[FLINK-22127][network] Enrich error message of read buffer request ti…

…meout to tell the user how to solve the problem when using sort-merge blocking shuffle

This closes apache#15499.

flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/SortMergeResultPartitionReadScheduler.java

@@ -20,6 +20,7 @@
 
 import org.apache.flink.annotation.VisibleForTesting;
 import org.apache.flink.api.common.time.Deadline;
+import org.apache.flink.configuration.TaskManagerOptions;
 import org.apache.flink.core.memory.MemorySegment;
 import org.apache.flink.runtime.io.disk.BatchShuffleReadBufferPool;
 import org.apache.flink.runtime.io.network.buffer.BufferRecycler;
@@ -163,7 +164,11 @@ private Queue<MemorySegment> allocateBuffers(
  }
 
  if (numRequestedBuffers <= 0) {
- throw new TimeoutException("Buffer request timeout.");
+ throw new TimeoutException(
+ String.format(
+ "Buffer request timeout, this means there is a fierce contention of"
+ + " the batch shuffle read memory, please increase '%s'.",
+ TaskManagerOptions.NETWORK_BATCH_SHUFFLE_READ_MEMORY.key()));
  }
  } catch (Throwable throwable) {
  // fail all pending subpartition readers immediately if any exception occurs