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From "Uwe Schindler (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
Date Fri, 16 Mar 2012 16:55:39 GMT

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

Uwe Schindler updated LUCENE-3867:
----------------------------------

    Attachment: LUCENE-3867.patch

Attached is a patch fixing several bugs and more:
- Removed the MemoryIndex VM class and the completely outdated and incorrect estimation there.
- Used Shai's new added methods also in Lucene's PackedInt classes
- Fixes overflows in Shai's new methods, as they can overflow if arrays are greater than 2
GB (casts to long missing)
- Fixed the up-rounding to multiples of 8 to work with longs

What's the reason why this rounding up to 8 bytes was added? I assume this information comes
from somewhere, but it was added by Shai without any explanation. Is this not also dependent
on the 64bitness if its 8 or 4?

Otherwise patch is ready.
                
> 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: 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
>
>
> 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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