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timestamp_cache.go
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timestamp_cache.go
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// Copyright 2014 The Cockroach Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
package storage
import (
"fmt"
"time"
"unsafe"
"github.com/google/btree"
"github.com/cockroachdb/cockroach/pkg/keys"
"github.com/cockroachdb/cockroach/pkg/roachpb"
"github.com/cockroachdb/cockroach/pkg/util/cache"
"github.com/cockroachdb/cockroach/pkg/util/hlc"
"github.com/cockroachdb/cockroach/pkg/util/interval"
"github.com/cockroachdb/cockroach/pkg/util/uuid"
)
const (
// MinTSCacheWindow specifies the minimum duration to hold entries in the
// cache before allowing eviction. After this window expires, transactions
// writing to this node with timestamps lagging by more than MinTSCacheWindow
// will necessarily have to advance their commit timestamp.
MinTSCacheWindow = 10 * time.Second
// defaultTimestampCacheSize is the default size in bytes for a store's
// timestamp cache. Note that the timestamp cache can use more memory than
// this because it holds on to all entries that are younger than
// MinTSCacheWindow.
defaultTimestampCacheSize = 64 << 20 // 64 MB
// Max entries in each btree node.
// TODO(peter): Not yet tuned.
btreeDegree = 64
)
// cacheRequest holds the timestamp cache data from a single batch request. The
// requests are stored in a btree keyed by the timestamp and are "expanded" to
// populate the read/write interval caches if a potential conflict is detected
// due to an earlier request (based on timestamp) arriving.
type cacheRequest struct {
span roachpb.RSpan
reads []roachpb.Span
writes []roachpb.Span
txn roachpb.Span
txnID uuid.UUID
timestamp hlc.Timestamp
// Used to distinguish requests with identical timestamps. For actual
// requests, the uniqueID value is >0. When probing the btree for requests
// later than a particular timestamp a value of 0 is used.
uniqueID int64
}
// Less implements the btree.Item interface.
func (cr *cacheRequest) Less(other btree.Item) bool {
otherReq := other.(*cacheRequest)
if cr.timestamp.Less(otherReq.timestamp) {
return true
}
if otherReq.timestamp.Less(cr.timestamp) {
return false
}
// Fallback to comparison of the uniqueID as a tie-breaker. This allows
// multiple requests with the same timestamp to exist in the requests btree.
return cr.uniqueID < otherReq.uniqueID
}
// numSpans returns the number of spans the request will expand into.
func (cr *cacheRequest) numSpans() int {
n := len(cr.reads) + len(cr.writes)
if cr.txn.Key != nil {
n++
}
return n
}
func (cr *cacheRequest) size() uint64 {
var n uint64
for i := range cr.reads {
s := &cr.reads[i]
n += cacheEntrySize(interval.Comparable(s.Key), interval.Comparable(s.EndKey))
}
for i := range cr.writes {
s := &cr.writes[i]
n += cacheEntrySize(interval.Comparable(s.Key), interval.Comparable(s.EndKey))
}
if cr.txn.Key != nil {
n += cacheEntrySize(interval.Comparable(cr.txn.Key), nil)
}
return n
}
var cacheEntryOverhead = uint64(unsafe.Sizeof(cache.IntervalKey{}) +
unsafe.Sizeof(cacheValue{}) + unsafe.Sizeof(cache.Entry{}))
func cacheEntrySize(start, end interval.Comparable) uint64 {
n := uint64(cap(start))
if end != nil && len(start) > 0 && len(end) > 0 && &end[0] != &start[0] {
// If the end key exists and is not sharing memory with the start key,
// account for its memory usage.
n += uint64(cap(end))
}
n += cacheEntryOverhead
return n
}
// A TimestampCache maintains an interval tree FIFO cache of keys or
// key ranges and the timestamps at which they were most recently read
// or written. If a timestamp was read or written by a transaction,
// the txn ID is stored with the timestamp to avoid advancing
// timestamps on successive requests from the same transaction.
