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。

freelist.go

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

// newFreelist returns an empty, initialized freelist.
func newFreelist() *freelist {
    return &freelist{
        pending: make(map[txid][]pgid),
        cache:   make(map[pgid]bool),
    }
}

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

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

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

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

    lenids := f.count()
    if lenids == 0 {
        p.count = uint16(lenids)
    } else if lenids < 0xFFFF {
        p.count = uint16(lenids)
        // 拷贝到page的ptr中
        f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:])
    } else {
        // 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
        p.count = 0xFFFF
        ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0] = pgid(lenids)
        // 从第一个元素位置拷贝
        f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[1:])
    }
    return nil
}

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

// mergepgids copies the sorted union of a and b into dst.
// If dst is too small, it panics.
// 将a和b按照有序合并成到dst中，a和b有序
func mergepgids(dst, a, b pgids) {
    if len(dst) < len(a)+len(b) {
        panic(fmt.Errorf("mergepgids bad len %d < %d + %d", len(dst), len(a), len(b)))
    }
    // Copy in the opposite slice if one is nil.
    if len(a) == 0 {
        copy(dst, b)
        return
    }
    if len(b) == 0 {
        copy(dst, a)
        return
    }

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

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

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

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

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

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

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

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

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

    // Rebuild the page cache.
    f.reindex()
}

// reindex rebuilds the free cache based on available and pending free lists.
func (f *freelist) reindex() {
    f.cache = make(map[pgid]bool, len(f.ids))
    for _, id := range f.ids {
        f.cache[id] = true
    }
    for _, pendingIDs := range f.pending {
        for _, pendingID := range pendingIDs {
            f.cache[pendingID] = true
        }
    }
}

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

// allocate returns the starting page id of a contiguous list of pages of a given size.
// If a contiguous block cannot be found then 0 is returned.
// [5,6,7,13,14,15,16,18,19,20,31,32]
// 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
func (f *freelist) allocate(n int) pgid {
    if len(f.ids) == 0 {
        return 0
    }

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

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

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

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

            return initial
        }

        previd = id
    }
    return 0
}

考虑是否需要补充其他的freelist的方法