第三节空闲列表页

第三节空闲列表页

空闲列表页中主要包含三个部分：所有已经可以重新利用的空闲页列表ids、将来很快被释放掉的事务关联的页列表pending、页id的缓存。详细定义在freelist.go文件中，下面给大家展示其空闲页的定义。

1. freelist.go

3. // freelist represents a list of all pages that are available for allocation.
4. // It also tracks pages that have been freed but are still in use by open transactions.
5. type freelist struct {
6. // 已经可以被分配的空闲页
7. ids     []pgid          // all free and available free page ids.
8. // 将来很快能被释放的空闲页，部分事务可能在读或者写
9. pending map[txid][]pgid // mapping of soon-to-be free page ids by tx.
10. cache   map[pgid]bool   // fast lookup of all free and pending page ids.
11. }

13. // newFreelist returns an empty, initialized freelist.
14. func newFreelist() *freelist {
15. return &freelist{
16. pending: make(map[txid][]pgid),
17. cache:   make(map[pgid]bool),
18. }
19. }

下图展示的是空闲列表的存储方式。

1. freelist->page

将空闲列表转换成页信息，写到磁盘中，此处需要注意一个问题，页头中的count字段是一个uint16，占两个字节，其最大可以表示2^16 即65536个数字，当空闲页的个数超过65535时时，需要将p.ptr中的第一个字节用来存储其空闲页的个数，同时将p.count设置为0xFFFF。否则不超过的情况下，直接用count来表示其空闲页的个数

1. // write writes the page ids onto a freelist page. All free and pending ids are
2. // saved to disk since in the event of a program crash, all pending ids will
3. // become free.
4. //将 freelist信息写入到p中
5. func (f *freelist) write(p *page) error {
6. // Combine the old free pgids and pgids waiting on an open transaction.

8. // Update the header flag.
9. // 设置页头中的页类型标识
10. p.flags |= freelistPageFlag

12. // The page.count can only hold up to 64k elements so if we overflow that
13. // number then we handle it by putting the size in the first element.

15. lenids := f.count()
16. if lenids == 0 {
17. p.count = uint16(lenids)
18. } else if lenids < 0xFFFF {
19. p.count = uint16(lenids)
20. // 拷贝到page的ptr中
21. f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:])
22. } else {
23. // 有溢出的情况下，后面第一个元素放置其长度，然后再存放所有的pgid列表
24. p.count = 0xFFFF
25. ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0] = pgid(lenids)
26. // 从第一个元素位置拷贝
27. f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[1:])
28. }
29. return nil
30. }

32. // copyall copies into dst a list of all free ids and all pending ids in one sorted list.
33. // f.count returns the minimum length required for dst.
34. func (f *freelist) copyall(dst []pgid) {
35. // 首先把pending状态的页放到一个数组中，并使其有序
36. m := make(pgids, 0, f.pending_count())
37. for _, list := range f.pending {
38. m = append(m, list...)
39. }
40. sort.Sort(m)
41. // 合并两个有序的列表，最后结果输出到dst中
42. mergepgids(dst, f.ids, m)
43. }

45. // mergepgids copies the sorted union of a and b into dst.
46. // If dst is too small, it panics.
47. // 将a和b按照有序合并成到dst中，a和b有序
48. func mergepgids(dst, a, b pgids) {
49. if len(dst) < len(a)+len(b) {
50. panic(fmt.Errorf("mergepgids bad len %d < %d + %d", len(dst), len(a), len(b)))
51. }
52. // Copy in the opposite slice if one is nil.
53. if len(a) == 0 {
54. copy(dst, b)
55. return
56. }
57. if len(b) == 0 {
58. copy(dst, a)
59. return
60. }

62. // Merged will hold all elements from both lists.
63. merged := dst[:0]

65. // Assign lead to the slice with a lower starting value, follow to the higher value.
66. lead, follow := a, b
67. if b[0] < a[0] {
68. lead, follow = b, a
69. }

