Github user shivaram commented on a diff in the pull request:
https://github.com/apache/spark/pull/4783#discussion_r25969639
--- 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
--- End diff --
`as a starting point`
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