Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/9084#discussion_r41825937
--- Diff:
core/src/main/scala/org/apache/spark/memory/UnifiedMemoryManager.scala ---
@@ -0,0 +1,138 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.memory
+
+import scala.collection.mutable
+
+import org.apache.spark.SparkConf
+import org.apache.spark.storage.{BlockStatus, BlockId}
+
+
+/**
+ * A [[MemoryManager]] that enforces a soft boundary between execution and
storage such that
+ * either side can borrow memory from the other.
+ *
+ * The region shared between execution and storage is a fraction of the
total heap space
+ * configurable through `spark.memory.fraction` (default 0.75). The
position of the boundary
+ * within this space is further determined by
`spark.memory.storageFraction` (default 0.5).
+ * This means the size of the storage region is 0.75 * 0.5 = 0.375 of the
heap space by default.
+ *
+ * Storage can borrow as much execution memory as is free until execution
reclaims its space.
+ * When this happens, cached blocks will be evicted from memory until
sufficient borrowed
+ * memory is released to satisfy the execution memory request.
+ *
+ * Similarly, execution can borrow as much storage memory as is free.
However, execution
+ * memory is *never* evicted by storage due to the complexities involved
in implementing this.
+ * The implication is that attempts to cache blocks may fail if execution
has already eaten
+ * up most of the storage space, in which case the new blocks will be
evicted directly.
--- End diff --
"evicted immediately"? I also wonder if this should clarify that the new
blocks will not be cached if their storage level is MEMORY_ONLY and will be
spilled to disk if their storage level permits that.
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