[
https://issues.apache.org/jira/browse/DERBY-3731?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12606927#action_12606927
]
Knut Anders Hatlen commented on DERBY-3731:
-------------------------------------------
Not directly relevant for this issue, but I'm posting some
observations here so that they are not lost.
I used DTrace to check how much memory that was allocated in JDK 6 on
OpenSolaris 2008.05 and compared the results to what ClassSize would
estimate. I found these differences (assuming that refSize was
correctly set to 4 on 32-bit VMs and 8 on 64-bit VMs):
1) a better name for minObjectSize would be minArraySize, as it
attempts to estimate the fixed overhead for arrays
2) minObjectSize is set to 4*refSize (16 bytes on 32-bit, 32 bytes on
64-bit). The actual value is 3*refSize, but because of 8-byte word
boundaries the size of an empty array is refSize*3+4 == 4*refSize on
32-bit VMs (and the unused space can be exploited, so byte[0] and
byte[4] use the exact same number of bytes). On 64-bit VMs, 3*refSize
matches the word boundaries, so the correct value is 24 (and byte[1]
uses 8 more bytes than byte[0]).
3) booleanSize should be 1, not 4
4) charSize and shortSize should be 2, not 4
5) estimateBaseFromCoefficients() uses minObjectSize as the minimum
object size, whereas it actually is objectOverhead*refSize.
> Improve calculation of refSize in ClassSize.java
> -------------------------------------------------
>
> Key: DERBY-3731
> URL: https://issues.apache.org/jira/browse/DERBY-3731
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Reporter: Kathey Marsden
> Priority: Minor
> Attachments: DERBY-3731_diff.txt
>
>
> java/engine/org/apache/derby/iapi/services/cache/ClassSize.java has a static
> code block which calculates the size of a reference for the architecture.
> This code could be improved by adding garbage collection before measuring
> memory, to give a consistent reading. Also there have been suggestions
> that we use os.arch or sun.arch.data.model to make the measurement more
> reliable, especially on 64bit machines.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
