asim: specify node capacity in cores, not nanos/sec

Previously, the datadriven asim tests expressed node capacity in
nanoseconds/core which are difficult to read due to lots of zeros.
Now, the node capacity is expressed in cores (float) which is much more
readable and less prone to user error. For example,
`node_cpu_rate_capacity=8000000000` is now expressed as `node_cpu_cores=8`.

Fixes #156845
Epic: CRDB-55052
Release note: none.

Author: angeladietz

pkg/kv/kvserver/asim/config/settings.go

@@ -31,10 +31,13 @@ const (
 	defaultLBRebalancingInterval   = time.Minute
 )
 
-const DefaultNodeCPURateCapacityNanos = 8 * 1e9        // 8 vcpus
-const DefaultStoreDiskCapacityBytes = 1024 << 30       // 1024 GiB
-const DoubleDefaultNodeCPURateCapacityNanos = 16 * 1e9 // 16 vcpus
-const DoubleDefaultStoreDiskCapacityBytes = 2048 << 30 // 2048 GiB
+const (
+	DefaultNodeCPUCores                   = 8.0                                 // 8 vcpus
+	DefaultNodeCPURateCapacityNanos       = DefaultNodeCPUCores * 1e9           // 8 vcpus
+	DefaultStoreDiskCapacityBytes         = 1024 << 30                          // 1024 GiB
+	DoubleDefaultNodeCPURateCapacityNanos = DefaultNodeCPURateCapacityNanos * 2 // 16 vcpus
+	DoubleDefaultStoreDiskCapacityBytes   = 2048 << 30                          // 2048 GiB
+)
 
 var (
 	// DefaultStartTime is used as the default beginning time for simulation

pkg/kv/kvserver/asim/state/BUILD.bazel

@@ -11,6 +11,7 @@ go_library(
         "load.go",
         "new_state.go",
         "new_state_test_helper.go",
+        "node_cpu_cores.go",
         "node_cpu_rate_capacities.go",
         "parser_replica_placement.go",
         "split_decider.go",
@@ -60,6 +61,7 @@ go_test(
         "change_test.go",
         "config_loader_test.go",
         "liveness_test.go",
+        "node_cpu_cores_test.go",
         "node_cpu_rate_capacities_test.go",
         "parser_replica_placement_test.go",
         "split_decider_test.go",

pkg/kv/kvserver/asim/state/node_cpu_cores.go

@@ -0,0 +1,19 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package state
+
+type NodeCPUCores []float64
+
+// ToRateCapacityNanos converts NodeCPUCores to node capacities in nanos
+// (NodeCPURateCapacities).
+func (nc NodeCPUCores) ToRateCapacityNanos() NodeCPURateCapacities {
+	res := make(NodeCPURateCapacities, len(nc))
+	const nanosPerSecond = 1e9
+	for i, cores := range nc {
+		res[i] = uint64(cores * nanosPerSecond)
+	}
+	return res
+}

pkg/kv/kvserver/asim/state/node_cpu_cores_test.go

@@ -0,0 +1,52 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package state
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNodeCPUCores_ToRateCapacityNanos(t *testing.T) {
+	testCases := []struct {
+		name     string
+		cores    NodeCPUCores
+		expected NodeCPURateCapacities
+	}{
+		{
+			name:     "empty",
+			cores:    NodeCPUCores{},
+			expected: NodeCPURateCapacities{},
+		},
+		{
+			name:     "single_core",
+			cores:    NodeCPUCores{1.0},
+			expected: NodeCPURateCapacities{1e9},
+		},
+		{
+			name:     "multiple_cores",
+			cores:    NodeCPUCores{1.0, 2.0, 3.0},
+			expected: NodeCPURateCapacities{1e9, 2e9, 3e9},
+		},
+		{
+			name:     "fractional_cores",
+			cores:    NodeCPUCores{1.5, 2.75, 3.25},
+			expected: NodeCPURateCapacities{1500e6, 2750e6, 3250e6},
+		},
+		{
+			name:     "round_down_to_nearest_nanosecond",
+			cores:    NodeCPUCores{1.4999999998, 2.1111111111999},
+			expected: NodeCPURateCapacities{1499999999, 2111111111},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			require.Equal(t, tc.expected, tc.cores.ToRateCapacityNanos())
+		})
+	}
+}

