PostgreSQL源码解读（248）-HTAB动态扩展图解#2
本节简单介绍了PostgreSQL中的HTAB如何动态扩展,这是第2部分,结合代码进行解析.
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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：
扩展后的结构如下图所示:
二、源码解读
主要的函数是expand_table
1.分配新桶,HTAB的最大桶数max_bucket+1
2.根据新桶号计算段号和段内编号
3.如需扩展段,则扩展(*2)
4.获取新桶号对应的原桶号,目的是为了把原桶号中的数据迁移到新桶中.新桶号和原桶号相差low_mask
5.扫描旧桶,重建旧桶元素链表,构造新桶元素链表
/*
* Expand the table by adding one more hash bucket.
* 通过增加一个或者多个hash bucket扩展hash表
*/
static bool
expand_table(HTAB *hashp)
{
HASHHDR *hctl = hashp->hctl;//hash控制结构
HASHSEGMENT old_seg,//原seg
new_seg;//新seg
long old_bucket,//原bucket
new_bucket;//新bucket
long new_segnum,//新seg号
new_segndx;//新seg索引(segment中的编号)
long old_segnum,//新seg号
old_segndx;//原seg索引
HASHBUCKET *oldlink,//原桶
*newlink;//新桶
HASHBUCKET currElement,//当前元素
nextElement;//下一元素
//#define IS_PARTITIONED(hctl) ((hctl)->num_partitions != 0)
Assert(!IS_PARTITIONED(hctl));
#ifdef HASH_STATISTICS
hash_expansions++;
#endif
new_bucket = hctl->max_bucket + 1;//新增加一个bucket
new_segnum = new_bucket >> hashp->sshift;//取商数
new_segndx = MOD(new_bucket, hashp->ssize);//取余数
if (new_segnum >= hctl->nsegs)
{
//扩展segment,每次扩展一倍
/* Allocate new segment if necessary -- could fail if dir full */
if (new_segnum >= hctl->dsize)
if (!dir_realloc(hashp))
return false;
if (!(hashp->dir[new_segnum] = seg_alloc(hashp)))//为新的seg对应的bucket分配空间
return false;
hctl->nsegs++;
}
/* OK, we created a new bucket */
//已完成创建
hctl->max_bucket++;
/*
* *Before* changing masks, find old bucket corresponding to same hash
* values; values in that bucket may need to be relocated to new bucket.
* Note that new_bucket is certainly larger than low_mask at this point,
* so we can skip the first step of the regular hash mask calc.
* 在修改掩码前,为新的bucket找到对应的原bucket,原bucket中的元素keneng需要迁移到新的bucket上.
* 注意new_bucket肯定会比low_mask要大,可以跳过常规的hash掩码计算的第一个步骤.
*/
old_bucket = (new_bucket & hctl->low_mask);
/*
* If we crossed a power of 2, readjust masks.
* 如果new_bucket是2的n次方,调整掩码
*/
if ((uint32) new_bucket > hctl->high_mask)
{
hctl->low_mask = hctl->high_mask;//如15->31
hctl->high_mask = (uint32) new_bucket | hctl->low_mask;//如31->63
}
/*
* Relocate records to the new bucket. NOTE: because of the way the hash
* masking is done in calc_bucket, only one old bucket can need to be
* split at this point. With a different way of reducing the hash value,
* that might not be true!
* 重定位记录到新的bucket上.
* 注意:由于通过方法calc_bucket计算hash掩码,这时只需要拆分一个bucket.
*
*/
old_segnum = old_bucket >> hashp->sshift;//计算原seg号
old_segndx = MOD(old_bucket, hashp->ssize);//计算原seg中的索引号
old_seg = hashp->dir[old_segnum];//旧seg
new_seg = hashp->dir[new_segnum];//新seg
oldlink = &old_seg[old_segndx];//原bucket指针
newlink = &new_seg[new_segndx];//新bucket指针
for (currElement = *oldlink;
currElement != NULL;
currElement = nextElement)//循环遍历
{
nextElement = currElement->link;
if ((long) calc_bucket(hctl, currElement->hashvalue) == old_bucket)
{
*oldlink = currElement;
oldlink = &currElement->link;//重新构造原bucket
}
else
{
*newlink = currElement;//构造新bucket
newlink = &currElement->link;
}
}
/* don't forget to terminate the rebuilt hash chains... */
//不要忘了终止重建后的hash链
*oldlink = NULL;
*newlink = NULL;
return true;
}
static bool
dir_realloc(HTAB *hashp)
{
HASHSEGMENT *p;
HASHSEGMENT *old_p;
long new_dsize;
long old_dirsize;
long new_dirsize;
if (hashp->hctl->max_dsize != NO_MAX_DSIZE)
return false;
/* Reallocate directory */
new_dsize = hashp->hctl->dsize << 1;
old_dirsize = hashp->hctl->dsize * sizeof(HASHSEGMENT);
new_dirsize = new_dsize * sizeof(HASHSEGMENT);
old_p = hashp->dir;
CurrentDynaHashCxt = hashp->hcxt;
p = (HASHSEGMENT *) hashp->alloc((Size) new_dirsize);
if (p != NULL)
{
memcpy(p, old_p, old_dirsize);
MemSet(((char *) p) + old_dirsize, 0, new_dirsize - old_dirsize);
hashp->dir = p;
hashp->hctl->dsize = new_dsize;
/* XXX assume the allocator is palloc, so we know how to free */
Assert(hashp->alloc == DynaHashAlloc);
pfree(old_p);
return true;
}
return false;
}
static HASHSEGMENT
seg_alloc(HTAB *hashp)
{
HASHSEGMENT segp;
CurrentDynaHashCxt = hashp->hcxt;
segp = (HASHSEGMENT) hashp->alloc(sizeof(HASHBUCKET) * hashp->ssize);
if (!segp)
return NULL;
MemSet(segp, 0, sizeof(HASHBUCKET) * hashp->ssize);
return segp;
}
三、跟踪分析
测试脚本
[local:/data/run/pg12]:5120 pg12@testdb=# \d t_expand;
Table "public.t_expand"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
id | integer | | |
[local:/data/run/pg12]:5120 pg12@testdb=# select count(*) from t_expand;
count
---------
2000000
(1 row)
[local:/data/run/pg12]:5120 pg12@testdb=# select * from t_expand;
...
