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From "Dawid Weiss (Commented) (JIRA)" <>
Subject [jira] [Commented] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
Date Sat, 17 Mar 2012 08:05:46 GMT


Dawid Weiss commented on LUCENE-3867:

bq. at least for this I have no idea

The management factory trick mentioned by Kris works for object alignment as well:
package spikes;

import java.lang.reflect.Method;
import java.util.List;


public class ObAlignment
    private static final String HOTSPOT_BEAN_NAME = "";
    private static HotSpotDiagnosticMXBean hotspotMBean;
    private static HotSpotDiagnosticMXBean getHotSpotMBean() {
      if (hotspotMBean == null) {
        try {
          hotspotMBean = ManagementFactory.newPlatformMXBeanProxy(
        } catch (IOException e) {
      return hotspotMBean;

    public static void main(String [] args)
        throws Exception
        // Just the object alignment.

        // Everything.
        Class<?> fc = Class.forName("");
        Method m = fc.getDeclaredMethod("getAllFlags");
        List<Object> flags = (List<Object>) m.invoke(null);
        for (Object f : flags) {
            Method dm = f.getClass().getDeclaredMethod("getVMOption");
            VMOption option = (VMOption) dm.invoke(f);

I don't think it is of much practical use for now (object alignment seems to be constant everywhere),
but we could as well probe it -- if it's available why not use it.

I'd also like to add a shallow size method (which wouldn't follow the fields, just return
the aligned object size). I'll be able to work on it in the evening though, not sooner.
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
> --------------------------------------------------------------------------
>                 Key: LUCENE-3867
>                 URL:
>             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.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:
> {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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