//
// The cache also maintains a low-water mark which is the most
// recently evicted entry's timestamp. This value always ratchets
// with monotonic increases. The low water mark is initialized to
// the current system time plus the maximum clock offset.
type timestampCache struct {
rCache, wCache *cache.IntervalCache
lowWater, latest hlc.Timestamp
// The requests tree contains cacheRequest entries keyed by timestamp. A
// request is "expanded" (i.e. the read/write spans are added to the
// read/write interval caches) when the timestamp cache is accessed on behalf
// of an earlier request.
requests *btree.BTree
tmpReq cacheRequest
reqIDAlloc int64
reqSpans int
bytes uint64
maxBytes uint64
}
// lowWaterTxnIDMarker is a special txn ID that identifies a cache entry as a
// low water mark. It is specified when a lease is acquired to clear the
// timestamp cache for a range. Also see timestampCache.getMax where this txn
// ID is checked in order to return whether the max read/write timestamp came
// from a regular entry or one of these low water mark entries.
var lowWaterTxnIDMarker = func() uuid.UUID {
// The specific txn ID used here isn't important. We use something that is a)
// non-zero and b) obvious.
u, err := uuid.FromString("11111111-1111-1111-1111-111111111111")
if err != nil {
panic(err)
}
return u
}()
// A cacheValue combines the timestamp with an optional txn ID.
type cacheValue struct {
timestamp hlc.Timestamp
txnID uuid.UUID // zero for no transaction
}
func makeCacheEntry(key cache.IntervalKey, value cacheValue) *cache.Entry {
alloc := struct {
key cache.IntervalKey
value cacheValue
entry cache.Entry
}{
key: key,
value: value,
}
alloc.entry.Key = &alloc.key
alloc.entry.Value = &alloc.value
return &alloc.entry
}
// newTimestampCache returns a new timestamp cache with supplied
// hybrid clock.
func newTimestampCache(clock *hlc.Clock) *timestampCache {
tc := ×tampCache{
rCache: cache.NewIntervalCache(cache.Config{Policy: cache.CacheFIFO}),
wCache: cache.NewIntervalCache(cache.Config{Policy: cache.CacheFIFO}),
maxBytes: uint64(defaultTimestampCacheSize),
}
tc.Clear(clock.Now())
tc.rCache.Config.ShouldEvict = tc.shouldEvict
tc.wCache.Config.ShouldEvict = tc.shouldEvict
onEvicted := func(k, v interface{}) {
ck := k.(*cache.IntervalKey)
reqSize := cacheEntrySize(ck.Start, ck.End)
if tc.bytes < reqSize {
panic(fmt.Sprintf("bad reqSize: %d < %d", tc.bytes, reqSize))
}
tc.bytes -= reqSize
}
tc.rCache.Config.OnEvicted = onEvicted
tc.wCache.Config.OnEvicted = onEvicted
return tc
}
// Clear clears the cache and resets the low-water mark.
func (tc *timestampCache) Clear(lowWater hlc.Timestamp) {
tc.requests = btree.New(btreeDegree)
tc.rCache.Clear()
tc.wCache.Clear()
tc.lowWater = lowWater
tc.latest = tc.lowWater
}
// len returns the total number of read and write intervals in the
// TimestampCache.
func (tc *timestampCache) len() int {
return tc.rCache.Len() + tc.wCache.Len() + tc.reqSpans
}
// add the specified timestamp to the cache as covering the range of
// keys from start to end. If end is nil, the range covers the start
// key only. txnID is nil for no transaction. readTSCache specifies
// whether the command adding this timestamp should update the read
// timestamp; false to update the write timestamp cache.
func (tc *timestampCache) add(
start, end roachpb.Key, timestamp hlc.Timestamp, txnID uuid.UUID, readTSCache bool,
) {
// This gives us a memory-efficient end key if end is empty.
if len(end) == 0 {
end = start.Next()
start = end[:len(start)]
}
tc.latest.Forward(timestamp)
// Only add to the cache if the timestamp is more recent than the
// low water mark.
