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From "Dawid Weiss (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
Date Thu, 22 Mar 2012 14:08:22 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3867?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13235588#comment-13235588
] 

Dawid Weiss commented on LUCENE-3867:
-------------------------------------

Yep, that assumption was wrong -- indeed:
{noformat}
WildClasses.Wild_2_5 wc = new WildClasses.Wild_2_5();
wc.fld_6_0_92 = 0x1122;
wc.fld_0_2_5 = Float.intBitsToFloat(0xa1a2a3a4);
wc.fld_0_1_85 = Double.longBitsToDouble(0xb1b2b3b4b5b6b7L);
System.out.println(ExpMemoryDumper.dumpObjectMem(wc));
{noformat}
results in:
{noformat}
0x0000 b0 3d 6f 01 00 00 00 00 0e 80 79 01 00 00 00 00
0x0010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0030 22 11 00 00 a4 a3 a2 a1 b7 b6 b5 b4 b3 b2 b1 00
0x0040 00 00 00 00 00 00 00 00
{noformat}
And you can see they are reordered and longs are aligned.

I'll provide a cumulative patch of changes in the evening, there's one more thing I wanted
to add (cache of fields) because this affects processing speed.
                
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
> --------------------------------------------------------------------------
>
>                 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: Uwe Schindler
>            Priority: Trivial
>             Fix For: 3.6, 4.0
>
>         Attachments: LUCENE-3867-3.x.patch, 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.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
>
>
> 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 the usual object
header. However, this object head is 12 bytes to accommodate 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 of bytes required for one element, depending on its type. The memory
usage for 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 feel about including
such helper methods in RUE, as static, stateless, methods? It's not perfect, there's some
room for improvement I'm sure, here it is:
> {code}
> 	/**
> 	 * Computes the approximate size of a String object. Note that if this object
> 	 * is also referenced by another object, you should add
> 	 * {@link RamUsageEstimator#NUM_BYTES_OBJECT_REF} to the result of this
> 	 * method.
> 	 */
> 	public static int sizeOf(String str) {
> 		return 2 * str.length() + 6 // chars + additional safeness for arrays alignment
> 				+ 3 * RamUsageEstimator.NUM_BYTES_INT // String maintains 3 integers
> 				+ RamUsageEstimator.NUM_BYTES_ARRAY_HEADER // char[] array
> 				+ RamUsageEstimator.NUM_BYTES_OBJECT_HEADER; // String object
> 	}
> {code}
> If people are not against it, I'd like to also add sizeOf(int[] / byte[] / long[] / double[]
... and String[]).

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