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https://issues.apache.org/jira/browse/LUCENE-3867?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13229460#comment-13229460
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Dawid Weiss commented on LUCENE-3867:
-------------------------------------
John Rose just replied to my question -- there are fields in Unsafe that allow
array scaling (1.7). Check these out:
{noformat}
ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale([Z);
ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale([B);
ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale([S);
ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale([C);
ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale([I);
ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale([J);
ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale([F);
ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale([D);
ARRAY_OBJECT_INDEX_SCALE =
theUnsafe.arrayIndexScale([Ljava/lang/Object;);
ADDRESS_SIZE = theUnsafe.addressSize();
{noformat}
So... there is a (theoretical?) possibility that, say, byte[] is machine
word-aligned :) I bet any RAM estimator written so far will be screwed if this
happens :)
> 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
>
>
> 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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