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Date: Fri, 16 Mar 2012 22:58:40 +0000 (UTC)
From: "Uwe Schindler (Commented) (JIRA)" <jira@apache.org>
To: dev@lucene.apache.org
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Subject: [jira] [Commented] (LUCENE-3867)
 RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
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    [ https://issues.apache.org/jira/browse/LUCENE-3867?page=3Dcom.atlassia=
n.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=3D132=
31719#comment-13231719 ]=20

Uwe Schindler commented on LUCENE-3867:
---------------------------------------

bq. I don't think it's "fixed" =E2=80=93 see the -XX:ObjectAlignmentInBytes=
=3D32 above. But the defaults seem to be the same on all systems.

I would like to have the rounding also dynamic, but this is not possible to=
 find out with Unsafe, at least for this I have no idea :(
               =20
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrec=
t
> -------------------------------------------------------------------------=
-
>
>                 Key: LUCENE-3867
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3867
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: core/index
>            Reporter: Shai Erera
>            Assignee: Shai Erera
>            Priority: Trivial
>             Fix For: 3.6, 4.0
>
>         Attachments: LUCENE-3867-compressedOops.patch, LUCENE-3867.patch,=
 LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch=
, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patc=
h
>
>
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER is computed like that: NUM_BYTES=
_OBJECT_HEADER + NUM_BYTES_INT + NUM_BYTES_OBJECT_REF. The NUM_BYTES_OBJECT=
_REF part should not be included, at least not according to this page: http=
://www.javamex.com/tutorials/memory/array_memory_usage.shtml
> {quote}
> A single-dimension array is a single object. As expected, the array has t=
he usual object header. However, this object head is 12 bytes to accommodat=
e a four-byte array length. Then comes the actual array data which, as you =
might expect, consists of the number of elements multiplied by the number o=
f bytes required for one element, depending on its type. The memory usage f=
or one element is 4 bytes for an object reference ...
> {quote}
> While on it, I wrote a sizeOf(String) impl, and I wonder how do people fe=
el about including such helper methods in RUE, as static, stateless, method=
s? It's not perfect, there's some room for improvement I'm sure, here it is=
:
> {code}
> =09/**
> =09 * Computes the approximate size of a String object. Note that if this=
 object
> =09 * is also referenced by another object, you should add
> =09 * {@link RamUsageEstimator#NUM_BYTES_OBJECT_REF} to the result of thi=
s
> =09 * method.
> =09 */
> =09public static int sizeOf(String str) {
> =09=09return 2 * str.length() + 6 // chars + additional safeness for arra=
ys alignment
> =09=09=09=09+ 3 * RamUsageEstimator.NUM_BYTES_INT // String maintains 3 i=
ntegers
> =09=09=09=09+ RamUsageEstimator.NUM_BYTES_ARRAY_HEADER // char[] array
> =09=09=09=09+ RamUsageEstimator.NUM_BYTES_OBJECT_HEADER; // String object
> =09}
> {code}
> If people are not against it, I'd like to also add sizeOf(int[] / byte[] =
/ long[] / double[] ... and String[]).

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