[GitHub] [spark] attilapiros commented on a change in pull request #34846: [SPARK-37593][CORE] Optimize HeapMemoryAllocator to avoid memory waste when using G1GC

[GitBox <gi...@apache.org>]

attilapiros commented on a change in pull request #34846:
URL: https://github.com/apache/spark/pull/34846#discussion_r769584860



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File path: core/src/main/scala/org/apache/spark/memory/MemoryManager.scala
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@@ -255,7 +256,17 @@ private[spark] abstract class MemoryManager(
     }
     val size = ByteArrayMethods.nextPowerOf2(maxTungstenMemory / cores / safetyFactor)
     val default = math.min(maxPageSize, math.max(minPageSize, size))
-    conf.get(BUFFER_PAGESIZE).getOrElse(default)
+    val sizeAsBytes = conf.get(BUFFER_PAGESIZE).getOrElse(default)
+    // If we are using G1 GC, it's better to take the LONG_ARRAY_OFFSET into consideration
+    // so that the requested memory size is power of 2 and can be divided by G1 region size
+    // to reduce memory waste within one G1 region
+    if (Utils.isG1GarbageCollector &&
+      tungstenMemoryMode == MemoryMode.ON_HEAP &&
+      sizeAsBytes % (1024 * 1024) == 0) {
+      sizeAsBytes - Platform.LONG_ARRAY_OFFSET

Review comment:
       >But when BUFFER_PAGESIZE is not set, I'm not quite sure if it's reasonable to choose it as pageSize = >G1HeapRegionSize - Platform.LONG_ARRAY_OFFSET, which seems a bit different with current logic to get default >page size.
   
   How the current logic for the default handles a case where a custom -XX:G1HeapRegionSize is given as extra java options?
   




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