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From "Shai Erera (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER is incorrect
Date Wed, 14 Mar 2012 13:28:38 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-3867?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Shai Erera updated LUCENE-3867:
-------------------------------

    Attachment: LUCENE-3867.patch

Patch adds RUE.sizeOf(String) and various sizeOf(arr[]) methods. Also fixes the ARRAY_HEADER.

Uwe, I merged with your patch, with one difference -- the System.out prints in the test are
printed only if VERBOSE.
                
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER is 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: Shai Erera
>            Priority: Trivial
>             Fix For: 3.6, 4.0
>
>         Attachments: LUCENE-3867-compressedOops.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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