if tc.lowWater.Less(timestamp) {
tcache := tc.wCache
if readTSCache {
tcache = tc.rCache
}
addRange := func(r interval.Range) {
value := cacheValue{timestamp: timestamp, txnID: txnID}
key := tcache.MakeKey(r.Start, r.End)
entry := makeCacheEntry(key, value)
tc.bytes += cacheEntrySize(r.Start, r.End)
tcache.AddEntry(entry)
}
addEntryAfter := func(entry, after *cache.Entry) {
ck := entry.Key.(*cache.IntervalKey)
tc.bytes += cacheEntrySize(ck.Start, ck.End)
tcache.AddEntryAfter(entry, after)
}
r := interval.Range{
Start: interval.Comparable(start),
End: interval.Comparable(end),
}
// Check existing, overlapping entries and truncate/split/remove if
// superseded and in the past. If existing entries are in the future,
// subtract from the range/ranges that need to be added to cache.
for _, entry := range tcache.GetOverlaps(r.Start, r.End) {
cv := entry.Value.(*cacheValue)
key := entry.Key.(*cache.IntervalKey)
sCmp := r.Start.Compare(key.Start)
eCmp := r.End.Compare(key.End)
// Some of the cases below adjust cv and key in-place (in a manner that
// maintains the IntervalCache invariants). These in-place modifications
// change the size of the entry. To capture all of these modifications we
// compute the current size of the entry and then use the new size at the
// end of this iteration to update TimestampCache.bytes.
oldSize := cacheEntrySize(key.Start, key.End)
if cv.timestamp.Less(timestamp) {
// The existing interval has a timestamp less than the new
// interval. Compare interval ranges to determine how to
// modify existing interval.
switch {
case sCmp == 0 && eCmp == 0:
// New and old are equal; replace old with new and avoid the need to insert new.
//
// New: ------------
// Old: ------------
//
// New: ------------
// Old:
*cv = cacheValue{timestamp: timestamp, txnID: txnID}
tcache.MoveToEnd(entry)
return
case sCmp <= 0 && eCmp >= 0:
// New contains or is equal to old; delete old.
//
// New: ------------ ------------ ------------
// Old: -------- or ---------- or ----------
//
// New: ------------ ------------ ------------
// Old:
tcache.DelEntry(entry)
continue // DelEntry adjusted tc.bytes, don't do it again
case sCmp > 0 && eCmp < 0:
// Old contains new; split up old into two.
//
// New: ----
// Old: ------------
//
// New: ----
// Old: ---- ----
oldEnd := key.End
key.End = r.Start
newKey := tcache.MakeKey(r.End, oldEnd)
newEntry := makeCacheEntry(newKey, *cv)
addEntryAfter(newEntry, entry)
case eCmp >= 0:
// Left partial overlap; truncate old end.
//
// New: -------- --------
// Old: -------- or ------------
//
// New: -------- --------
// Old: ---- ----
key.End = r.Start
case sCmp <= 0:
// Right partial overlap; truncate old start.
//
// New: -------- --------
// Old: -------- or ------------
//
// New: -------- --------
// Old: ---- ----
key.Start = r.End
default:
panic(fmt.Sprintf("no overlap between %v and %v", key.Range, r))
}
} else if timestamp.Less(cv.timestamp) {
// The existing interval has a timestamp greater than the new interval.
// Compare interval ranges to determine how to modify new interval before
// adding it to the timestamp cache.
switch {
case sCmp >= 0 && eCmp <= 0:
// Old contains or is equal to new; no need to add.
//
// Old: ----------- ----------- ----------- -----------
// New: ----- or ----------- or -------- or --------
//
// Old: ----------- ----------- ----------- -----------
// New:
return
case sCmp < 0 && eCmp > 0:
// New contains old; split up old into two. We can add the left piece
// immediately because it is guaranteed to be before the rest of the
// overlaps.
//
// Old: ------
// New: ------------
//
// Old: ------
// New: --- ---
lr := interval.Range{Start: r.Start, End: key.Start}
addRange(lr)
r.Start = key.End
case eCmp > 0:
// Left partial overlap; truncate new start.
