eolivelli commented on code in PR #4709:
URL: https://github.com/apache/bookkeeper/pull/4709#discussion_r2749017022
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bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/BookieImpl.java:
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@@ -940,8 +942,30 @@ LedgerDescriptor getLedgerForEntry(ByteBuf entry, final
byte[] masterKey)
return handles.getHandle(ledgerId, masterKey, false);
}
+ /**
+ * Constant for Fibonacci hashing. Multiplying by this constant adds
randomness, breaking
+ * patterns in ledger IDs that would otherwise cause uneven journal
distribution.
+ *
+ * <p>This is the same constant used as {@code GOLDEN_GAMMA} in
+ * {@link java.util.concurrent.ThreadLocalRandom} and {@link
java.util.SplittableRandom}
+ * for seed mixing.
+ */
+ private static final long FIBONACCI_HASH_CONSTANT = 0x9E3779B97F4A7C15L;
+
+ /**
+ * Returns the journal to use for the given ledger.
+ *
+ * <p>When hash-based selection is enabled, uses Fibonacci hashing to
distribute ledgers
+ * across journals. The xor with the right-shifted value folds the high 32
bits into
+ * the low 32 bits, adding more mixing prior to the modulo.
+ */
private Journal getJournal(long ledgerId) {
- return journals.get(MathUtils.signSafeMod(ledgerId, journals.size()));
+ long index = ledgerId;
+ if (journalHashBasedSelection) {
+ index = ledgerId * FIBONACCI_HASH_CONSTANT;
Review Comment:
what about putting this block into a static method so that we can add unit
tests ?
The function gets the index and the number of journals and computes the
journal index
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