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/*
* Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
* or more contributor license agreements. Licensed under the Elastic License
* 2.0 and the Server Side Public License, v 1; you may not use this file except
* in compliance with, at your election, the Elastic License 2.0 or the Server
* Side Public License, v 1.
*/
package org.elasticsearch.index.engine;
import org.apache.lucene.search.ReferenceManager;
import org.apache.lucene.util.Accountable;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.RamUsageEstimator;
import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
import org.elasticsearch.common.util.concurrent.KeyedLock;
import org.elasticsearch.core.Releasable;
import java.io.IOException;
import java.util.Collections;
import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;
/** Maps _uid value to its version information. */
public final class LiveVersionMap implements ReferenceManager.RefreshListener, Accountable {
private final KeyedLock keyedLock = new KeyedLock<>();
private final LiveVersionMapArchive archive;
LiveVersionMap() {
this(LiveVersionMapArchive.NOOP_ARCHIVE);
}
LiveVersionMap(LiveVersionMapArchive archive) {
this.archive = archive;
}
public static final class VersionLookup {
/** Tracks bytes used by current map, i.e. what is freed on refresh. For deletes, which are also added to tombstones,
* we only account for the CHM entry here, and account for BytesRef/VersionValue against the tombstones, since refresh would not
* clear this from RAM. */
final AtomicLong ramBytesUsed = new AtomicLong();
private static final VersionLookup EMPTY = new VersionLookup(Collections.emptyMap());
private final Map map;
// each version map has a notion of safe / unsafe which allows us to apply certain optimization in the auto-generated ID usecase
// where we know that documents can't have any duplicates so we can skip the version map entirely. This reduces
// the memory pressure significantly for this use-case where we often get a massive amount of small document (metrics).
// if the version map is in safeAccess mode we track all version in the version map. yet if a document comes in that needs
// safe access but we are not in this mode we force a refresh and make the map as safe access required. All subsequent ops will
// respect that and fill the version map. The nice part here is that we are only really requiring this for a single ID and since
// we hold the ID lock in the engine while we do all this it's safe to do it globally unlocked.
// NOTE: these values can both be non-volatile since it's ok to read a stale value per doc ID. We serialize changes in the engine
// that will prevent concurrent updates to the same document ID and therefore we can rely on the happens-before guanratee of the
// map reference itself.
private boolean unsafe;
// minimum timestamp of delete operations that were made while this map was active. this is used to make sure they are kept in
// the tombstone
private final AtomicLong minDeleteTimestamp = new AtomicLong(Long.MAX_VALUE);
// Modifies the map of this instance by merging with the given VersionLookup
public void merge(VersionLookup versionLookup) {
long existingEntriesSize = 0;
for (var entry : versionLookup.map.entrySet()) {
var existingValue = map.get(entry.getKey());
existingEntriesSize += existingValue == null ? 0 : mapEntryBytesUsed(entry.getKey(), existingValue);
}
map.putAll(versionLookup.map);
adjustRamUsage(versionLookup.ramBytesUsed() - existingEntriesSize);
minDeleteTimestamp.accumulateAndGet(versionLookup.minDeleteTimestamp(), Math::min);
}
// Visible for testing
VersionLookup(Map map) {
this.map = map;
}
public VersionValue get(BytesRef key) {
return map.get(key);
}
VersionValue put(BytesRef key, VersionValue value) {
long ramAccounting = mapEntryBytesUsed(key, value);
VersionValue previousValue = map.put(key, value);
ramAccounting += previousValue == null ? 0 : -mapEntryBytesUsed(key, previousValue);
adjustRamUsage(ramAccounting);
return previousValue;
}
public boolean isEmpty() {
return map.isEmpty();
}
int size() {
return map.size();
}
public boolean isUnsafe() {
return unsafe;
}
void markAsUnsafe() {
unsafe = true;
}
VersionValue remove(BytesRef uid) {
VersionValue previousValue = map.remove(uid);
if (previousValue != null) {
adjustRamUsage(-mapEntryBytesUsed(uid, previousValue));
}
return previousValue;
}
public void updateMinDeletedTimestamp(DeleteVersionValue delete) {
minDeleteTimestamp.accumulateAndGet(delete.time, Math::min);
}
public long minDeleteTimestamp() {
return minDeleteTimestamp.get();
}
void adjustRamUsage(long value) {
if (value != 0) {
long v = ramBytesUsed.addAndGet(value);
assert v >= 0 : "bytes=" + v;
}
}
public long ramBytesUsed() {
return ramBytesUsed.get();
}
public static long mapEntryBytesUsed(BytesRef key, VersionValue value) {
return (BASE_BYTES_PER_BYTESREF + key.bytes.length) + (BASE_BYTES_PER_CHM_ENTRY + value.ramBytesUsed());
}
// Used only for testing
Map getMap() {
return map;
}
}
private static final class Maps {
// All writes (adds and deletes) go into here:
final VersionLookup current;
// Used while refresh is running, and to hold adds/deletes until refresh finishes. We read from both current and old on lookup:
final VersionLookup old;
// this is not volatile since we don't need to maintain a happens before relationship across doc IDs so it's enough to
// have the volatile read of the Maps reference to make it visible even across threads.
