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https://issues.apache.org/jira/browse/DERBY-3794?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12615978#action_12615978
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Knut Anders Hatlen commented on DERBY-3794:
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Copying my observations from DERBY-3731:
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 estimates in iapi.services.cache.ClassSize
> --------------------------------------------------
>
> Key: DERBY-3794
> URL: https://issues.apache.org/jira/browse/DERBY-3794
> Project: Derby
> Issue Type: Improvement
> Components: Services
> Reporter: Knut Anders Hatlen
> Priority: Minor
>
> In a comment in DERBY-3731 I mentioned some discrepancies I found between the
> estimated memory usage and the actual memory usage. We should try to improve
> the estimates so that they match what is actually going to be used. The
> actual memory usage may vary between different platforms, so it would be good
> to check the actual memory usage on a range of platforms.
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