Speed up DDL execution by send DDL concurrently (pingcap#377)

* task, restore: send DDLs parallelly

* restore: use moded id to index sessionpool

* store: return nil DB when no TiDB session provided

* *: use isolated DB instead of Session

* *: fix some mis-pushed files :|

* *: rename session pool to DB pool

* restore: rename some sessions to dbs

* restore: add some todos

Co-authored-by: 3pointer <luancheng@pingcap.com>

pkg/restore/client.go

@@ -59,9 +59,18 @@ type Client struct {
 	workerPool   *utils.WorkerPool
 	tlsConf      *tls.Config
 
-	databases       map[string]*utils.Database
-	ddlJobs         []*model.Job
-	backupMeta      *backup.BackupMeta
+	databases  map[string]*utils.Database
+	ddlJobs    []*model.Job
+	backupMeta *backup.BackupMeta
+	// TODO Remove this field or replace it with a []*DB,
+	// since https://github.com/pingcap/br/pull/377 needs more DBs to speed up DDL execution.
+	// And for now, we must inject a pool of DBs to `Client.GoCreateTables`, otherwise there would be a race condition.
+	// Which is dirty: why we need DBs from different sources?
+	// By replace it with a []*DB, we can remove the dirty parameter of `Client.GoCreateTable`,
+	// along with them in some private functions.
+	// Before you do it, you can firstly read discussions at
+	// https://github.com/pingcap/br/pull/377#discussion_r446594501,
+	// this probably isn't as easy and it seems like (however, not hard, too :D)
 	db              *DB
 	rateLimit       uint64
 	isOnline        bool
@@ -337,7 +346,7 @@ func (rc *Client) CreateTables(
 	for i, t := range tables {
 		tbMapping[t.Info.Name.String()] = i
 	}
-	dataCh := rc.GoCreateTables(context.TODO(), dom, tables, newTS, errCh)
+	dataCh := rc.GoCreateTables(context.TODO(), dom, tables, newTS, nil, errCh)
 	for et := range dataCh {
 		rules := et.RewriteRule
 		rewriteRules.Table = append(rewriteRules.Table, rules.Table...)
@@ -359,11 +368,24 @@ func (rc *Client) CreateTables(
 	return rewriteRules, newTables, nil
 }
 
-func (rc *Client) createTable(dom *domain.Domain, table *utils.Table, newTS uint64) (CreatedTable, error) {
+func (rc *Client) createTable(
+	ctx context.Context,
+	db *DB,
+	dom *domain.Domain,
+	table *utils.Table,
+	newTS uint64,
+) (CreatedTable, error) {
+	if db == nil {
+		db = rc.db
+	}
+
 	if rc.IsSkipCreateSQL() {
 		log.Info("skip create table and alter autoIncID", zap.Stringer("table", table.Info.Name))
 	} else {
-		err := rc.db.CreateTable(rc.ctx, table)
+		// don't use rc.ctx here...
+		// remove the ctx field of Client would be a great work,
+		// we just take a small step here :<
+		err := db.CreateTable(ctx, table)
 		if err != nil {
 			return CreatedTable{}, err
 		}
@@ -382,22 +404,25 @@ func (rc *Client) createTable(dom *domain.Domain, table *utils.Table, newTS uint
 }
 