//
// Old: -------- --------
// New: -------- or ------------
//
// Old: -------- --------
// New: ---- ----
r.Start = key.End
case sCmp < 0:
// Right partial overlap; truncate new end.
//
// Old: -------- --------
// New: -------- or ------------
//
// Old: -------- --------
// New: ---- ----
r.End = key.Start
default:
panic(fmt.Sprintf("no overlap between %v and %v", key.Range, r))
}
} else if cv.txnID == txnID {
// The existing interval has a timestamp equal to the new
// interval, and the same transaction ID.
switch {
case sCmp >= 0 && eCmp <= 0:
// Old contains or is equal to new; no need to add.
//
// New: ----- or ----------- or -------- or --------
// Old: ----------- ----------- ----------- -----------
//
// New:
// Old: ----------- ----------- ----------- -----------
return
case sCmp <= 0 && eCmp >= 0:
// New contains old; delete old.
//
// New: ------------ ------------ ------------
// Old: -------- or ---------- or ----------
//
// New: ------------ ------------ ------------
// Old:
tcache.DelEntry(entry)
continue // DelEntry adjusted tc.bytes, don't do it again
case eCmp >= 0:
// Left partial overlap; truncate old end.
//
// New: -------- --------
// Old: -------- or ------------
//
// New: -------- --------
// Old: ---- ----
key.End = r.Start
case sCmp <= 0:
// Right partial overlap; truncate old start.
//
// New: -------- --------
// Old: -------- or ------------
//
// New: -------- --------
// Old: ---- ----
key.Start = r.End
default:
panic(fmt.Sprintf("no overlap between %v and %v", key.Range, r))
}
} else {
// The existing interval has a timestamp equal to the new
// interval and a different transaction ID.
switch {
case sCmp == 0 && eCmp == 0:
// New and old are equal. Segment is no longer owned by any
// transaction.
//
// New: ------------
// Old: ------------
//
// New:
// Nil: ============
// Old:
cv.txnID = uuid.UUID{}
tc.bytes += cacheEntrySize(key.Start, key.End) - oldSize
return
case sCmp == 0 && eCmp > 0:
// New contains old, left-aligned. Clear ownership of the
// existing segment and truncate new.
//
// New: ------------
// Old: ----------
//
// New: --
// Nil: ==========
// Old:
cv.txnID = uuid.UUID{}
r.Start = key.End
case sCmp < 0 && eCmp == 0:
// New contains old, right-aligned. Clear ownership of the
// existing segment and truncate new.
//
// New: ------------
// Old: ----------
//
// New: --
// Nil: ==========
// Old:
cv.txnID = uuid.UUID{}
r.End = key.Start
case sCmp < 0 && eCmp > 0:
// New contains old; split into three segments with the
// overlap owned by no txn.
//
// New: ------------
// Old: --------
//
// New: -- --
// Nil: ========
// Old:
cv.txnID = uuid.UUID{}
newKey := tcache.MakeKey(r.Start, key.Start)
newEntry := makeCacheEntry(newKey, cacheValue{timestamp: timestamp, txnID: txnID})
addEntryAfter(newEntry, entry)
r.Start = key.End
case sCmp > 0 && eCmp < 0:
// Old contains new; split up old into two. New segment is
// owned by no txn.
//
// New: ----
// Old: ------------
//
// New:
// Nil: ====
// Old: ---- ----
txnID = uuid.UUID{}
oldEnd := key.End
key.End = r.Start
newKey := tcache.MakeKey(r.End, oldEnd)
newEntry := makeCacheEntry(newKey, *cv)
addEntryAfter(newEntry, entry)
case eCmp == 0:
// Old contains new, right-aligned; truncate old end and clear
// ownership of new segment.
//
// New: --------
// Old: ------------
//
// New:
// Nil: ========
// Old: ----
txnID = uuid.UUID{}
key.End = r.Start
case sCmp == 0:
// Old contains new, left-aligned; truncate old start and
// clear ownership of new segment.
