Github user advancedxy commented on a diff in the pull request:
https://github.com/apache/spark/pull/4783#discussion_r25840206
--- Diff: core/src/main/scala/org/apache/spark/util/SizeEstimator.scala ---
@@ -257,21 +262,56 @@ private[spark] object SizeEstimator extends Logging {
val parent = getClassInfo(cls.getSuperclass)
var shellSize = parent.shellSize
var pointerFields = parent.pointerFields
+
+ // we have only four sizes: 1, 2, 4, 8. pointerSize can be 4 or 8.
+ // we can just use an Array.fill(8 + 1) to hold all the size counting,
but we
+ // can also use a HashMap to store size counting; Here we uses Array.
+ val sizeCount = Array.fill(fieldSizes.max + 1)(0)
+ // iterate through the fields of this class and gather information.
for (field <- cls.getDeclaredFields) {
if (!Modifier.isStatic(field.getModifiers)) {
val fieldClass = field.getType
if (fieldClass.isPrimitive) {
- shellSize += primitiveSize(fieldClass)
+ sizeCount(primitiveSize(fieldClass)) += 1
} else {
field.setAccessible(true) // Enable future get()'s on this field
- shellSize += pointerSize
+ sizeCount(pointerSize) += 1
pointerFields = field :: pointerFields
}
}
}
- shellSize = alignSize(shellSize)
+ // Based on the simulated field layout code in Aleksey Shipilev's
report:
+ //
http://cr.openjdk.java.net/~shade/papers/2013-shipilev-fieldlayout-latest.pdf
+ // The code is in Figure 9. Aleksey Shiplev also leaders the
development of JOL (Java Object
+ // Layout) tool. The simulated code is also in the JOL. The simplified
idea of field layout
+ // consists of these parts (see more details in the report):
+ // 1. field alignment: HotSpot lay out the fields aligned by their
size.
+ // 2. object alignment: HotSpot rounds instance size up to 8 bytes
+ // 3. consistent field layouts throughout the hierarchy: This means we
should layout
+ // superclass first. And we use superclass's shellSize as a start
point to layout the
+ // other fields in this class.
+ // 4. class alignment: HotSpot rounds field blocks upto to HeapOopSize
not 4 bytes, confirmed
+ // with Aleksey. see
https://bugs.openjdk.java.net/browse/CODETOOLS-7901322
+ //
+ // The real world field layout is much more complicated. There are
three kinds of fields
+ // order in Java 8. And we don't consider the @contended annotation
introduced by Java 8.
+ // see the HotSpot classloader code, layout_fields method for more
details.
+ //
hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/classfile/classFileParser.cpp
--- End diff --
It exceeds the 100 characters. So, I remove the first "http://"; characters.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
