WangGuangxin commented on a change in pull request #34846:
URL: https://github.com/apache/spark/pull/34846#discussion_r769493813
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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:
Yes, it's better to make sure `G1HeapRegionSize % (pageSize +
Platform.LONG_ARRAY_OFFSET) == 0`.
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.
By following the current logic to calculte default page size and then minus
`Platform.LONG_ARRAY_OFFSET` can also make sure `G1HeapRegionSize % (pageSize +
Platform.LONG_ARRAY_OFFSET) == 0`
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