// New: --------
// Old: ------------
//
// New:
// Nil: ========
// Old: ----
txnID = uuid.UUID{}
key.Start = r.End
case eCmp > 0:
// Left partial overlap; truncate old end and split new into
// segments owned by no txn (the overlap) and the new txn.
//
// New: --------
// Old: --------
//
// New: ----
// Nil: ====
// Old: ----
key.End, r.Start = r.Start, key.End
newKey := tcache.MakeKey(key.End, r.Start)
newCV := cacheValue{timestamp: cv.timestamp}
newEntry := makeCacheEntry(newKey, newCV)
addEntryAfter(newEntry, entry)
case sCmp < 0:
// Right partial overlap; truncate old start and split new into
// segments owned by no txn (the overlap) and the new txn.
//
// New: --------
// Old: --------
//
// New: ----
// Nil: ====
// Old: ----
key.Start, r.End = r.End, key.Start
newKey := tcache.MakeKey(r.End, key.Start)
newCV := cacheValue{timestamp: cv.timestamp}
newEntry := makeCacheEntry(newKey, newCV)
addEntryAfter(newEntry, entry)
default:
panic(fmt.Sprintf("no overlap between %v and %v", key.Range, r))
}
}
tc.bytes += cacheEntrySize(key.Start, key.End) - oldSize
}
addRange(r)
}
}
// AddRequest adds the specified request to the cache in an unexpanded state.
func (tc *timestampCache) AddRequest(req cacheRequest) {
if len(req.reads) == 0 && len(req.writes) == 0 && req.txn.Key == nil {
// The request didn't contain any spans for the timestamp cache.
return
}
if !tc.lowWater.Less(req.timestamp) {
// Request too old to be added.
return
}
tc.reqIDAlloc++
req.uniqueID = tc.reqIDAlloc
tc.requests.ReplaceOrInsert(&req)
tc.reqSpans += req.numSpans()
tc.bytes += req.size()
// Bump the latest timestamp and evict any requests that are now too old.
tc.latest.Forward(req.timestamp)
edge := tc.latest
edge.WallTime -= MinTSCacheWindow.Nanoseconds()
// Evict requests as long as the number of cached spans (both in the requests
// queue and the interval caches) is larger than the eviction threshold.
for tc.bytes > tc.maxBytes {
// TODO(peter): It might be more efficient to gather up the requests to
// delete using BTree.AscendLessThan rather than calling Min
// repeatedly. Maybe.
minItem := tc.requests.Min()
if minItem == nil {
break
}
minReq := minItem.(*cacheRequest)
if edge.Less(minReq.timestamp) {
break
}
tc.lowWater = minReq.timestamp
tc.requests.DeleteMin()
if tc.reqSpans < minReq.numSpans() {
panic(fmt.Sprintf("bad reqSpans: %d < %d", tc.reqSpans, minReq.numSpans()))
}
tc.reqSpans -= minReq.numSpans()
minReqSize := minReq.size()
if tc.bytes < minReqSize {
panic(fmt.Sprintf("bad reqSize: %d < %d", tc.bytes, minReqSize))
}
tc.bytes -= minReqSize
}
}
// ExpandRequests expands any request that is newer than the specified
// timestamp and which overlaps the specified span.
func (tc *timestampCache) ExpandRequests(timestamp hlc.Timestamp, span roachpb.RSpan) {
// Find all of the requests that have a timestamp greater than or equal to
// the specified timestamp. Note that we can't delete the requests during the
// btree iteration.
var reqs []*cacheRequest
tc.tmpReq.timestamp = timestamp
tc.requests.AscendGreaterOrEqual(&tc.tmpReq, func(i btree.Item) bool {
cr := i.(*cacheRequest)
if cr.span.Overlaps(span) {
reqs = append(reqs, cr)
}
return true
})
// Expand the requests, inserting the spans into either the read or write
// interval caches.
