store/copr: set upper limit for extra concurrency

store/copr/coprocessor.go

@@ -70,6 +70,7 @@ const (
 	copNextMaxBackoff      = 20000
 	CopSmallTaskRow        = 32 // 32 is the initial batch size of TiKV
 	smallTaskSigma         = 0.5
+	smallConcPerCore       = 20
 )
 
 // CopClient is coprocessor client.
@@ -200,7 +201,7 @@ func (c *CopClient) BuildCopIterator(ctx context.Context, req *kv.Request, vars
 	}
 	if tryRowHint {
 		var smallTasks int
-		smallTasks, it.smallTaskConcurrency = smallTaskConcurrency(tasks)
+		smallTasks, it.smallTaskConcurrency = smallTaskConcurrency(tasks, c.store.numcpu)
 		if len(tasks)-smallTasks < it.concurrency {
 			it.concurrency = len(tasks) - smallTasks
 		}
@@ -580,7 +581,7 @@ func isSmallTask(task *copTask) bool {
 
 // smallTaskConcurrency counts the small tasks of tasks,
 // then returns the task count and extra concurrency for small tasks.
-func smallTaskConcurrency(tasks []*copTask) (int, int) {
+func smallTaskConcurrency(tasks []*copTask, numcpu int) (int, int) {
 	res := 0
 	for _, task := range tasks {
 		if isSmallTask(task) {
@@ -592,8 +593,15 @@ func smallTaskConcurrency(tasks []*copTask) (int, int) {
 	}
 	// Calculate the extra concurrency for small tasks
 	// extra concurrency = tasks / (1 + sigma * sqrt(log(tasks ^ 2)))
-	extraConc := float64(res) / (1 + smallTaskSigma*math.Sqrt(2*math.Log(float64(res))))
-	return res, int(extraConc)
+	extraConc := int(float64(res) / (1 + smallTaskSigma*math.Sqrt(2*math.Log(float64(res)))))
+	if numcpu <= 0 {
+		numcpu = 1
+	}
+	smallTaskConcurrencyLimit := smallConcPerCore * numcpu
+	if extraConc > smallTaskConcurrencyLimit {
+		extraConc = smallTaskConcurrencyLimit
+	}
+	return res, extraConc
 }
 
 type copIterator struct {

store/copr/coprocessor_test.go

@@ -696,7 +696,7 @@ func TestBasicSmallTaskConc(t *testing.T) {
 	require.True(t, isSmallTask(&copTask{RowCountHint: 6}))
 	require.True(t, isSmallTask(&copTask{RowCountHint: CopSmallTaskRow}))
 	require.False(t, isSmallTask(&copTask{RowCountHint: CopSmallTaskRow + 1}))
-	_, conc := smallTaskConcurrency([]*copTask{})
+	_, conc := smallTaskConcurrency([]*copTask{}, 16)
 	require.GreaterOrEqual(t, conc, 0)
 }
 
@@ -734,7 +734,7 @@ func TestBuildCopTasksWithRowCountHint(t *testing.T) {
 	require.Equal(t, tasks[2].RowCountHint, 3)
 	// task[3] ["t"-"x", "y"-"z"]
 	require.Equal(t, tasks[3].RowCountHint, 3+CopSmallTaskRow)
-	_, conc := smallTaskConcurrency(tasks)
+	_, conc := smallTaskConcurrency(tasks, 16)
 	require.Equal(t, conc, 1)
 
 	ranges = buildCopRanges("a", "c", "d", "e", "h", "x", "y", "z")
@@ -753,7 +753,7 @@ func TestBuildCopTasksWithRowCountHint(t *testing.T) {
 	require.Equal(t, tasks[2].RowCountHint, 3)
 	// task[3] ["t"-"x", "y"-"z"]
 	require.Equal(t, tasks[3].RowCountHint, 6)
-	_, conc = smallTaskConcurrency(tasks)
+	_, conc = smallTaskConcurrency(tasks, 16)
 	require.Equal(t, conc, 2)
 
 	// cross-region long range
@@ -774,3 +774,20 @@ func TestBuildCopTasksWithRowCountHint(t *testing.T) {
 	// task[3] ["t"-"z"]
 	require.Equal(t, tasks[3].RowCountHint, 10)
 }
+
+func TestSmallTaskConcurrencyLimit(t *testing.T) {
+	smallTaskCount := 1000
+	tasks := make([]*copTask, 0, smallTaskCount)
+	for i := 0; i < smallTaskCount; i++ {
+		tasks = append(tasks, &copTask{
+			RowCountHint: 1,
+		})
+	}
+	count, conc := smallTaskConcurrency(tasks, 1)
+	require.Equal(t, smallConcPerCore, conc)
+	require.Equal(t, smallTaskCount, count)
+	// also handle 0 value.
+	count, conc = smallTaskConcurrency(tasks, 0)
+	require.Equal(t, smallConcPerCore, conc)
+	require.Equal(t, smallTaskCount, count)
+}

store/copr/store.go

@@ -17,6 +17,7 @@ package copr
 import (
 	"context"
 	"math/rand"
+	"runtime"
 	"sync/atomic"
 	"time"
 
@@ -76,6 +77,7 @@ type Store struct {
 	*kvStore
 	coprCache       *coprCache
 	replicaReadSeed uint32
+	numcpu          int
 }
 
 // NewStore creates a new store instance.
@@ -90,6 +92,7 @@ func NewStore(s *tikv.KVStore, coprCacheConfig *config.CoprocessorCache) (*Store
 		kvStore:         &kvStore{store: s},
 		coprCache:       coprCache,
 		replicaReadSeed: rand.Uint32(),
+		numcpu:          runtime.GOMAXPROCS(0),
 	}, nil
 }