[FLINK-4094] [core] Add a warning to only use off heap memory with pr…

…e-allocation=true

This closes apache#2336

docs/setup/config.md

@@ -83,7 +83,7 @@ The default fraction for managed memory can be adjusted using the `taskmanager.m
 
 - `taskmanager.memory.segment-size`: The size of memory buffers used by the memory manager and the network stack in bytes (DEFAULT: 32768 (= 32 KiBytes)).
 
-- `taskmanager.memory.preallocate`: Can be either of `true` or `false`. Specifies whether task managers should allocate all managed memory when starting up. (DEFAULT: false)
+- `taskmanager.memory.preallocate`: Can be either of `true` or `false`. Specifies whether task managers should allocate all managed memory when starting up. (DEFAULT: false). When `taskmanager.memory.off-heap` is set to `true`, then it is advised that this configuration is also set to `true`.  If this configuration is set to `false` cleaning up of the allocated offheap memory happens only when the configured JVM parameter MaxDirectMemorySize is reached by triggering a full GC.
 
 ### Memory and Performance Debugging
 

flink-runtime/src/main/java/org/apache/flink/runtime/memory/MemoryManager.java

@@ -34,6 +34,8 @@
 import org.apache.flink.core.memory.MemorySegment;
 import org.apache.flink.core.memory.MemoryType;
 import org.apache.flink.util.MathUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * The memory manager governs the memory that Flink uses for sorting, hashing, and caching. Memory
@@ -53,6 +55,7 @@
  */
 public class MemoryManager {
 
+	private static final Logger LOG = LoggerFactory.getLogger(MemoryManager.class);
 	/** The default memory page size. Currently set to 32 KiBytes. */
 	public static final int DEFAULT_PAGE_SIZE = 32 * 1024;
 
@@ -163,6 +166,10 @@ public MemoryManager(long memorySize, int numberOfSlots, int pageSize,
 				this.memoryPool = new HeapMemoryPool(memToAllocate, pageSize);
 				break;
 			case OFF_HEAP:
+				if(!preAllocateMemory) {
+					LOG.warn("It is advisable to set 'taskmanager.memory.preallocate' to true when" +
+						" the memory type 'taskmanager.memory.off-heap' is set to true.");
+				}
 				this.memoryPool = new HybridOffHeapMemoryPool(memToAllocate, pageSize);
 				break;
 			default: