Tomasz Grabiec 66439bb753 Merge 'load_balancer: apply balance threshold to intranode shard balancing' from Ferenc Szili ...

- Fix intranode shard balancing to respect the size-based balance threshold, preventing unnecessary migrations when load difference between shards is negligible
- Add a regression test that verifies the threshold is respected for intranode balancing

The intranode shard balancing loop only stopped when the algorithm exhausted the migration candidates or when a migration would go against convergence (it would increase imbalance instead of decrease it). This caused unnecessary tablet migrations for negligible imbalances (e.g., 0.78% difference between shards).

The inter-node balancer already uses `is_balanced()` to stop when the relative load difference is within the configured `size_based_balance_threshold`, but this check was missing from the intranode path.

Apply the same `is_balanced()` threshold check that is already used for inter-node balancing to the intranode convergence loop. When the relative load difference between the most-loaded and least-loaded shards on a node is within the threshold, the balancer now stops without issuing further migrations.

The test creates a single node with 2 shards and 512 tablets:
1. **Balanced scenario** (257 vs 255 tablets, same size): relative diff = 0.78% < 1% threshold → verifies no intranode migration is emitted
2. **Unbalanced scenario** (307 vs 205 tablets, same size): relative diff = 33% >> 1% threshold → verifies intranode migration IS emitted

Fixes: SCYLLADB-1775

This is a performance improvement which reduces the number of intranode migrations issued, and needs to be backported to versions with size-based load balancing: 2026.1 and 2026.2

Closes scylladb/scylladb#29756

* github.com:scylladb/scylladb:
  test: add test for intranode balance threshold in size-based mode
  tablet_allocator: apply balance threshold to intranode shard balancing

2026-05-13 13:09:52 +02:00

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