for _, req := range reqs {
tc.requests.Delete(req)
if tc.reqSpans < req.numSpans() {
panic(fmt.Sprintf("bad reqSpans: %d < %d", tc.reqSpans, req.numSpans()))
}
tc.reqSpans -= req.numSpans()
reqSize := req.size()
if tc.bytes < reqSize {
panic(fmt.Sprintf("bad reqSize: %d < %d", tc.bytes, reqSize))
}
tc.bytes -= reqSize
for i := range req.reads {
sp := &req.reads[i]
tc.add(sp.Key, sp.EndKey, req.timestamp, req.txnID, true /* readTSCache */)
}
for i := range req.writes {
sp := &req.writes[i]
tc.add(sp.Key, sp.EndKey, req.timestamp, req.txnID, false /* !readTSCache */)
}
if req.txn.Key != nil {
// Make the transaction key from the request key. We're guaranteed
// req.txnID != nil because we only hit this code path for
// EndTransactionRequests.
key := keys.TransactionKey(req.txn.Key, req.txnID)
// We set txnID=nil because we want hits for same txn ID.
tc.add(key, nil, req.timestamp, uuid.UUID{}, false /* !readTSCache */)
}
}
}
// GetMaxRead returns the maximum read timestamp which overlaps the
// interval spanning from start to end. If that timestamp belongs to a
// single transaction, that transaction's ID is returned. If no part
// of the specified range is overlapped by timestamps from different
// transactions in the cache, the low water timestamp is returned for
// the read timestamps. Also returns an "ok" bool, indicating whether
// an explicit match of the interval was found in the cache (as
// opposed to using the low-water mark).
func (tc *timestampCache) GetMaxRead(start, end roachpb.Key) (hlc.Timestamp, uuid.UUID, bool) {
return tc.getMax(start, end, true)
}
// GetMaxWrite returns the maximum write timestamp which overlaps the
// interval spanning from start to end. If that timestamp belongs to a
// single transaction, that transaction's ID is returned. If no part
// of the specified range is overlapped by timestamps from different
// transactions in the cache, the low water timestamp is returned for
// the write timestamps. Also returns an "ok" bool, indicating whether
// an explicit match of the interval was found in the cache (as
// opposed to using the low-water mark).
func (tc *timestampCache) GetMaxWrite(start, end roachpb.Key) (hlc.Timestamp, uuid.UUID, bool) {
return tc.getMax(start, end, false)
}
func (tc *timestampCache) getMax(
start, end roachpb.Key, readTSCache bool,
) (hlc.Timestamp, uuid.UUID, bool) {
if len(end) == 0 {
end = start.Next()
}
var ok bool
maxTS := tc.lowWater
var maxTxnID uuid.UUID
cache := tc.wCache
if readTSCache {
cache = tc.rCache
}
for _, o := range cache.GetOverlaps(start, end) {
ce := o.Value.(*cacheValue)
if maxTS.Less(ce.timestamp) {
ok = true
maxTS = ce.timestamp
maxTxnID = ce.txnID
} else if maxTS == ce.timestamp && maxTxnID != ce.txnID {
maxTxnID = uuid.UUID{}
}
}
if maxTxnID == lowWaterTxnIDMarker {
ok = false
}
return maxTS, maxTxnID, ok
}
// shouldEvict returns true if the cache entry's timestamp is no
// longer within the MinTSCacheWindow.
func (tc *timestampCache) shouldEvict(size int, key, value interface{}) bool {
if tc.bytes <= tc.maxBytes {
return false
}
ce := value.(*cacheValue)
// In case low water mark was set higher, evict any entries
// which occurred before it.
if ce.timestamp.Less(tc.lowWater) {
return true
}
// Compute the edge of the cache window.
edge := tc.latest
edge.WallTime -= MinTSCacheWindow.Nanoseconds()
// We evict and update the low water mark if the proposed evictee's
// timestamp is <= than the edge of the window.
if !edge.Less(ce.timestamp) {
tc.lowWater = ce.timestamp
return true
}
return false
}