boolean needsSafeAccess;
final boolean previousMapsNeededSafeAccess;
Maps(VersionLookup current, VersionLookup old, boolean previousMapsNeededSafeAccess) {
this.current = current;
this.old = old;
this.previousMapsNeededSafeAccess = previousMapsNeededSafeAccess;
}
Maps() {
this(new VersionLookup(ConcurrentCollections.newConcurrentMapWithAggressiveConcurrency()), VersionLookup.EMPTY, false);
}
boolean isSafeAccessMode() {
return needsSafeAccess || previousMapsNeededSafeAccess;
}
boolean shouldInheritSafeAccess() {
final boolean mapHasNotSeenAnyOperations = current.isEmpty() && current.isUnsafe() == false;
return needsSafeAccess
// we haven't seen any ops and map before needed it so we maintain it
|| (mapHasNotSeenAnyOperations && previousMapsNeededSafeAccess);
}
/**
* Builds a new map for the refresh transition this should be called in beforeRefresh()
*/
Maps buildTransitionMap() {
return new Maps(
new VersionLookup(ConcurrentCollections.newConcurrentMapWithAggressiveConcurrency(current.size())),
current,
shouldInheritSafeAccess()
);
}
/**
* similar to `invalidateOldMap` but used only for the `unsafeKeysMap` used for assertions
*/
Maps invalidateOldMapForAssert() {
return new Maps(current, VersionLookup.EMPTY, previousMapsNeededSafeAccess);
}
/**
* builds a new map that invalidates the old map but maintains the current. This should be called in afterRefresh()
*/
Maps invalidateOldMap(LiveVersionMapArchive archive) {
archive.afterRefresh(old);
return new Maps(current, VersionLookup.EMPTY, previousMapsNeededSafeAccess);
}
void put(BytesRef uid, VersionValue version) {
current.put(uid, version);
}
void remove(BytesRef uid, DeleteVersionValue deleted) {
current.remove(uid);
current.updateMinDeletedTimestamp(deleted);
if (old != VersionLookup.EMPTY) {
// we also need to remove it from the old map here to make sure we don't read this stale value while
// we are in the middle of a refresh. Most of the time the old map is an empty map so we can skip it there.
old.remove(uid);
}
}
long getMinDeleteTimestamp() {
return Math.min(current.minDeleteTimestamp.get(), old.minDeleteTimestamp.get());
}
long ramBytesUsed() {
return current.ramBytesUsed.get() + old.ramBytesUsed.get();
}
}
// All deletes also go here, and delete "tombstones" are retained after refresh:
private final Map tombstones = ConcurrentCollections.newConcurrentMapWithAggressiveConcurrency();
private volatile Maps maps = new Maps();
// we maintain a second map that only receives the updates that we skip on the actual map (unsafe ops)
// this map is only maintained if assertions are enabled
private volatile Maps unsafeKeysMap = new Maps();
/**
* Bytes consumed for each BytesRef UID:
* In this base value, we account for the {@link BytesRef} object itself as
* well as the header of the byte[] array it holds, and some lost bytes due
* to object alignment. So consumers of this constant just have to add the
* length of the byte[] (assuming it is not shared between multiple
* instances).
