ceremonyclient/pebble/read_compaction_queue.go

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2024-01-03 07:31:42 +00:00
package pebble
import "github.com/cockroachdb/pebble/internal/base"
// The maximum number of elements in the readCompactions queue.
// We want to limit the number of elements so that we only do
// compactions for ranges which are being read recently.
const readCompactionMaxQueueSize = 5
// The readCompactionQueue is a queue of read compactions with
// 0 overlapping ranges.
type readCompactionQueue struct {
// Invariant: A contiguous prefix of the queue contains
// all the elements in the queue, in order of insertion.
// When we remove duplicates from the queue, we break
// the invariant that a contiguous prefix of the queue
// has all the elements in it. To fix this, we shift
// the elements of the queue to the left. This is cheap
// because the queue has a max length of 5.
queue [readCompactionMaxQueueSize]*readCompaction
// The size of the queue which is occupied.
// A size of k, implies that the first k elements
// of the queue are occupied.
// The size will be <= readCompactionMaxQueueSize.
size int
}
// combine should be used to combine an older queue with a newer
// queue.
func (qu *readCompactionQueue) combine(newQu *readCompactionQueue, cmp base.Compare) {
for i := 0; i < newQu.size; i++ {
qu.add(newQu.queue[i], cmp)
}
}
// add adds read compactions to the queue, while maintaining the invariant
// that there are no overlapping ranges in the queue.
func (qu *readCompactionQueue) add(rc *readCompaction, cmp base.Compare) {
sz := qu.size
for i := 0; i < sz; i++ {
left := qu.queue[i]
right := rc
if cmp(left.start, right.start) > 0 {
left, right = right, left
}
if cmp(right.start, left.end) <= 0 {
qu.queue[i] = nil
qu.size--
}
}
// Get rid of the holes which may have been formed
// in the queue.
qu.shiftLeft()
if qu.size == readCompactionMaxQueueSize {
// Make space at the end.
copy(qu.queue[0:], qu.queue[1:])
qu.queue[qu.size-1] = rc
} else {
qu.size++
qu.queue[qu.size-1] = rc
}
}
// Shifts the non-nil elements of the queue to the left so
// that a continguous prefix of the queue is non-nil.
func (qu *readCompactionQueue) shiftLeft() {
nilPos := -1
for i := 0; i < readCompactionMaxQueueSize; i++ {
if qu.queue[i] == nil && nilPos == -1 {
nilPos = i
} else if qu.queue[i] != nil && nilPos != -1 {
qu.queue[nilPos] = qu.queue[i]
qu.queue[i] = nil
nilPos++
}
}
}
// remove will remove the oldest element from the queue.
func (qu *readCompactionQueue) remove() *readCompaction {
if qu.size == 0 {
return nil
}
c := qu.queue[0]
copy(qu.queue[0:], qu.queue[1:])
qu.queue[qu.size-1] = nil
qu.size--
return c
}