[GitHub] [lucene] rmuir commented on a change in pull request #69: LUCENE-9850: Use PFOR encoding for doc IDs (instead of FOR)

[GitBox]

rmuir commented on a change in pull request #69:
URL: https://github.com/apache/lucene/pull/69#discussion_r609185490



##########
File path: lucene/core/src/java/org/apache/lucene/codecs/lucene90/PForUtil.java
##########
@@ -121,4 +167,146 @@ void skip(DataInput in) throws IOException {
       in.skipBytes(forUtil.numBytes(bitsPerValue) + (numExceptions << 1));
     }
   }
+
+  /**
+   * Fill {@code longs} with the final values for the case of all deltas being 
1. Note this assumes
+   * there are no exceptions to apply.
+   */
+  private static void prefixSumOfOnes(long[] longs, long base) {
+    System.arraycopy(IDENTITY_PLUS_ONE, 0, longs, 0, ForUtil.BLOCK_SIZE);
+    // This loop gets auto-vectorized
+    for (int i = 0; i < ForUtil.BLOCK_SIZE; ++i) {
+      longs[i] += base;
+    }
+  }
+
+  /**
+   * Fill {@code longs} with the final values for the case of all deltas being 
{@code val}. Note
+   * this assumes there are no exceptions to apply.
+   */
+  private static void prefixSumOf(long[] longs, long base, long val) {
+    for (int i = 0; i < ForUtil.BLOCK_SIZE; i++) {
+      longs[i] = (i + 1) * val + base;

Review comment:
       > I came across this 
[read](http://psy-lob-saw.blogspot.com/2015/07/jmh-perfasm.html) that seemed to 
indicate the need to run on "real hardware."
   
   I dont know about this. For me it works fine both on real hardware and 
inside QEMU vm, i see similar results for a simple test command passing the 
same arguments that jmh does behind the scenes:
   ```
   perf record --freq 1000 --event cycles,instructions --output test.tmp dd 
if=/dev/urandom of=/dev/null bs=1M count=100
   ```
   If it says 0 samples, then something is off.
   
   hardware wise, I see the following config:
   ```
   host$ journalctl -k | grep PMU
   kernel: Performance Events: PEBS fmt3+, Skylake events, 32-deep LBR, 
full-width counters, Intel PMU driver.
   
   guest$ journalctl -k | grep PMU
   kernel: Performance Events: Skylake events, full-width counters, Intel PMU 
driver.
   ```




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