use segregated hashmap to boost the freelist allocate and release performance

Makefile

@@ -5,7 +5,9 @@ GOLDFLAGS="-X main.branch $(BRANCH) -X main.commit $(COMMIT)"
 default: build
 
 race:
-	@go test -v -race -test.run="TestSimulate_(100op|1000op)"
+	@TEST_FREELIST_TYPE=hashmap go test -v -race -test.run="TestSimulate_(100op|1000op)"
+	@echo "array freelist test"
+	@TEST_FREELIST_TYPE=array go test -v -race -test.run="TestSimulate_(100op|1000op)"
 
 fmt:
 	!(gofmt -l -s -d $(shell find . -name \*.go) | grep '[a-z]')
@@ -23,8 +25,14 @@ errcheck:
 	@errcheck -ignorepkg=bytes -ignore=os:Remove go.etcd.io/bbolt
 
 test:
-	go test -timeout 20m -v -coverprofile cover.out -covermode atomic
+	TEST_FREELIST_TYPE=hashmap go test -timeout 20m -v -coverprofile cover.out -covermode atomic
 	# Note: gets "program not an importable package" in out of path builds
-	go test -v ./cmd/bbolt
+	TEST_FREELIST_TYPE=hashmap go test -v ./cmd/bbolt
+
+	@echo "array freelist test"
+
+	@TEST_FREELIST_TYPE=array go test -timeout 20m -v -coverprofile cover.out -covermode atomic
+	# Note: gets "program not an importable package" in out of path builds
+	@TEST_FREELIST_TYPE=array go test -v ./cmd/bbolt
 
 .PHONY: race fmt errcheck test gosimple unused

allocate_test.go

@@ -5,7 +5,7 @@ import (
 )
 
 func TestTx_allocatePageStats(t *testing.T) {
-	f := newFreelist()
+	f := newTestFreelist()
 	ids := []pgid{2, 3}
 	f.readIDs(ids)
 

db.go

@@ -43,6 +43,16 @@ var defaultPageSize = os.Getpagesize()
 // The time elapsed between consecutive file locking attempts.
 const flockRetryTimeout = 50 * time.Millisecond
 
+// FreelistType is the type of the freelist backend
+type FreelistType string
+
+const (
+	// FreelistArrayType indicates backend freelist type is array
+	FreelistArrayType = FreelistType("array")
+	// FreelistMapType indicates backend freelist type is hashmap
+	FreelistMapType = FreelistType("hashmap")
+)
+
 // DB represents a collection of buckets persisted to a file on disk.
 // All data access is performed through transactions which can be obtained through the DB.
 // All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
@@ -70,6 +80,13 @@ type DB struct {
 	// re-sync during recovery.
 	NoFreelistSync bool
 
+	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
+	// dramatic performance degradation if database is large and framentation in freelist is common.
+	// The alternative one is using hashmap, it is faster in almost all circumstances
+	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
+	// The default type is array
+	FreelistType FreelistType
+
 	// When true, skips the truncate call when growing the database.
 	// Setting this to true is only safe on non-ext3/ext4 systems.
 	// Skipping truncation avoids preallocation of hard drive space and
@@ -169,6 +186,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	db.NoGrowSync = options.NoGrowSync
 	db.MmapFlags = options.MmapFlags
 	db.NoFreelistSync = options.NoFreelistSync
+	db.FreelistType = options.FreelistType
 
 	// Set default values for later DB operations.
 	db.MaxBatchSize = DefaultMaxBatchSize
@@ -283,15 +301,15 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 // concurrent accesses being made to the freelist.
 func (db *DB) loadFreelist() {
 	db.freelistLoad.Do(func() {
-		db.freelist = newFreelist()
+		db.freelist = newFreelist(db.FreelistType)
 		if !db.hasSyncedFreelist() {
 			// Reconstruct free list by scanning the DB.
 			db.freelist.readIDs(db.freepages())
 		} else {
 			// Read free list from freelist page.
 			db.freelist.read(db.page(db.meta().freelist))
 		}
-		db.stats.FreePageN = len(db.freelist.getFreePageIDs())
+		db.stats.FreePageN = db.freelist.free_count()
 	})
 }
 
@@ -1005,6 +1023,13 @@ type Options struct {
 	// under normal operation, but requires a full database re-sync during recovery.
 	NoFreelistSync bool
 
