WangGuangxin commented on a change in pull request #34846:
URL: https://github.com/apache/spark/pull/34846#discussion_r772120430
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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:
@attilapiros Since -XX:G1HeapRegionSize can only be set to N-th power of
2, and the default size calculated here is also N-th power of 2, so it can be
guanranteed that `G1HeapRegionSize % (pageSize + Platform.LONG_ARRAY_OFFSET)
== 0` or `(pageSize + Platform.LONG_ARRAY_OFFSET) % G1HeapRegionSize == 0`,
right?
Such a change has little effect on the current logic.
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