public Map<K, V> getLatestAccessedItems(int n) {
// we need to grab the lock since we are changing lastAccessedCopy
markAndSweepLock.lock();
Map<K, V> result = new LinkedHashMap<K, V>();
TreeSet<CacheEntry> tree = new TreeSet<CacheEntry>();
try {
for (Map.Entry<Object, CacheEntry> entry : map.entrySet()) {
CacheEntry ce = entry.getValue();
ce.lastAccessedCopy = ce.lastAccessed;
if (tree.size() < n) {
tree.add(ce);
} else {
if (ce.lastAccessedCopy > tree.last().lastAccessedCopy) {
tree.remove(tree.last());
tree.add(ce);
}
}
}
} finally {
markAndSweepLock.unlock();
}
for (CacheEntry<K, V> e : tree) {
result.put(e.key, e.value);
}
return result;
}
/**
* Removes items from the cache to bring the size down to an acceptable value
* ('acceptableWaterMark').
*
* <p>It is done in two stages. In the first stage, least recently used items are evicted. If,
* after the first stage, the cache size is still greater than 'acceptableSize' config parameter,
* the second stage takes over.
*
* <p>The second stage is more intensive and tries to bring down the cache size to the
* 'lowerWaterMark' config parameter.
*/
private void markAndSweep() {
// if we want to keep at least 1000 entries, then timestamps of
// current through current-1000 are guaranteed not to be the oldest (but that does
// not mean there are 1000 entries in that group... it's acutally anywhere between
// 1 and 1000).
// Also, if we want to remove 500 entries, then
// oldestEntry through oldestEntry+500 are guaranteed to be
// removed (however many there are there).
if (!markAndSweepLock.tryLock()) return;
try {
long oldestEntry = this.oldestEntry;
isCleaning = true;
this.oldestEntry = oldestEntry; // volatile write to make isCleaning visible
long timeCurrent = stats.accessCounter.get();
int sz = stats.size.get();
int numRemoved = 0;
int numKept = 0;
long newestEntry = timeCurrent;
long newNewestEntry = -1;
long newOldestEntry = Integer.MAX_VALUE;
int wantToKeep = lowerWaterMark;
int wantToRemove = sz - lowerWaterMark;
CacheEntry<K, V>[] eset = new CacheEntry[sz];
int eSize = 0;
// System.out.println("newestEntry="+newestEntry + " oldestEntry="+oldestEntry);
// System.out.println("items removed:" + numRemoved + " numKept=" + numKept + " esetSz="+
// eSize + " sz-numRemoved=" + (sz-numRemoved));
for (CacheEntry<K, V> ce : map.values()) {
// set lastAccessedCopy to avoid more volatile reads
ce.lastAccessedCopy = ce.lastAccessed;
long thisEntry = ce.lastAccessedCopy;
// since the wantToKeep group is likely to be bigger than wantToRemove, check it first
if (thisEntry > newestEntry - wantToKeep) {
// this entry is guaranteed not to be in the bottom
// group, so do nothing.
numKept++;
newOldestEntry = Math.min(thisEntry, newOldestEntry);
} else if (thisEntry < oldestEntry + wantToRemove) { // entry in bottom group?
// this entry is guaranteed to be in the bottom group
// so immediately remove it from the map.
evictEntry(ce.key);
numRemoved++;
} else {
// This entry *could* be in the bottom group.
// Collect these entries to avoid another full pass... this is wasted
// effort if enough entries are normally removed in this first pass.
// An alternate impl could make a full second pass.
if (eSize < eset.length - 1) {
eset[eSize++] = ce;
newNewestEntry = Math.max(thisEntry, newNewestEntry);
newOldestEntry = Math.min(thisEntry, newOldestEntry);
}
}
}
// System.out.println("items removed:" + numRemoved + " numKept=" + numKept + " esetSz="+
// eSize + " sz-numRemoved=" + (sz-numRemoved));
// TODO: allow this to be customized in the constructor?
int numPasses = 1; // maximum number of linear passes over the data
// if we didn't remove enough entries, then make more passes
// over the values we collected, with updated min and max values.
while (sz - numRemoved > acceptableWaterMark && --numPasses >= 0) {
oldestEntry = newOldestEntry == Integer.MAX_VALUE ? oldestEntry : newOldestEntry;
newOldestEntry = Integer.MAX_VALUE;
newestEntry = newNewestEntry;
newNewestEntry = -1;
wantToKeep = lowerWaterMark - numKept;
wantToRemove = sz - lowerWaterMark - numRemoved;
// iterate backward to make it easy to remove items.