*/
private static final long BASE_BYTES_PER_BYTESREF =
// shallow memory usage of the BytesRef object
RamUsageEstimator.shallowSizeOfInstance(BytesRef.class) +
// header of the byte[] array
RamUsageEstimator.NUM_BYTES_ARRAY_HEADER +
// with an alignment size (-XX:ObjectAlignmentInBytes) of 8 (default),
// there could be between 0 and 7 lost bytes, so we account for 3
// lost bytes on average
3;
/**
* Bytes used by having CHM point to a key/value.
*/
private static final long BASE_BYTES_PER_CHM_ENTRY;
static {
// use the same impl as the Maps does
Map map = ConcurrentCollections.newConcurrentMapWithAggressiveConcurrency();
map.put(0, 0);
long chmEntryShallowSize = RamUsageEstimator.shallowSizeOf(map.entrySet().iterator().next());
// assume a load factor of 50%
// for each entry, we need two object refs, one for the entry itself
// and one for the free space that is due to the fact hash tables can
// not be fully loaded
BASE_BYTES_PER_CHM_ENTRY = chmEntryShallowSize + 2 * RamUsageEstimator.NUM_BYTES_OBJECT_REF;
}
/**
* Tracks bytes used by tombstones (deletes)
*/
private final AtomicLong ramBytesUsedForTombstones = new AtomicLong();
@Override
public void beforeRefresh() throws IOException {
// Start sending all updates after this point to the new
// map. While reopen is running, any lookup will first
// try this new map, then fallback to old, then to the
// current searcher:
maps = maps.buildTransitionMap();
assert (unsafeKeysMap = unsafeKeysMap.buildTransitionMap()) != null;
// This is not 100% correct, since concurrent indexing ops can change these counters in between our execution of the previous
// line and this one, but that should be minor, and the error won't accumulate over time:
}
@Override
public void afterRefresh(boolean didRefresh) throws IOException {
// We can now drop old because these operations are now visible via the newly opened searcher. Even if didRefresh is false, which
// means Lucene did not actually open a new reader because it detected no changes, it's possible old has some entries in it, which
// is fine: it means they were actually already included in the previously opened reader, so we can still safely drop them in that
// case. This is because we assign new maps (in beforeRefresh) slightly before Lucene actually flushes any segments for the
// reopen, and so any concurrent indexing requests can still sneak in a few additions to that current map that are in fact
// reflected in the previous reader. We don't touch tombstones here: they expire on their own index.gc_deletes timeframe:
maps = maps.invalidateOldMap(archive);
assert (unsafeKeysMap = unsafeKeysMap.invalidateOldMapForAssert()) != null;
}
/**
* Returns the live version (add or delete) for this uid.
*/
VersionValue getUnderLock(final BytesRef uid) {
return getUnderLock(uid, maps);
}
private VersionValue getUnderLock(final BytesRef uid, Maps currentMaps) {
assert assertKeyedLockHeldByCurrentThread(uid);
// First try to get the "live" value:
VersionValue value = currentMaps.current.get(uid);
if (value != null) {
return value;
}
value = currentMaps.old.get(uid);
if (value != null) {
return value;
}
// We first check the tombstone then the archive since the archive accumulates ids from the old map, and we
// makes sure in `putDeleteUnderLock` the old map does not hold an entry that is in tombstone, archive also wouldn't have them.
value = tombstones.get(uid);
if (value != null) {
return value;
}
return archive.get(uid);
}
VersionValue getVersionForAssert(final BytesRef uid) {
VersionValue value = getUnderLock(uid, maps);
if (value == null) {
value = getUnderLock(uid, unsafeKeysMap);
}
return value;
}
boolean isUnsafe() {
return maps.current.isUnsafe() || maps.old.isUnsafe() || archive.isUnsafe();
}
void enforceSafeAccess() {
maps.needsSafeAccess = true;
}
boolean isSafeAccessRequired() {
return maps.isSafeAccessMode();
}
/**
* Adds this uid/version to the pending adds map iff the map needs safe access.
