internal/lsp/cache: optimize Snapshot.clone

This change replaces the single large map used for snapshot.goFiles
by a map of 256 stripes, each of which becomes immutable once shared.
This optimizes the common case in which the copy is nearly identical
to the original.

We still need to visit each map entry to see whether it needs to be
deleted (which is rare) and to inherit the handle in the usual case.
This is now done concurrently.

Also, share the (logically immutable) []PackageIDs slices across
old and new snapshots. This was worth 5% of CPU and 1/3 of allocations
(all small).

Benchmark on darwin/arm64 shows a 29% reduction for DidChange.

$ go test -v ./gopls/internal/regtest/bench -run=TestBenchmarkDidChange -didchange_dir=$HOME/w/kubernetes -didchange_file=pkg/util/hash/hash.go

Before:
BenchmarkStatistics	     100	  22955469 ns/op	11308095 B/op	   47412 allocs/op
BenchmarkStatistics	     100	  23454630 ns/op	11226742 B/op	   46882 allocs/op
BenchmarkStatistics	     100	  23618532 ns/op	11258619 B/op	   47068 allocs/op

After goFilesMap:
BenchmarkStatistics	     100	  16643972 ns/op	 8770787 B/op	   46238 allocs/op
BenchmarkStatistics	     100	  17805864 ns/op	 8862926 B/op	   46762 allocs/op
BenchmarkStatistics	     100	  18618255 ns/op	 9308864 B/op	   49776 allocs/op

After goFilesMap and ids sharing:
BenchmarkStatistics          100          16703623 ns/op         8772626 B/op      33812 allocs/op
BenchmarkStatistics          100          16927378 ns/op         8529491 B/op      32328 allocs/op
BenchmarkStatistics          100          16632762 ns/op         8557533 B/op      32497 allocs/op

Also:
- Add comments documenting findings of profiling.
- preallocate slice for knownSubdirs.
- remove unwanted loop over slice in Generation.Inherit

Updates golang/go#45686

Change-Id: Id953699191b8404cf36ba3a7ab9cd78b1d19c0a2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/410176
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>

internal/lsp/cache/cache.go

@@ -183,8 +183,14 @@ func (h *fileHandle) Read() ([]byte, error) {
 	return h.bytes, h.err
 }
 
+// hashContents returns a string of hex digits denoting the hash of contents.
+//
+// TODO(adonovan): opt: use [32]byte array as a value more widely and convert
+// to hex digits on demand (rare). The array is larger when it appears as a
+// struct field (32B vs 16B) but smaller overall (string data is 64B), has
+// better locality, and is more efficiently hashed by runtime maps.
 func hashContents(contents []byte) string {
-	return fmt.Sprintf("%x", sha256.Sum256(contents))
+	return fmt.Sprintf("%64x", sha256.Sum256(contents))
 }
 
 var cacheIndex, sessionIndex, viewIndex int64

internal/lsp/cache/session.go

@@ -234,7 +234,7 @@ func (s *Session) createView(ctx context.Context, name string, folder span.URI,
 		packages:          make(map[packageKey]*packageHandle),
 		meta:              NewMetadataGraph(),
 		files:             make(map[span.URI]source.VersionedFileHandle),
-		goFiles:           make(map[parseKey]*parseGoHandle),
+		goFiles:           newGoFileMap(),
 		symbols:           make(map[span.URI]*symbolHandle),
 		actions:           make(map[actionKey]*actionHandle),
 		workspacePackages: make(map[PackageID]PackagePath),

internal/lsp/cache/snapshot.go

@@ -76,7 +76,7 @@ type snapshot struct {
 	files map[span.URI]source.VersionedFileHandle
 
 	// goFiles maps a parseKey to its parseGoHandle.
-	goFiles map[parseKey]*parseGoHandle
+	goFiles *goFileMap
 