71. // Continue while there are elements in the lead.
72. for len(lead) > 0 {
73. // Merge largest prefix of lead that is ahead of follow[0].
74. n := sort.Search(len(lead), func(i int) bool { return lead[i] > follow[0] })
75. merged = append(merged, lead[:n]...)
76. if n >= len(lead) {
77. break
78. }

80. // Swap lead and follow.
81. lead, follow = follow, lead[n:]
82. }

84. // Append what's left in follow.
85. _ = append(merged, follow...)
86. }

2. page->freelist

从磁盘中加载空闲页信息，并转为freelist结构，转换时，也需要注意其空闲页的个数的判断逻辑，当p.count为0xFFFF时，需要读取p.ptr中的第一个字节来计算其空闲页的个数。否则则直接读取p.ptr中存放的数据为空闲页ids列表

1. //从磁盘中的page初始化freelist
2. // read initializes the freelist from a freelist page.
3. func (f *freelist) read(p *page) {
4. // If the page.count is at the max uint16 value (64k) then it's considered
5. // an overflow and the size of the freelist is stored as the first element.
6. idx, count := 0, int(p.count)
7. if count == 0xFFFF {
8. idx = 1
9. // 用第一个uint64来存储整个count的值
10. count = int(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0])
11. }

13. // Copy the list of page ids from the freelist.
14. if count == 0 {
15. f.ids = nil
16. } else {
17. ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx:count]
18. f.ids = make([]pgid, len(ids))
19. copy(f.ids, ids)

21. // Make sure they're sorted.
22. sort.Sort(pgids(f.ids))
23. }

25. // Rebuild the page cache.
26. f.reindex()
27. }

29. // reindex rebuilds the free cache based on available and pending free lists.
30. func (f *freelist) reindex() {
31. f.cache = make(map[pgid]bool, len(f.ids))
32. for _, id := range f.ids {
33. f.cache[id] = true
34. }
35. for _, pendingIDs := range f.pending {
36. for _, pendingID := range pendingIDs {
37. f.cache[pendingID] = true
38. }
39. }
40. }

3. allocate

开始分配一段连续的n个页。其中返回值为初始的页id。如果无法分配，则返回0即可

1. // allocate returns the starting page id of a contiguous list of pages of a given size.
2. // If a contiguous block cannot be found then 0 is returned.
3. // [5,6,7,13,14,15,16,18,19,20,31,32]
4. // 开始分配一段连续的n个页。其中返回值为初始的页id。如果无法分配，则返回0即可
5. func (f *freelist) allocate(n int) pgid {
6. if len(f.ids) == 0 {
7. return 0
8. }

10. var initial, previd pgid
11. for i, id := range f.ids {
12. if id <= 1 {
13. panic(fmt.Sprintf("invalid page allocation: %d", id))
14. }

16. // Reset initial page if this is not contiguous.
17. // id-previd != 1 来判断是否连续
18. if previd == 0 || id-previd != 1 {
19. // 第一次不连续时记录一下第一个位置
20. initial = id
21. }

23. // If we found a contiguous block then remove it and return it.
24. // 找到了连续的块，然后将其返回即可
25. if (id-initial)+1 == pgid(n) {
26. // If we're allocating off the beginning then take the fast path
27. // and just adjust the existing slice. This will use extra memory
28. // temporarily but the append() in free() will realloc the slice
29. // as is necessary.
30. if (i + 1) == n {
31. // 找到的是前n个连续的空间
32. f.ids = f.ids[i+1:]
33. } else {
34. copy(f.ids[i-n+1:], f.ids[i+1:])
35. f.ids = f.ids[:len(f.ids)-n]
36. }

38. // Remove from the free cache.
39. // 同时更新缓存
40. for i := pgid(0); i < pgid(n); i++ {
41. delete(f.cache, initial+i)
42. }

44. return initial
45. }

47. previd = id
48. }
49. return 0
50. }

考虑是否需要补充其他的freelist的方法

第三节 空闲列表页

第三节 空闲列表页

第三节空闲列表页

第三节空闲列表页