*/
void maybePutIndexUnderLock(BytesRef uid, IndexVersionValue version) {
assert assertKeyedLockHeldByCurrentThread(uid);
Maps maps = this.maps;
if (maps.isSafeAccessMode()) {
putIndexUnderLock(uid, version);
} else {
// Even though we don't store a record of the indexing operation (and mark as unsafe),
// we should still remove any previous delete for this uuid (avoid accidental accesses).
// Not this should not hurt performance because the tombstone is small (or empty) when unsafe is relevant.
removeTombstoneUnderLock(uid);
maps.current.markAsUnsafe();
assert putAssertionMap(uid, version);
}
}
void putIndexUnderLock(BytesRef uid, IndexVersionValue version) {
assert assertKeyedLockHeldByCurrentThread(uid);
assert uid.bytes.length == uid.length : "Oversized _uid! UID length: " + uid.length + ", bytes length: " + uid.bytes.length;
maps.put(uid, version);
removeTombstoneUnderLock(uid);
}
private boolean putAssertionMap(BytesRef uid, IndexVersionValue version) {
assert assertKeyedLockHeldByCurrentThread(uid);
assert uid.bytes.length == uid.length : "Oversized _uid! UID length: " + uid.length + ", bytes length: " + uid.bytes.length;
unsafeKeysMap.put(uid, version);
return true;
}
void putDeleteUnderLock(BytesRef uid, DeleteVersionValue version) {
assert assertKeyedLockHeldByCurrentThread(uid);
assert uid.bytes.length == uid.length : "Oversized _uid! UID length: " + uid.length + ", bytes length: " + uid.bytes.length;
putTombstone(uid, version);
maps.remove(uid, version);
}
private void putTombstone(BytesRef uid, DeleteVersionValue version) {
long uidRamBytesUsed = BASE_BYTES_PER_BYTESREF + uid.bytes.length;
// Also enroll the delete into tombstones, and account for its RAM usage too:
final VersionValue prevTombstone = tombstones.put(uid, version);
long ramBytes = (BASE_BYTES_PER_CHM_ENTRY + version.ramBytesUsed() + uidRamBytesUsed);
// Deduct tombstones bytes used for the version we just removed or replaced:
if (prevTombstone != null) {
ramBytes -= (BASE_BYTES_PER_CHM_ENTRY + prevTombstone.ramBytesUsed() + uidRamBytesUsed);
}
if (ramBytes != 0) {
long v = ramBytesUsedForTombstones.addAndGet(ramBytes);
assert v >= 0 : "bytes=" + v;
}
}
/**
* Removes this uid from the pending deletes map.
*/
void removeTombstoneUnderLock(BytesRef uid) {
assert assertKeyedLockHeldByCurrentThread(uid);
long uidRamBytesUsed = BASE_BYTES_PER_BYTESREF + uid.bytes.length;
final VersionValue prev = tombstones.remove(uid);
if (prev != null) {
assert prev.isDelete();
long v = ramBytesUsedForTombstones.addAndGet(-(BASE_BYTES_PER_CHM_ENTRY + prev.ramBytesUsed() + uidRamBytesUsed));
assert v >= 0 : "bytes=" + v;
}
}
private boolean canRemoveTombstone(long maxTimestampToPrune, long maxSeqNoToPrune, DeleteVersionValue versionValue) {
// check if the value is old enough and safe to be removed
final boolean isTooOld = versionValue.time < maxTimestampToPrune;
final boolean isSafeToPrune = versionValue.seqNo <= maxSeqNoToPrune;
// version value can't be removed it's
// not yet flushed to lucene ie. it's part of this current maps object
final boolean isNotTrackedByCurrentMaps = versionValue.time < maps.getMinDeleteTimestamp();
final boolean isNotTrackedByArchive = versionValue.time < archive.getMinDeleteTimestamp();
return isTooOld && isSafeToPrune && isNotTrackedByCurrentMaps & isNotTrackedByArchive;
}
/**
* Try to prune tombstones whose timestamp is less than maxTimestampToPrune and seqno at most the maxSeqNoToPrune.
