[PR] Reduce long[] array allocation for bitset in readBitSetIterator [lucene]

[via GitHub]

easyice opened a new pull request, #13828:
URL: https://github.com/apache/lucene/pull/13828

   I got a memory allocation flamegraph from my cluster, It shows the 
`DocIdsWriter#readBitSetIterator` allocating about ~14GB RAM in 30 seconds,  
this proposal uses a `scratchLongs[]` array instead of allocating a new array.
   
   
![image](https://github.com/user-attachments/assets/bbf16618-05f6-44d3-8cc1-67540ee47628)
   
[mem.html.zip](https://github.com/user-attachments/files/17146431/mem.html.zip)
   
   
   When using some mock data that let `DocIdsWriter#readBitSetIterator` 
allocate a long[] array of size 96, the JMH output shows:
   
   main:
   ```
   Benchmark                                Mode  Cnt        Score     Error   
Units
   BKDWithBitMap.search                     avgt    5       65.001 ±   0.579   
us/op
   BKDWithBitMap.search:gc.alloc.rate       avgt    5    18626.467 ± 166.049  
MB/sec
   BKDWithBitMap.search:gc.alloc.rate.norm  avgt    5  1269568.090 ±   0.001    
B/op
   BKDWithBitMap.search:gc.count            avgt    5      423.000            
counts
   BKDWithBitMap.search:gc.time             avgt    5      218.000              
  ms
   ```
   pr:
   ```
   Benchmark                                Mode  Cnt       Score     Error   
Units
   BKDWithBitMap.search                     avgt    5      63.622 ±   0.784   
us/op
   BKDWithBitMap.search:gc.alloc.rate       avgt    5    9582.419 ± 118.030  
MB/sec
   BKDWithBitMap.search:gc.alloc.rate.norm  avgt    5  639272.088 ±   0.001    
B/op
   BKDWithBitMap.search:gc.count            avgt    5     209.000            
counts
   BKDWithBitMap.search:gc.time             avgt    5     111.000               
 ms
   ```
   
   
   
   <details>
   <summary >JMH Code</summary>
   
   ```
   @BenchmarkMode(Mode.AverageTime)
   @OutputTimeUnit(TimeUnit.MICROSECONDS)
   @State(Scope.Benchmark)
   @Warmup(iterations = 3, time = 3)
   @Measurement(iterations = 5, time = 5)
   @Fork(
       value = 1,
       jvmArgsPrepend = {"--add-modules=jdk.unsupported"})
   public class BKDWithBitMap {
     IndexReader reader;
     IndexSearcher searcher;
     Query q = LongPoint.newSetQuery("f", 0);
   
     @Setup
     public void setup() throws IOException {
       Path path = Files.createTempDirectory("points");
       Directory dir = MMapDirectory.open(path);
   
       try (IndexWriter w =
           new IndexWriter(
               dir,
               new IndexWriterConfig()
                   .setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH)
                   .setRAMBufferSizeMB(256.0))) {
         int actualIndexed = 0;
         for (int i = 0; i < 5_000_000; ++i) {
           Document doc = new Document();
           doc.add(new StringField("id", Long.toString(i), Field.Store.NO));
           if (i % 12 == 0) {
             doc.add(new LongPoint("f", 0));
           }
           w.addDocument(doc);
           if (actualIndexed++ % 1_000_000 == 0) {
             System.out.println("Indexed: " + actualIndexed);
           }
         }
         w.commit();
         w.forceMerge(1);
         w.commit();
       }
   
       reader = DirectoryReader.open(dir);
       searcher = new IndexSearcher(reader);
       searcher.setQueryCache(null);
     }
   
     @Benchmark
     public void search(Blackhole bh) throws IOException {
       Weight weight = searcher.createWeight(searcher.rewrite(q), 
ScoreMode.COMPLETE_NO_SCORES, 1f);
       ScorerSupplier ss = weight.scorerSupplier(reader.leaves().get(0));
       if (ss != null) {
         Scorer scorer = ss.get(Long.MAX_VALUE);
         bh.consume(scorer);
       }
     }
   }
   ```
   </details>


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