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From "Uwe Schindler (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER is incorrect
Date Thu, 15 Mar 2012 09:41:37 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

Hi,
here new patch using Unsafe to get the bitness (with the well-known fallback) and for compressedOops
detection. Looks much cleaner.
I also like it more, that the addressSize is now detected natively and not from sysprops.

The constants mentioned by Dawid are only availabe in Java 7, so i reflected the underlying
methods from theUnsafe. I also changed the boolean JRE_USES_COMPRESSED_OOPS to an integer
JRE_REFERENCE_SIZE that is used by RamUsageEstimator. We might do the same for all other native
types... (this is just a start).

Shai: Can you test with your JVMs and also enable/disable compressed oops/refs?
                
> 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, 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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