*/
void pruneTombstones(long maxTimestampToPrune, long maxSeqNoToPrune) {
for (Map.Entry entry : tombstones.entrySet()) {
// we do check before we actually lock the key - this way we don't need to acquire the lock for tombstones that are not
// prune-able. If the tombstone changes concurrently we will re-read and step out below since if we can't collect it now w
// we won't collect the tombstone below since it must be newer than this one.
if (canRemoveTombstone(maxTimestampToPrune, maxSeqNoToPrune, entry.getValue())) {
final BytesRef uid = entry.getKey();
try (Releasable lock = keyedLock.tryAcquire(uid)) {
// we use tryAcquire here since this is a best effort and we try to be least disruptive
// this method is also called under lock in the engine under certain situations such that this can lead to deadlocks
// if we do use a blocking acquire. see #28714
if (lock != null) { // did we get the lock?
// Must re-get it here, vs using entry.getValue(), in case the uid was indexed/deleted since we pulled the iterator:
final DeleteVersionValue versionValue = tombstones.get(uid);
if (versionValue != null) {
if (canRemoveTombstone(maxTimestampToPrune, maxSeqNoToPrune, versionValue)) {
removeTombstoneUnderLock(uid);
}
}
}
}
}
}
}
/**
* Called when this index is closed.
*/
synchronized void clear() {
maps = new Maps();
tombstones.clear();
// NOTE: we can't zero this here, because a refresh thread could be calling InternalEngine.pruneDeletedTombstones at the same time,
// and this will lead to an assert trip. Presumably it's fine if our ramBytesUsedForTombstones is non-zero after clear since the
// index is being closed:
// ramBytesUsedForTombstones.set(0);
}
@Override
public long ramBytesUsed() {
return maps.ramBytesUsed() + ramBytesUsedForTombstones.get() + ramBytesUsedForArchive();
}
/**
* Returns how much RAM is used by refresh. This is the RAM usage of the current and old version maps, and the RAM usage of the
* archive, if any.
*/
long ramBytesUsedForRefresh() {
return maps.ramBytesUsed() + archive.getRamBytesUsed();
}
/**
* Returns how much RAM could be reclaimed from the version map.
*
* In stateful, this is the RAM usage of the current version map, and could be reclaimed by refreshing. It doesn't include tombstones
* since they don't get cleared on refresh, nor the old version map that is being reclaimed.
*
* In stateless, this is the RAM usage of current and old version map plus the RAM usage of the parts of the archive that require
* a new unpromotable refresh. To reclaim all three components we need to refresh AND flush.
*/
long reclaimableRefreshRamBytes() {
return archive == LiveVersionMapArchive.NOOP_ARCHIVE
? maps.current.ramBytesUsed.get()
: maps.ramBytesUsed() + archive.getReclaimableRamBytes();
}
/**
* Returns how much RAM would be freed up by cleaning out the LiveVersionMapArchive.
*/
long ramBytesUsedForArchive() {
return archive.getRamBytesUsed();
}
/**
* Returns how much RAM is current being freed up by refreshing. In Stateful, this is the RAM usage of the previous version map
* that needs to stay around until operations are safely recorded in the Lucene index. In Stateless, this is the RAM usage of a
* fraction of the Archive entries that are kept around until an ongoing unpromotable refresh is finished.
*/
long getRefreshingBytes() {
return archive == LiveVersionMapArchive.NOOP_ARCHIVE ? maps.old.ramBytesUsed.get() : archive.getRefreshingRamBytes();
}
/**
* Returns the current internal versions as a point in time snapshot
*/
Map getAllCurrent() {
return maps.current.map;
}
/** Iterates over all deleted versions, including new ones (not yet exposed via reader) and old ones
* (exposed via reader but not yet GC'd). */
Map getAllTombstones() {
return tombstones;
}
/**
* Acquires a releaseable lock for the given uId. All *UnderLock methods require
* this lock to be hold by the caller otherwise the visibility guarantees of this version
* map are broken. We assert on this lock to be hold when calling these methods.
* @see KeyedLock
*/
Releasable acquireLock(BytesRef uid) {
return keyedLock.acquire(uid);
}
boolean assertKeyedLockHeldByCurrentThread(BytesRef uid) {
assert keyedLock.isHeldByCurrentThread(uid) : "Thread [" + Thread.currentThread().getName() + "], uid [" + uid.utf8ToString() + "]";
return true;
}
// visible for testing purposes only
LiveVersionMapArchive getArchive() {
return archive;
}
}