attilapiros commented on a change in pull request #34846:
URL: https://github.com/apache/spark/pull/34846#discussion_r769417875
##########
File path: core/src/main/scala/org/apache/spark/util/Utils.scala
##########
@@ -3204,6 +3204,14 @@ private[spark] object Utils extends Logging {
}
files.toSeq
}
+
+ val isG1GarbageCollector: Boolean = {
+ ManagementFactory.getGarbageCollectorMXBeans
Review comment:
When I looked for how to find out what the garbage collector type is I
bumped into this several times:
```java
HotSpotDiagnosticMXBean diagnostic =
ManagementFactoryHelper.getDiagnosticMXBean();
VMOption option = diagnostic.getVMOption("UseG1GC");
if (option.getValue().equals("false")) {
...
}
```
For example at the [OpenJDK
tests](https://www.programcreek.com/java-api-examples/?code=AdoptOpenJDK%2Fopenjdk-jdk8u%2Fopenjdk-jdk8u-master%2Fhotspot%2Ftest%2Fgc%2Farguments%2FTestG1HeapRegionSize.java
).
Is there any reason why a different solution have been chosen here?
##########
File path: core/src/main/scala/org/apache/spark/memory/MemoryManager.scala
##########
@@ -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:
If I get this right in case of G1GC the best would be if we choose a
pageSize where the following holds:
```
G1HeapRegionSize % (pageSize + Platform.LONG_ARRAY_OFFSET) == 0
```
And when When BUFFER_PAGESIZE is not set we are free to choose it as:
```
pageSize = G1HeapRegionSize - Platform.LONG_ARRAY_OFFSET;
```
And with the above code we just try to calculate G1HeapRegionSize with our
own way. But what about accessing this value?
Like the same way used in the [OpenJDK
tests](https://www.programcreek.com/java-api-examples/?code=AdoptOpenJDK%2Fopenjdk-jdk8u%2Fopenjdk-jdk8u-master%2Fhotspot%2Ftest%2Fgc%2Farguments%2FTestG1HeapRegionSize.java):
```java
HotSpotDiagnosticMXBean diagnostic =
ManagementFactoryHelper.getDiagnosticMXBean();
option = diagnostic.getVMOption("G1HeapRegionSize");
```
As I see the OpenJDK test was executed like:
```
run main/othervm -Xmx64m TestG1HeapRegionSize 1048576
```
So `diagnostic.getVMOption("G1HeapRegionSize")` gives back the calculated
region size.
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