 // GoCreateTables create tables, and generate their information.
+// this function will use workers as the same number of sessionPool,
+// leave sessionPool nil to send DDLs sequential.
 func (rc *Client) GoCreateTables(
 	ctx context.Context,
 	dom *domain.Domain,
 	tables []*utils.Table,
 	newTS uint64,
+	dbPool []*DB,
 	errCh chan<- error,
 ) <-chan CreatedTable {
 	// Could we have a smaller size of tables?
 	outCh := make(chan CreatedTable, len(tables))
-	createOneTable := func(t *utils.Table) error {
+	createOneTable := func(db *DB, t *utils.Table) error {
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
 		default:
 		}
-		rt, err := rc.createTable(dom, t, newTS)
+		rt, err := rc.createTable(ctx, db, dom, t, newTS)
 		if err != nil {
 			log.Error("create table failed",
 				zap.Error(err),
@@ -412,16 +437,46 @@ func (rc *Client) GoCreateTables(
 		outCh <- rt
 		return nil
 	}
+	startWork := func(t *utils.Table, done func()) {
+		defer done()
+		if err := createOneTable(nil, t); err != nil {
+			errCh <- err
+			return
+		}
+	}
+	if len(dbPool) > 0 {
+		workers := utils.NewWorkerPool(uint(len(dbPool)), "DDL workers")
+		startWork = func(t *utils.Table, done func()) {
+			workers.ApplyWithID(func(id uint64) {
+				defer done()
+				selectedDB := int(id) % len(dbPool)
+				if err := createOneTable(dbPool[selectedDB], t); err != nil {
+					errCh <- err
+					return
+				}
+			})
+		}
+	}
+
 	go func() {
+		// TODO replace it with an errgroup
+		wg := new(sync.WaitGroup)
 		defer close(outCh)
 		defer log.Info("all tables created")
+		defer func() {
+			if len(dbPool) > 0 {
+				for _, db := range dbPool {
+					db.Close()
+				}
+			}
+		}()
 
 		for _, table := range tables {
-			if err := createOneTable(table); err != nil {
-				errCh <- err
-				return
-			}
+			tbl := table
+			wg.Add(1)
+			startWork(tbl, wg.Done)
 		}
+		wg.Wait()
 	}()
 	return outCh
 }

pkg/restore/util.go

@@ -141,6 +141,19 @@ func GetSSTMetaFromFile(
 	}
 }
 
+// MakeDBPool makes a session pool with specficated size by sessionFactory.
+func MakeDBPool(size uint, dbFactory func() (*DB, error)) ([]*DB, error) {
+	dbPool := make([]*DB, 0, size)
+	for i := uint(0); i < size; i++ {
+		db, e := dbFactory()
+		if e != nil {
+			return dbPool, e
+		}
+		dbPool = append(dbPool, db)
+	}
+	return dbPool, nil
+}
+
 // EstimateRangeSize estimates the total range count by file.
 func EstimateRangeSize(files []*backup.File) int {
 	result := 0

pkg/task/restore.go

@@ -31,6 +31,7 @@ const (
 
 	defaultRestoreConcurrency = 128
 	maxRestoreBatchSizeLimit  = 256
+	defaultDDLConcurrency     = 16
 )
 
 var (
@@ -192,7 +193,21 @@ func RunRestore(c context.Context, g glue.Glue, cmdName string, cfg *RestoreConf
 
 	// We make bigger errCh so we won't block on multi-part failed.
 	errCh := make(chan error, 32)
-	tableStream := client.GoCreateTables(ctx, mgr.GetDomain(), tables, newTS, errCh)
+	// Maybe allow user modify the DDL concurrency isn't necessary,
+	// because executing DDL is really I/O bound (or, algorithm bound?),
+	// and we cost most of time at waiting DDL jobs be enqueued.
+	// So these jobs won't be faster or slower when machine become faster or slower,
+	// hence make it a fixed value would be fine.
+	dbPool, err := restore.MakeDBPool(defaultDDLConcurrency, func() (*restore.DB, error) {
+		return restore.NewDB(g, mgr.GetTiKV())
+	})
+	if err != nil {
+		log.Warn("create session pool failed, we will send DDLs only by created sessions",
+			zap.Error(err),
+			zap.Int("sessionCount", len(dbPool)),
+		)
+	}
+	tableStream := client.GoCreateTables(ctx, mgr.GetDomain(), tables, newTS, dbPool, errCh)
 	if len(files) == 0 {
 		log.Info("no files, empty databases and tables are restored")
 		summary.SetSuccessStatus(true)

pkg/utils/worker.go

@@ -21,7 +21,7 @@ type Worker struct {
 }
 
 type taskFunc func()
-type identifiedTaskFunc func(id uint64)
+type identifiedTaskFunc func(uint64)
 
 // NewWorkerPool returns a WorkPool.
 func NewWorkerPool(limit uint, name string) *WorkerPool {