sjvanrossum commented on PR #34764:
URL: https://github.com/apache/beam/pull/34764#issuecomment-2841577401
@kennknowles the default object layout on recent JVMs (64 bit, 8 bytes
alignment, 12 bytes object header, compressed references and class pointers)
leaves 4 bytes of padding in `SimpleWindowedValue`:
```
org.apache.beam.sdk.util.WindowedValue$SimpleWindowedValue object internals:
OFF SZ TYPE DESCRIPTION
VALUE
0 8 (object header:
mark) N/A
8 4 (object header:
class) N/A
12 4 java.lang.Object
SimpleWindowedValue.value N/A
16 4 org.apache.beam.sdk.transforms.windowing.PaneInfo
SimpleWindowedValue.pane N/A
20 4 (object
alignment gap)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
```
`MinTimestampWindowedValue` and `TimestampedValueInGlobalWindow` also have
the same size, but `TimestampedValueInGlobalWindow` packs the `timestamp` field
in the alignment gap.
```
org.apache.beam.sdk.util.WindowedValue$MinTimestampWindowedValue object
internals:
OFF SZ TYPE DESCRIPTION
VALUE
0 8 (object header:
mark) N/A
8 4 (object header:
class) N/A
12 4 java.lang.Object
SimpleWindowedValue.value N/A
16 4 org.apache.beam.sdk.transforms.windowing.PaneInfo
SimpleWindowedValue.pane N/A
20 4 (object
alignment gap)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
org.apache.beam.sdk.util.WindowedValue$TimestampedValueInGlobalWindow object
internals:
OFF SZ TYPE DESCRIPTION
VALUE
0 8 (object header:
mark) N/A
8 4 (object header:
class) N/A
12 4 java.lang.Object
SimpleWindowedValue.value N/A
16 4 org.apache.beam.sdk.transforms.windowing.PaneInfo
SimpleWindowedValue.pane N/A
20 4 org.joda.time.Instant
TimestampedWindowedValue.timestamp N/A
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
```
And for `TimestampedValueInSingleWindow` and
`TimestampedValueInMultipleWindows` it spills over into the next word
(`timestamp` fills the alignment gap, `window`/`windows` spills into the next
word):
```
org.apache.beam.sdk.util.WindowedValue$TimestampedValueInSingleWindow object
internals:
OFF SZ TYPE DESCRIPTION
VALUE
0 8 (object
header: mark) N/A
8 4 (object
header: class) N/A
12 4 java.lang.Object
SimpleWindowedValue.value N/A
16 4 org.apache.beam.sdk.transforms.windowing.PaneInfo
SimpleWindowedValue.pane N/A
20 4 org.joda.time.Instant
TimestampedWindowedValue.timestamp N/A
24 4 org.apache.beam.sdk.transforms.windowing.BoundedWindow
TimestampedValueInSingleWindow.window N/A
28 4 (object
alignment gap)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
org.apache.beam.sdk.util.WindowedValue$TimestampedValueInMultipleWindows
object internals:
OFF SZ TYPE DESCRIPTION
VALUE
0 8 (object header:
mark) N/A
8 4 (object header:
class) N/A
12 4 java.lang.Object
SimpleWindowedValue.value N/A
16 4 org.apache.beam.sdk.transforms.windowing.PaneInfo
SimpleWindowedValue.pane N/A
20 4 org.joda.time.Instant
TimestampedWindowedValue.timestamp N/A
24 4 java.util.Collection
TimestampedValueInMultipleWindows.windows N/A
28 4 (object
alignment gap)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
```
With default object layout settings the addition of an `extensions` field
would only increase the size of `TimestampedValueInGlobalWindow` by 8 bytes
since it would spill into the next word, all the other subclasses should pack
that field into the alignment gap. I'm not sure whether users of Beam commonly
disable `UseCompressedPointers` or `UseCompressedOops`, but it would seem
somewhat unlikely in my opinion since both of them would generally increase
memory usage.
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