 	// TODO(rfindley): consider merging this with files to reduce burden on clone.
 	symbols map[span.URI]*symbolHandle
@@ -663,16 +663,17 @@ func (s *snapshot) transitiveReverseDependencies(id PackageID, ids map[PackageID
 func (s *snapshot) getGoFile(key parseKey) *parseGoHandle {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	return s.goFiles[key]
+	return s.goFiles.get(key)
 }
 
 func (s *snapshot) addGoFile(key parseKey, pgh *parseGoHandle) *parseGoHandle {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	if existing, ok := s.goFiles[key]; ok {
-		return existing
+
+	if prev := s.goFiles.get(key); prev != nil {
+		return prev
 	}
-	s.goFiles[key] = pgh
+	s.goFiles.set(key, pgh)
 	return pgh
 }
 
@@ -811,6 +812,8 @@ func (s *snapshot) fileWatchingGlobPatterns(ctx context.Context) map[string]stru
 	patterns := map[string]struct{}{
 		fmt.Sprintf("**/*.{%s}", extensions): {},
 	}
+
+	// Add a pattern for each Go module in the workspace that is not within the view.
 	dirs := s.workspace.dirs(ctx, s)
 	for _, dir := range dirs {
 		dirName := dir.Filename()
@@ -830,14 +833,19 @@ func (s *snapshot) fileWatchingGlobPatterns(ctx context.Context) map[string]stru
 	// contain Go code (golang/go#42348). To handle this, explicitly watch all
 	// of the directories in the workspace. We find them by adding the
 	// directories of every file in the snapshot's workspace directories.
-	var dirNames []string
-	for _, uri := range s.getKnownSubdirs(dirs) {
-		dirNames = append(dirNames, uri.Filename())
-	}
-	sort.Strings(dirNames)
-	if len(dirNames) > 0 {
+	// There may be thousands.
+	knownSubdirs := s.getKnownSubdirs(dirs)
+	if n := len(knownSubdirs); n > 0 {
+		dirNames := make([]string, 0, n)
+		for _, uri := range knownSubdirs {
+			dirNames = append(dirNames, uri.Filename())
+		}
+		sort.Strings(dirNames)
+		// The double allocation of Sprintf(Join()) accounts for 8%
+		// of DidChange, but specializing doesn't appear to help. :(
 		patterns[fmt.Sprintf("{%s}", strings.Join(dirNames, ","))] = struct{}{}
 	}
+
 	return patterns
 }
 
@@ -874,7 +882,7 @@ func (s *snapshot) getKnownSubdirs(wsDirs []span.URI) []span.URI {
 	}
 	s.unprocessedSubdirChanges = nil
 
-	var result []span.URI
+	result := make([]span.URI, 0, len(s.knownSubdirs))
 	for uri := range s.knownSubdirs {
 		result = append(result, uri)
 	}
@@ -1719,7 +1727,7 @@ func (s *snapshot) clone(ctx, bgCtx context.Context, changes map[span.URI]*fileC
 		packages:          make(map[packageKey]*packageHandle, len(s.packages)),
 		actions:           make(map[actionKey]*actionHandle, len(s.actions)),
 		files:             make(map[span.URI]source.VersionedFileHandle, len(s.files)),
-		goFiles:           make(map[parseKey]*parseGoHandle, len(s.goFiles)),
+		goFiles:           s.goFiles.clone(),
 		symbols:           make(map[span.URI]*symbolHandle, len(s.symbols)),
 		workspacePackages: make(map[PackageID]PackagePath, len(s.workspacePackages)),
 		unloadableFiles:   make(map[span.URI]struct{}, len(s.unloadableFiles)),
@@ -1764,12 +1772,27 @@ func (s *snapshot) clone(ctx, bgCtx context.Context, changes map[span.URI]*fileC
 		result.parseWorkHandles[k] = v
 	}
 