for (int i = eSize - 1; i >= 0; i--) {
CacheEntry<K, V> ce = eset[i];
long thisEntry = ce.lastAccessedCopy;
if (thisEntry > newestEntry - wantToKeep) {
// this entry is guaranteed not to be in the bottom
// group, so do nothing but remove it from the eset.
numKept++;
// remove the entry by moving the last element to it's position
eset[i] = eset[eSize - 1];
eSize--;
newOldestEntry = Math.min(thisEntry, newOldestEntry);
} else if (thisEntry < oldestEntry + wantToRemove) { // entry in bottom group?
// this entry is guaranteed to be in the bottom group
// so immediately remove it from the map.
evictEntry(ce.key);
numRemoved++;
// remove the entry by moving the last element to it's position
eset[i] = eset[eSize - 1];
eSize--;
} else {
// This entry *could* be in the bottom group, so keep it in the eset,
// and update the stats.
newNewestEntry = Math.max(thisEntry, newNewestEntry);
newOldestEntry = Math.min(thisEntry, newOldestEntry);
}
}
// System.out.println("items removed:" + numRemoved + " numKept=" + numKept + " esetSz="+
// eSize + " sz-numRemoved=" + (sz-numRemoved));
}
// if we still didn't remove enough entries, then make another pass while
// inserting into a priority queue
if (sz - numRemoved > acceptableWaterMark) {
oldestEntry = newOldestEntry == Integer.MAX_VALUE ? oldestEntry : newOldestEntry;
newOldestEntry = Integer.MAX_VALUE;
newestEntry = newNewestEntry;
newNewestEntry = -1;
wantToKeep = lowerWaterMark - numKept;
wantToRemove = sz - lowerWaterMark - numRemoved;
PQueue queue = new PQueue(wantToRemove);
for (int i = eSize - 1; i >= 0; i--) {
CacheEntry<K, V> ce = eset[i];
long thisEntry = ce.lastAccessedCopy;
if (thisEntry > newestEntry - wantToKeep) {
// this entry is guaranteed not to be in the bottom
// group, so do nothing but remove it from the eset.
numKept++;
// removal not necessary on last pass.
// eset[i] = eset[eSize-1];
// eSize--;
newOldestEntry = Math.min(thisEntry, newOldestEntry);
} else if (thisEntry < oldestEntry + wantToRemove) { // entry in bottom group?
// this entry is guaranteed to be in the bottom group
// so immediately remove it.
evictEntry(ce.key);
numRemoved++;
// removal not necessary on last pass.
// eset[i] = eset[eSize-1];
// eSize--;
} else {
// This entry *could* be in the bottom group.
// add it to the priority queue
// everything in the priority queue will be removed, so keep track of
// the lowest value that ever comes back out of the queue.
// first reduce the size of the priority queue to account for
// the number of items we have already removed while executing
// this loop so far.
queue.myMaxSize = sz - lowerWaterMark - numRemoved;
while (queue.size() > queue.myMaxSize && queue.size() > 0) {
CacheEntry otherEntry = (CacheEntry) queue.pop();
newOldestEntry = Math.min(otherEntry.lastAccessedCopy, newOldestEntry);
}
if (queue.myMaxSize <= 0) break;
Object o = queue.myInsertWithOverflow(ce);
if (o != null) {
newOldestEntry = Math.min(((CacheEntry) o).lastAccessedCopy, newOldestEntry);
}
}
}
// Now delete everything in the priority queue.
// avoid using pop() since order doesn't matter anymore
for (Object o : queue.getValues()) {
if (o == null) continue;
CacheEntry<K, V> ce = (CacheEntry) o;
evictEntry(ce.key);
numRemoved++;
}
// System.out.println("items removed:" + numRemoved + " numKept=" + numKept + "
// initialQueueSize="+ wantToRemove + " finalQueueSize=" + queue.size() + " sz-numRemoved="
// + (sz-numRemoved));
}
oldestEntry = newOldestEntry == Integer.MAX_VALUE ? oldestEntry : newOldestEntry;
this.oldestEntry = oldestEntry;
} finally {
isCleaning = false; // set before markAndSweep.unlock() for visibility
markAndSweepLock.unlock();
}
}