+	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
+	// dramatic performance degradation if database is large and framentation in freelist is common.
+	// The alternative one is using hashmap, it is faster in almost all circumstances
+	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
+	// The default type is array
+	FreelistType FreelistType
+
 	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
 	// grab a shared lock (UNIX).
 	ReadOnly bool
@@ -1034,8 +1059,9 @@ type Options struct {
 // DefaultOptions represent the options used if nil options are passed into Open().
 // No timeout is used which will cause Bolt to wait indefinitely for a lock.
 var DefaultOptions = &Options{
-	Timeout:    0,
-	NoGrowSync: false,
+	Timeout:      0,
+	NoGrowSync:   false,
+	FreelistType: FreelistArrayType,
 }
 
 // Stats represents statistics about the database.

db_test.go

@@ -1605,6 +1605,16 @@ func MustOpenDB() *DB {
 // MustOpenDBWithOption returns a new, open DB at a temporary location with given options.
 func MustOpenWithOption(o *bolt.Options) *DB {
 	f := tempfile()
+	if o == nil {
+		o = bolt.DefaultOptions
+	}
+
+	freelistType := bolt.FreelistArrayType
+	if env := os.Getenv(bolt.TestFreelistType); env == string(bolt.FreelistMapType) {
+		freelistType = bolt.FreelistMapType
+	}
+	o.FreelistType = freelistType
+
 	db, err := bolt.Open(f, 0666, o)
 	if err != nil {
 		panic(err)

freelist.go

@@ -14,22 +14,54 @@ type txPending struct {
 	lastReleaseBegin txid   // beginning txid of last matching releaseRange
 }
 
+// pidSet holds the set of starting pgids which have the same span size
+type pidSet map[pgid]struct{}
+
 // freelist represents a list of all pages that are available for allocation.
 // It also tracks pages that have been freed but are still in use by open transactions.
 type freelist struct {
-	ids     []pgid              // all free and available free page ids.
-	allocs  map[pgid]txid       // mapping of txid that allocated a pgid.
-	pending map[txid]*txPending // mapping of soon-to-be free page ids by tx.
-	cache   map[pgid]bool       // fast lookup of all free and pending page ids.
+	freelistType   FreelistType                // freelist type
+	ids            []pgid                      // all free and available free page ids.
+	allocs         map[pgid]txid               // mapping of txid that allocated a pgid.
+	pending        map[txid]*txPending         // mapping of soon-to-be free page ids by tx.
+	cache          map[pgid]bool               // fast lookup of all free and pending page ids.
+	freemaps       map[uint64]pidSet           // key is the size of continuous pages(span), value is a set which contains the starting pgids of same size
+	forwardMap     map[pgid]uint64             // key is start pgid, value is its span size
+	backwardMap    map[pgid]uint64             // key is end pgid, value is its span size
+	allocate       func(txid txid, n int) pgid // the freelist allocate func
+	free_count     func() int                  // the function which gives you free page number
+	mergeSpans     func(ids pgids)             // the mergeSpan func
+	getFreePageIDs func() []pgid               // get free pgids func
+	readIDs        func(pgids []pgid)          // readIDs func reads list of pages and init the freelist
 }
 
 // newFreelist returns an empty, initialized freelist.
-func newFreelist() *freelist {
-	return &freelist{
-		allocs:  make(map[pgid]txid),
-		pending: make(map[txid]*txPending),
-		cache:   make(map[pgid]bool),
+func newFreelist(freelistType FreelistType) *freelist {
+	f := &freelist{
+		freelistType: freelistType,
+		allocs:       make(map[pgid]txid),
+		pending:      make(map[txid]*txPending),
+		cache:        make(map[pgid]bool),
+		freemaps:     make(map[uint64]pidSet),
+		forwardMap:   make(map[pgid]uint64),
+		backwardMap:  make(map[pgid]uint64),
+	}
+
+	if freelistType == FreelistMapType {
+		f.allocate = f.hashmapAllocate
+		f.free_count = f.hashmapFreeCount
+		f.mergeSpans = f.hashmapMergeSpans
+		f.getFreePageIDs = f.hashmapGetFreePageIDs
+		f.readIDs = f.hashmapReadIDs
+	} else {
+		f.allocate = f.arrayAllocate
+		f.free_count = f.arrayFreeCount
+		f.mergeSpans = f.arrayMergeSpans
+		f.getFreePageIDs = f.arrayGetFreePageIDs
+		f.readIDs = f.arrayReadIDs
 	}
+
+	return f
 }
 
