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From "Nadav Har'El (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-1899) Inefficient growth of OpenBitSet
Date Tue, 08 Sep 2009 13:32:57 GMT
Inefficient growth of OpenBitSet
--------------------------------

                 Key: LUCENE-1899
                 URL: https://issues.apache.org/jira/browse/LUCENE-1899
             Project: Lucene - Java
          Issue Type: Bug
          Components: Store
    Affects Versions: 2.9
            Reporter: Nadav Har'El
            Priority: Minor


Hi, I found a potentially serious efficiency problem with OpenBitSet.

One typical (I think) way to build a bit set is to set() the bits one by one -
e.g., have a HitCollector set() the bit for each matching document.
The underlying array of longs needs to grow as more as more bits are set, of
course.

But looking at the code, it appears to me that the array grows very
ineefficiently - in the worst case (when doc ids are sorted, as they would
normally be in the HitCollector case for example), copying the array again
and again for every added bit... The relevant code in OpenBitSet.java is:

  public void set(long index) {
    int wordNum = expandingWordNum(index);
    ...
  }

  protected int expandingWordNum(long index) {
    int wordNum = (int)(index >> 6);
    if (wordNum>=wlen) {
      ensureCapacity(index+1);
    ...
  }
  public void ensureCapacityWords(int numWords) {
    if (bits.length < numWords) {
      long[] newBits = new long[numWords];
      System.arraycopy(bits,0,newBits,0,wlen);
      bits = newBits;
    }
  }

As you can see, if the bits array is not long enough, a new one is
allocated at exactly the right size - and in the worst case it can grow
just one word every time...

Shouldn't the growth be more exponential in nature, e.g., grow to the maximum
of index+1 and twice the existing size?

Alternatively, if the growth is so inefficient, this should be documented,
and it should be recommended to use the variant of the constructor with the
correct initial size (e.g., in the HitCollector case, the number of documents
in the index). and the fastSet() method instead of set().

Thanks,
Nadav.


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