启动gdb跟踪
(gdb) b hash_search_with_hash_value
Breakpoint 2 at 0xa790f2: file dynahash.c, line 925.
(gdb) c
Continuing.
Breakpoint 2, hash_search_with_hash_value (hashp=0x224eac8, keyPtr=0x7fffed717700, hashvalue=2252448879, action=HASH_ENTER, foundPtr=0x7fffed7176ff) at dynahash.c:925
925 HASHHDR *hctl = hashp->hctl; --> hash控制结构体
(gdb) n
926 int freelist_idx = FREELIST_IDX(hctl, hashvalue);--> 空闲链表
(gdb) p *hctl
$1 = {freeList = {{mutex = 0 '\000', nentries = 0, freeList = 0x22504d0}, {mutex = 0 '\000', nentries = 0, freeList = 0x0} }, dsize = 256, nsegs = 1, max_bucket = 15,
high_mask = 31, low_mask = 15, keysize = 20, entrysize = 72, num_partitions = 0, ffactor = 1, max_dsize = -1, ssize = 256, sshift = 8, nelem_alloc = 46}
(gdb) n
949 if (action == HASH_ENTER || action == HASH_ENTER_NULL)
(gdb)
956 if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
(gdb)
957 hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor && --> 判断是否需要扩展
(gdb)
956 if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
(gdb)
965 bucket = calc_bucket(hctl, hashvalue);-->计算hash桶
(gdb) step
calc_bucket (hctl=0x224eb60, hash_val=2252448879) at dynahash.c:871
871 bucket = hash_val & hctl->high_mask;-->先行与high_mask(31)进行掩码运算
(gdb) n
872 if (bucket > hctl->max_bucket)-->得到的结果如何比max_bucket还大,那要跟low_mask(15)进行掩码运算
(gdb) p bucket
$2 = 15
(gdb) n
875 return bucket;
(gdb) l
870
871 bucket = hash_val & hctl->high_mask;
872 if (bucket > hctl->max_bucket)
873 bucket = bucket & hctl->low_mask;
874
875 return bucket;
876 }
877
878 /*
879 * hash_search -- look up key in table and perform action
(gdb) n
876 }
(gdb)
hash_search_with_hash_value (hashp=0x224eac8, keyPtr=0x7fffed717700, hashvalue=2252448879, action=HASH_ENTER, foundPtr=0x7fffed7176ff) at dynahash.c:967
967 segment_num = bucket >> hashp->sshift;-->seg号,相当于15/256,结果为0
(gdb)
968 segment_ndx = MOD(bucket, hashp->ssize);-->seg内编号,相当于15/256取模,结果为15
(gdb)
970 segp = hashp->dir[segment_num];
(gdb)
972 if (segp == NULL)
(gdb) p segment_num
$3 = 0
(gdb) p segment_ndx
$4 = 15
(gdb) n
975 prevBucketPtr = &segp[segment_ndx];
(gdb)
976 currBucket = *prevBucketPtr;
(gdb)
981 match = hashp->match; /* save one fetch in inner loop */
(gdb)
982 keysize = hashp->keysize; /* ditto */
(gdb)
984 while (currBucket != NULL)
(gdb)
997 if (foundPtr)
(gdb)
998 *foundPtr = (bool) (currBucket != NULL);
(gdb)
1003 switch (action)
(gdb)
1047 if (currBucket != NULL)
(gdb)
1051 if (hashp->frozen)
(gdb)
1055 currBucket = get_hash_entry(hashp, freelist_idx);
(gdb)
1056 if (currBucket == NULL)
(gdb)
1073 *prevBucketPtr = currBucket;
(gdb)
1074 currBucket->link = NULL;
(gdb)
1077 currBucket->hashvalue = hashvalue;
(gdb)
1078 hashp->keycopy(ELEMENTKEY(currBucket), keyPtr, keysize);
(gdb)
1087 return (void *) ELEMENTKEY(currBucket);
(gdb)
1093 }
(gdb)
hash_search (hashp=0x224eac8, keyPtr=0x7fffed717700, action=HASH_ENTER, foundPtr=0x7fffed7176ff) at dynahash.c:916
916 }
(gdb)
四、参考资料
N/A
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