roachtest/tpcc: reduce load during rebalance period

Relates to #58298.

One thing I've noticed when looking into #58298 is that we were often
badly overloading the cluster during the rebalance wait period. During
this time, we just want to apply a small amount of load to help instruct
load-based splitting and rebalancing. But in some cases, we were
completely overloading the cluster. We also weren't ramping up the load,
as we had intended to.

This commit fixes both of these issues. It adds a ramp period for the
first quarter of the rebalance time and it scales the txn rate based on
the expected max warehouse count instead of the loaded warehouse count.

pkg/cmd/roachtest/tpcc.go

@@ -677,12 +677,18 @@ func loadTPCCBench(
 		return err
 	}
 
-	// Split and scatter the tables. Ramp up to the expected load in the desired
-	// distribution. This should allow for load-based rebalancing to help
-	// distribute load. Optionally pass some load configuration-specific flags.
+	// Split and scatter the tables. Ramp up to the half of the expected load in
+	// the desired distribution. This should allow for load-based rebalancing to
+	// help distribute load. Optionally pass some load configuration-specific
+	// flags.
+	const txnsPerWarehousePerSecond = 12.8 * (23.0 / 10.0) * (1.0 / 60.0) // max_tpmC/warehouse * all_txns/new_order_txns * minutes/seconds
+	rateAtExpected := txnsPerWarehousePerSecond * float64(b.EstimatedMax)
+	maxRate := int(rateAtExpected / 2)
+	rampTime := (1 * rebalanceWait) / 4
+	loadTime := (3 * rebalanceWait) / 4
 	cmd = fmt.Sprintf("./cockroach workload run tpcc --warehouses=%d --workers=%d --max-rate=%d "+
-		"--wait=false --duration=%s --scatter --tolerate-errors {pgurl%s}",
-		b.LoadWarehouses, b.LoadWarehouses, b.LoadWarehouses/2, rebalanceWait, roachNodes)
+		"--wait=false --ramp=%s --duration=%s --scatter --tolerate-errors {pgurl%s}",
+		b.LoadWarehouses, b.LoadWarehouses, maxRate, rampTime, loadTime, roachNodes)
 	if out, err := c.RunWithBuffer(ctx, c.l, loadNode, cmd); err != nil {
 		return errors.Wrapf(err, "failed with output %q", string(out))
 	}