 // size returns the size of the page after serialization.
@@ -47,8 +79,8 @@ func (f *freelist) count() int {
 	return f.free_count() + f.pending_count()
 }
 
-// free_count returns count of free pages
-func (f *freelist) free_count() int {
+// arrayFreeCount returns count of free pages(array version)
+func (f *freelist) arrayFreeCount() int {
 	return len(f.ids)
 }
 
@@ -72,9 +104,9 @@ func (f *freelist) copyall(dst []pgid) {
 	mergepgids(dst, f.getFreePageIDs(), m)
 }
 
-// allocate returns the starting page id of a contiguous list of pages of a given size.
+// arrayAllocate returns the starting page id of a contiguous list of pages of a given size.
 // If a contiguous block cannot be found then 0 is returned.
-func (f *freelist) allocate(txid txid, n int) pgid {
+func (f *freelist) arrayAllocate(txid txid, n int) pgid {
 	if len(f.ids) == 0 {
 		return 0
 	}
@@ -160,8 +192,7 @@ func (f *freelist) release(txid txid) {
 			delete(f.pending, tid)
 		}
 	}
-	sort.Sort(m)
-	f.ids = pgids(f.ids).merge(m)
+	f.mergeSpans(m)
 }
 
 // releaseRange moves pending pages allocated within an extent [begin,end] to the free list.
@@ -194,8 +225,7 @@ func (f *freelist) releaseRange(begin, end txid) {
 			delete(f.pending, tid)
 		}
 	}
-	sort.Sort(m)
-	f.ids = pgids(f.ids).merge(m)
+	f.mergeSpans(m)
 }
 
 // rollback removes the pages from a given pending tx.
@@ -222,8 +252,7 @@ func (f *freelist) rollback(txid txid) {
 	}
 	// Remove pages from pending list and mark as free if allocated by txid.
 	delete(f.pending, txid)
-	sort.Sort(m)
-	f.ids = pgids(f.ids).merge(m)
+	f.mergeSpans(m)
 }
 
 // freed returns whether a given page is in the free list.
@@ -249,24 +278,24 @@ func (f *freelist) read(p *page) {
 		f.ids = nil
 	} else {
 		ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx : idx+count]
-		f.ids = make([]pgid, len(ids))
-		copy(f.ids, ids)
 
+		// copy the ids, so we don't modify on the freelist page directly
+		idsCopy := make([]pgid, count)
+		copy(idsCopy, ids)
 		// Make sure they're sorted.
-		sort.Sort(pgids(f.ids))
-	}
+		sort.Sort(pgids(idsCopy))
 
-	// Rebuild the page cache.
-	f.reindex()
+		f.readIDs(idsCopy)
+	}
 }
 
-// readIDs initializes the freelist from a given list of ids.
-func (f *freelist) readIDs(ids []pgid) {
+// arrayReadIDs initializes the freelist from a given list of ids.
+func (f *freelist) arrayReadIDs(ids []pgid) {
 	f.ids = ids
 	f.reindex()
 }
 
-func (f *freelist) getFreePageIDs() []pgid {
+func (f *freelist) arrayGetFreePageIDs() []pgid {
 	return f.ids
 }
 
@@ -322,8 +351,9 @@ func (f *freelist) reload(p *page) {
 
 // reindex rebuilds the free cache based on available and pending free lists.
 func (f *freelist) reindex() {
-	f.cache = make(map[pgid]bool, len(f.getFreePageIDs()))
-	for _, id := range f.getFreePageIDs() {
+	ids := f.getFreePageIDs()
+	f.cache = make(map[pgid]bool, len(ids))
+	for _, id := range ids {
 		f.cache[id] = true
 	}
 	for _, txp := range f.pending {
@@ -332,3 +362,9 @@ func (f *freelist) reindex() {
 		}
 	}
 }
+
+// arrayMergeSpans try to merge list of pages(represented by pgids) with existing spans but using array
+func (f *freelist) arrayMergeSpans(ids pgids) {
+	sort.Sort(ids)
+	f.ids = pgids(f.ids).merge(ids)
+}