PostgreSQL源码解读（247）-HTAB动态扩展图解#1

本节简单介绍了PostgreSQL中的HTAB如何动态扩展,这是第1部分.

创新互联建站制作网站网页找三站合一网站制作公司,专注于网页设计,成都网站建设、成都做网站,网站设计,企业网站搭建,网站开发,建网站业务,680元做网站,已为上千余家服务,创新互联建站网站建设将一如既往的为我们的客户提供最优质的网站建设、网络营销推广服务!

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：

扩展后的结构如下图所示:

二、跟踪分析

N/A

三、参考资料

N/A