hash", isshared = true, isfixed = false, frozen = false, keysize = 16, ssize = 256, sshift = 8} (gdb) p *hashp->dir $3 = (HASHSEGMENT) 0x7fd906aaf500 (gdb) p hashp->dir $4 = (HASHSEGMENT *) 0x7fd906aaecd8 (gdb) p **hashp->dir $5 = (HASHBUCKET) 0x7fd907212dd0 (gdb) p ***hashp->dir $6 = {link = 0x7fd9071a7b90, hashvalue = 1748602880} (gdb) n 949 if (action == HASH_ENTER || action == HASH_ENTER_NULL) (gdb) 956 if (!IS_PARTITIONED(hctl) && !hashp->frozen && (gdb) 965 bucket = calc_bucket(hctl, hashvalue); --> hash桶 (gdb) 967 segment_num = bucket >> hashp->sshift; --> 桶号右移8位得到段号 (gdb) 968 segment_ndx = MOD(bucket, hashp->ssize); --> 桶号取模得到段内偏移 (gdb) 970 segp = hashp->dir[segment_num]; --> 获取段(HASHELEMENT **) (gdb) 972 if (segp == NULL) (gdb) p bucket $7 = 2072 (gdb) p segment_num $8 = 8 (gdb) p segment_ndx $9 = 24 (gdb) p segp --> $10 = (HASHSEGMENT) 0x7fd906ab3500 (gdb) (gdb) n 975 prevBucketPtr = &segp[segment_ndx]; --> HASHELEMENT ** (gdb) 976 currBucket = *prevBucketPtr; --> HASHELEMENT * (gdb) 981 match = hashp->match; /* save one fetch in inner loop */ (gdb) p prevBucketPtr $12 = (HASHBUCKET *) 0x7fd906ab35c0 (gdb) p currBucket $13 = (HASHBUCKET) 0x7fd90714da68 (gdb)
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文章标题:分析PostgreSQL中的数据结构HTAB
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