[SPARK-12181] Check Cached unaligned-access capability before using Unsafe

## What changes were proposed in this pull request?

For MemoryMode.OFF_HEAP, Unsafe.getInt etc. are used with no restriction.

However, the Oracle implementation uses these methods only if the class variable unaligned (commented as "Cached unaligned-access capability") is true, which seems to be calculated whether the architecture is i386, x86, amd64, or x86_64.

I think we should perform similar check for the use of Unsafe.

Reference: https://github.com/netty/netty/blob/4.1/common/src/main/java/io/netty/util/internal/PlatformDependent0.java#L112

## How was this patch tested?

Unit test suite

Author: tedyu <yuzhihong@gmail.com>

Closes #11943 from tedyu/master.

1 files changed, 3 insertions, 0 deletions

diff --git a/core/src/main/scala/org/apache/spark/memory/MemoryManager.scala b/core/src/main/scala/org/apache/spark/memory/MemoryManager.scala
index 10656bc8c8..0210217e41 100644
--- a/core/src/main/scala/org/apache/spark/memory/MemoryManager.scala
+++ b/core/src/main/scala/org/apache/spark/memory/MemoryManager.scala
@@ -23,6 +23,7 @@ import org.apache.spark.SparkConf
 import org.apache.spark.internal.Logging
 import org.apache.spark.storage.BlockId
 import org.apache.spark.storage.memory.MemoryStore
+import org.apache.spark.unsafe.Platform
 import org.apache.spark.unsafe.array.ByteArrayMethods
 import org.apache.spark.unsafe.memory.MemoryAllocator
 
@@ -190,6 +191,8 @@ private[spark] abstract class MemoryManager(
     if (conf.getBoolean("spark.memory.offHeap.enabled", false)) {
       require(conf.getSizeAsBytes("spark.memory.offHeap.size", 0) > 0,
         "spark.memory.offHeap.size must be > 0 when spark.memory.offHeap.enabled == true")
+      require(Platform.unaligned(),
+        "No support for unaligned Unsafe. Set spark.memory.offHeap.enabled to false.")
       MemoryMode.OFF_HEAP
     } else {
       MemoryMode.ON_HEAP