Method evictBasedOffMemoryUsage

db/revision_cache_lru.go:896–950

(ctx context.Context)

}

func (rc *LRURevisionCache) evictBasedOffMemoryUsage(ctx context.Context) int64 {
	var numItemsRemoved, numBytesRemoved int64
	rc.lock.Lock()
	defer rc.lock.Unlock()
	// check if we are over memory capacity after holding rev cache mutex (protect against another goroutine evicting whilst waiting for mutex above)
	if currMemoryUsage := rc.currMemoryUsage.Value(); currMemoryUsage > rc.memoryCapacity {
		// find amount of bytes needed to evict till below threshold
		bytesNeededToRemove := currMemoryUsage - rc.memoryCapacity
		for bytesNeededToRemove > numBytesRemoved {
			value := rc._findEvictionValue()
			if value == nil {
				if rc.lruList.Len() > 0 {
					// no more values ready for eviction
					break
				} else {
					// list is empty, nothing more to evict but stats are wrong so zero stats and return
					base.DebugfCtx(ctx, base.KeyCache, "Revision cache memory stats inconsistent for this shard, resetting to zero")
					correctionVal := rc.currMemoryUsage.Value()
					// Correct overall memory stat across shards to remove this shards memory usage, and set this shard to zero
					rc.cacheMemoryBytesStat.Add(-correctionVal)
					rc.currMemoryUsage.Set(0)
					return numItemsRemoved
				}
			}
			revKey := IDAndRev{DocID: value.id, RevID: value.revID, CollectionID: value.collectionID}
			hlvKey := IDandCV{DocID: value.id, Source: value.cv.SourceID, Version: value.cv.Value, CollectionID: value.collectionID}
			// same below but for hlv lookup map
			if elem := rc.hlvCache[hlvKey]; elem != nil {
				revValue := elem.Value.(*revCacheValue)
				// we need to check if the value pointed to by the cv lookup map is the same value we're evicting, this is
				// because we can currently have two items with the same docID and CV, but different revIDs due to
				// local wins conflict resolution not generating a new CV but generating a new revID.
				if revValue.revID == value.revID {
					// this cv lookup item matches the value we're evicting, so remove it
					delete(rc.hlvCache, hlvKey)
				}
			}
			if elem := rc.cache[revKey]; elem != nil {
				revValue := elem.Value.(*revCacheValue)
				// we need to check if the value pointed to by the rev lookup map is the same value we're evicting, this is
				// because we can have can currently have two items with the same docID and revID, but different CVs due to
				// a new HLV being generated for user xattr updates where we don't generate a new revID.
				if revValue.cv.String() == value.cv.String() {
					// this rev lookup item matches the value we're evicting, so remove it
					delete(rc.cache, revKey)
				}
			}
			numItemsRemoved++
			valueBytes := value.getItemBytes()
			numBytesRemoved += valueBytes
		}
	}
	rc._decrRevCacheMemoryUsage(ctx, -numBytesRemoved) // need update rev cache memory stats before release lock to stop other goroutines evicting based on outdated stats
	return numItemsRemoved
}

// _decrRevCacheMemoryUsage atomically decreases overall memory usage for cache and the actual rev cache objects usage.
// You should be holding rev cache lock in using this function to avoid eviction processes over evicting items