GH-2998 back off when many concurrent writes

core/sail/base/src/main/java/org/eclipse/rdf4j/sail/base/SailSourceBranch.java

@@ -40,6 +40,8 @@
  */
 class SailSourceBranch implements SailSource {
 
+	private static final long BACKOFF_START = Runtime.getRuntime().maxMemory() / 1024 / 1024 / 1024 * 10;
+
 	private static final Logger logger = LoggerFactory.getLogger(SailSourceBranch.class);
 
 	/**
@@ -404,6 +406,31 @@ private boolean isChanged(Changeset change) {
 	private SailDataset derivedFromSerializable(IsolationLevel level) throws SailException {
 		try {
 			semaphore.lock();
+
+			if (changes.size() > BACKOFF_START) {
+				int size = changes.size();
+				Changeset peekLast = changes.peekLast();
+				compressChanges();
+				int backoff = Math.min(5, changes.size());
+				while (peekLast == changes.peekLast() && backoff-- > 0) {
+					peekLast = changes.peekLast();
+					try {
+						semaphore.unlock();
+						if (semaphore.isHeldByCurrentThread()) {
+							System.out.println();
+						}
+						System.out.println("Backing off: " + changes.size());
+						Thread.sleep(10);
+						compressChanges();
+					} catch (InterruptedException e) {
+						Thread.currentThread().interrupt();
+						throw new SailException(e);
+					} finally {
+						semaphore.lock();
+					}
+				}
+			}
+
 			if (serializable == null && level.isCompatibleWith(IsolationLevels.SERIALIZABLE)) {
 				serializable = backingSource.sink(level);
 			}