pkg/kv/kvserver/asim/tests/datadriven_simulation_test.go

@@ -61,11 +61,11 @@ var runAsimTests = envutil.EnvOrDefaultBool("COCKROACH_RUN_ASIM_TESTS", false)
 //     cpu_per_access=0 raft_cpu_per_write=0
 //
 //   - "gen_cluster" [nodes=<int>] [stores_per_node=<int>]
-//     [store_byte_capacity_gib=<int>] [node_cpu_rate_capacity=<int>]
+//     [store_byte_capacity_gib=<int>] [node_cpu_cores=<float>]
 //     Initialize the cluster generator parameters. On the next call to eval,
 //     the cluster generator is called to create the initial state used in the
 //     simulation. The default values are: nodes=3 stores_per_node=1
-//     store_byte_capacity_gib=256, node_cpu_rate_capacity=0.
+//     store_byte_capacity_gib=256, node_cpu_cores=8.0.
 //
 //   - "load_cluster": config=<name>
 //     Load a defined cluster configuration to be the generated cluster in the
@@ -342,7 +342,7 @@ func TestDataDriven(t *testing.T) {
 				case "gen_cluster":
 					var nodes = 3
 					var storesPerNode = 1
-					var nodeCPURateCapacity = []uint64{config.DefaultNodeCPURateCapacityNanos}
+					var nodeCPUCores = []float64{config.DefaultNodeCPUCores}
 					var region []string
 					var nodesPerRegion []int
 					var storeByteCapacityGiB int64 = 256
@@ -351,16 +351,16 @@ func TestDataDriven(t *testing.T) {
 					scanIfExists(t, d, "store_byte_capacity_gib", &storeByteCapacityGiB)
 					scanIfExists(t, d, "region", &region)
 					scanIfExists(t, d, "nodes_per_region", &nodesPerRegion)
-					scanIfExists(t, d, "node_cpu_rate_capacity", &nodeCPURateCapacity)
+					scanIfExists(t, d, "node_cpu_cores", &nodeCPUCores)
 
 					var buf strings.Builder
-					require.NotEmpty(t, nodeCPURateCapacity)
+					require.NotEmpty(t, nodeCPUCores)
 					{
-						n := len(nodeCPURateCapacity)
-						require.True(t, n == 1 || n == nodes, "need to specify node_cpu_rate_capacity for each node")
+						n := len(nodeCPUCores)
+						require.True(t, n == 1 || n == nodes, "need to specify node_cpu_cores for each node")
 
-						for _, cpct := range nodeCPURateCapacity {
-							if cores := float64(cpct) / 1e9; cores < 1 {
+						for _, cores := range nodeCPUCores {
+							if cores < 1.0 {
 								// TODO(mma): fix up the tests that trigger this warning.
 								// TODO(mma): print a warning whenever the measured CPU utilization
 								// on a node exceeds this capacity, as that's likely not what the test
@@ -377,7 +377,7 @@ func TestDataDriven(t *testing.T) {
 						StoreByteCapacity:   storeByteCapacityGiB << 30,
 						Region:              region,
 						NodesPerRegion:      nodesPerRegion,
-						NodeCPURateCapacity: nodeCPURateCapacity,
+						NodeCPURateCapacity: state.NodeCPUCores(nodeCPUCores).ToRateCapacityNanos(),
 					}
 					return buf.String()
 				case "load_cluster":

pkg/kv/kvserver/asim/tests/testdata/non_rand/mma/heterogeneous_cpu.txt

@@ -9,7 +9,7 @@
 # balance on absolute cpu-nanos. n3 should handle more load due to its higher
 # capacity, but the current implementation doesn't account for this. This is
 # tracked in issue: https://github.com/cockroachdb/cockroach/issues/153777.
-gen_cluster nodes=3 node_cpu_rate_capacity=(8000000000,8000000000,16000000000)
+gen_cluster nodes=3 node_cpu_cores=(8,8,16)
 ----
 
 gen_ranges ranges=200 min_key=1 max_key=10000 placement_type=even

pkg/kv/kvserver/asim/tests/testdata/non_rand/mma/high_cpu.txt

@@ -7,7 +7,7 @@
 #
 # Expected outcome: The allocator should rebalance both replicas and leases to
 # distribute the high-cpu workload more evenly across all 10 nodes.
-gen_cluster nodes=10 node_cpu_rate_capacity=8000000000
+gen_cluster nodes=10 node_cpu_cores=8
 ----
 
 # TODO(wenyihu6): why didn't we balance more replicas/leases - is it because of a very high cpu per range 

pkg/kv/kvserver/asim/tests/testdata/non_rand/mma/high_cpu_25nodes.txt

@@ -2,7 +2,7 @@
 # there is high CPU load imbalance across a large cluster. The test set-up is
 # similar to high_cpu.txt but is on 25 nodes and with 3x the load for two
 # gen_load commands.
-gen_cluster nodes=25 node_cpu_rate_capacity=8000000000
+gen_cluster nodes=25 node_cpu_cores=8
 ----
 
 setting split_queue_enabled=false

pkg/kv/kvserver/asim/tests/testdata/non_rand/mma/high_cpu_able_to_shed_leases.txt

@@ -4,7 +4,7 @@
 # will be able to shed its own leases because it is the leaseholer. There should
 # be a period of lease-rebalancing activity before replica-rebalancing.
 
-gen_cluster nodes=5 node_cpu_rate_capacity=9000000000
+gen_cluster nodes=5 node_cpu_cores=9
 ----
 
 setting split_queue_enabled=false

pkg/kv/kvserver/asim/tests/testdata/non_rand/mma/high_cpu_unable_to_shed_leases.txt

@@ -16,7 +16,7 @@
 # the CPU overloaded s1, so we should observe a period of lease transfers before
 # any replica based rebalancing away from the store occurs.
 
-gen_cluster nodes=5 node_cpu_rate_capacity=9000000000
+gen_cluster nodes=5 node_cpu_cores=9
 ----
 
 setting split_queue_enabled=false