lightning: calculate concurrency of schema import from region-concurrency (#58202)

close #58141

Author: D3Hunter

lightning/pkg/importer/import.go

@@ -593,7 +593,9 @@ func (rc *Controller) restoreSchema(ctx context.Context) error {
 	// we can handle the duplicated created with createIfNotExist statement
 	// and we will check the schema in TiDB is valid with the datafile in DataCheck later.
 	logger := log.FromContext(ctx)
-	concurrency := min(rc.cfg.App.RegionConcurrency, 8)
+	// the minimum 4 comes the fact that when connect to non-owner TiDB, the max
+	// QPS is 2 per connection due to polling every 500ms.
+	concurrency := max(2*rc.cfg.App.RegionConcurrency, 4)
 	// sql.DB is a connection pool, we set it to concurrency + 1(for job generator)
 	// to reuse connections, as we might call db.Conn/conn.Close many times.
 	// there's no API to get sql.DB.MaxIdleConns, so we revert to its default which is 2