[SPARK-12081] Make unified memory manager work with small heaps

core/src/main/scala/org/apache/spark/memory/UnifiedMemoryManager.scala

@@ -26,7 +26,7 @@ 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
+ * The region shared between execution and storage is a fraction of (the total heap space - 300MB)
  * 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.
@@ -48,7 +48,7 @@ import org.apache.spark.storage.{BlockStatus, BlockId}
  */
 private[spark] class UnifiedMemoryManager private[memory] (
     conf: SparkConf,
-    maxMemory: Long,
+    val maxMemory: Long,
     private val storageRegionSize: Long,
     numCores: Int)
   extends MemoryManager(
@@ -130,6 +130,12 @@ private[spark] class UnifiedMemoryManager private[memory] (
 
 object UnifiedMemoryManager {
 
+  // Set aside a fixed amount of memory for non-storage, non-execution purposes.
+  // This serves a function similar to `spark.memory.fraction`, but guarantees that we reserve
+  // sufficient memory for the system even for small heaps. E.g. if we have a 1GB JVM, then
+  // the memory used for execution and storage will be (1024 - 300) * 0.75 = 543MB by default.
+  private val RESERVED_SYSTEM_MEMORY_BYTES = 300 * 1024 * 1024
+
   def apply(conf: SparkConf, numCores: Int): UnifiedMemoryManager = {
     val maxMemory = getMaxMemory(conf)
     new UnifiedMemoryManager(
@@ -144,8 +150,16 @@ object UnifiedMemoryManager {
    * Return the total amount of memory shared between execution and storage, in bytes.
    */
   private def getMaxMemory(conf: SparkConf): Long = {
-    val systemMaxMemory = conf.getLong("spark.testing.memory", Runtime.getRuntime.maxMemory)
+    val systemMemory = conf.getLong("spark.testing.memory", Runtime.getRuntime.maxMemory)
+    val reservedMemory = conf.getLong("spark.testing.reservedMemory",
+      if (conf.contains("spark.testing")) 0 else RESERVED_SYSTEM_MEMORY_BYTES)
+    val minSystemMemory = reservedMemory * 1.5
+    if (systemMemory < minSystemMemory) {
+      throw new IllegalArgumentException(s"System memory $systemMemory must " +
+        s"be at least $minSystemMemory. Please use a larger heap size.")
+    }
+    val usableMemory = systemMemory - reservedMemory
     val memoryFraction = conf.getDouble("spark.memory.fraction", 0.75)
-    (systemMaxMemory * memoryFraction).toLong
+    (usableMemory * memoryFraction).toLong
   }
 }

core/src/test/scala/org/apache/spark/memory/UnifiedMemoryManagerSuite.scala

@@ -182,4 +182,24 @@ class UnifiedMemoryManagerSuite extends MemoryManagerSuite with PrivateMethodTes
     assertEnsureFreeSpaceCalled(ms, 850L)
   }
 
+  test("small heap") {
+    val systemMemory = 1024 * 1024
+    val reservedMemory = 300 * 1024
+    val memoryFraction = 0.8
+    val conf = new SparkConf()
+      .set("spark.memory.fraction", memoryFraction.toString)
+      .set("spark.testing.memory", systemMemory.toString)
+      .set("spark.testing.reservedMemory", reservedMemory.toString)
+    val mm = UnifiedMemoryManager(conf, numCores = 1)
+    val expectedMaxMemory = ((systemMemory - reservedMemory) * memoryFraction).toLong
+    assert(mm.maxMemory === expectedMaxMemory)
+
+    // Try using a system memory that's too small
+    val conf2 = conf.clone().set("spark.testing.memory", (reservedMemory / 2).toString)
+    val exception = intercept[IllegalArgumentException] {
+      UnifiedMemoryManager(conf2, numCores = 1)
+    }
+    assert(exception.getMessage.contains("larger heap size"))
+  }
+
 }

docs/configuration.md

@@ -719,8 +719,8 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.memory.fraction</code></td>
   <td>0.75</td>
   <td>
-    Fraction of the heap space used for execution and storage. The lower this is, the more
-    frequently spills and cached data eviction occur. The purpose of this config is to set
+    Fraction of (heap space - 300MB) used for execution and storage. The lower this is, the
+    more frequently spills and cached data eviction occur. The purpose of this config is to set
     aside memory for internal metadata, user data structures, and imprecise size estimation
     in the case of sparse, unusually large records. Leaving this at the default value is
     recommended. For more detail, see <a href="tuning.html#memory-management-overview">

docs/tuning.md

@@ -114,7 +114,7 @@ variety of workloads without requiring user expertise of how memory is divided i
 Although there are two relevant configurations, the typical user should not need to adjust them
 as the default values are applicable to most workloads:
 
-* `spark.memory.fraction` expresses the size of `M` as a fraction of the total JVM heap space
+* `spark.memory.fraction` expresses the size of `M` as a fraction of the (JVM heap space - 300MB)
 (default 0.75). The rest of the space (25%) is reserved for user data structures, internal
 metadata in Spark, and safeguarding against OOM errors in the case of sparse and unusually
 large records.