-	for k, v := range s.goFiles {
-		if _, ok := changes[k.file.URI]; ok {
-			continue
+	// Copy the handles of all Go source files.
+	// There may be tens of thousands of files,
+	// but changes are typically few, so we
+	// use a striped map optimized for this case
+	// and visit its stripes in parallel.
+	var (
+		toDeleteMu sync.Mutex
+		toDelete   []parseKey
+	)
+	s.goFiles.forEachConcurrent(func(k parseKey, v *parseGoHandle) {
+		if changes[k.file.URI] == nil {
+			// no change (common case)
+			newGen.Inherit(v.handle)
+		} else {
+			toDeleteMu.Lock()
+			toDelete = append(toDelete, k)
+			toDeleteMu.Unlock()
 		}
-		newGen.Inherit(v.handle)
-		result.goFiles[k] = v
+	})
+	for _, k := range toDelete {
+		result.goFiles.delete(k)
 	}
 
 	// Copy all of the go.mod-related handles. They may be invalidated later,
@@ -1975,21 +1998,34 @@ func (s *snapshot) clone(ctx, bgCtx context.Context, changes map[span.URI]*fileC
 	deleteInvalidMetadata := forceReloadMetadata || workspaceModeChanged
 	idsInSnapshot := map[PackageID]bool{} // track all known IDs
 	for uri, ids := range s.meta.ids {
-		var resultIDs []PackageID
-		for _, id := range ids {
+		// Optimization: ids slices are typically numerous, short (<3),
+		// and rarely modified by this loop, so don't allocate copies
+		// until necessary.
+		var resultIDs []PackageID // nil implies equal to ids[:i:i]
+		for i, id := range ids {
 			if skipID[id] || deleteInvalidMetadata && idsToInvalidate[id] {
+				resultIDs = ids[:i:i] // unshare
 				continue
 			}
 			// The ID is not reachable from any workspace package, so it should
 			// be deleted.
 			if !reachableID[id] {
+				resultIDs = ids[:i:i] // unshare
 				continue
 			}
 			idsInSnapshot[id] = true
-			resultIDs = append(resultIDs, id)
+			if resultIDs != nil {
+				resultIDs = append(resultIDs, id)
+			}
+		}
+		if resultIDs == nil {
+			resultIDs = ids
 		}
 		result.meta.ids[uri] = resultIDs
 	}
+	// TODO(adonovan): opt: represent PackageID as an index into a process-global
+	// dup-free list of all package names ever seen, then use a bitmap instead of
+	// a hash table for "PackageSet" (e.g. idsInSnapshot).
 
 	// Copy the package metadata. We only need to invalidate packages directly
 	// containing the affected file, and only if it changed in a relevant way.
@@ -2259,7 +2295,7 @@ func metadataChanges(ctx context.Context, lockedSnapshot *snapshot, oldFH, newFH
 // lockedSnapshot must be locked.
 func peekOrParse(ctx context.Context, lockedSnapshot *snapshot, fh source.FileHandle, mode source.ParseMode) (*source.ParsedGoFile, error) {
 	key := parseKey{file: fh.FileIdentity(), mode: mode}
-	if pgh := lockedSnapshot.goFiles[key]; pgh != nil {
+	if pgh := lockedSnapshot.goFiles.get(key); pgh != nil {
 		cached := pgh.handle.Cached(lockedSnapshot.generation)
 		if cached != nil {
 			cached := cached.(*parseGoData)
@@ -2547,3 +2583,107 @@ func readGoSum(dst map[module.Version][]string, file string, data []byte) error
 	}
 	return nil
 }
+
+// -- goFileMap --
+
+// A goFileMap is conceptually a map[parseKey]*parseGoHandle,
+// optimized for cloning all or nearly all entries.
+type goFileMap struct {
+	// The map is represented as a map of 256 stripes, one per
+	// distinct value of the top 8 bits of key.file.Hash.
+	// Each stripe has an associated boolean indicating whether it
+	// is shared, and thus immutable, and thus must be copied before any update.
+	// (The bits could be packed but it hasn't been worth it yet.)
+	stripes   [256]map[parseKey]*parseGoHandle
+	exclusive [256]bool // exclusive[i] means stripe[i] is not shared and may be safely mutated
+}
+
+// newGoFileMap returns a new empty goFileMap.
+func newGoFileMap() *goFileMap {
+	return new(goFileMap) // all stripes are shared (non-exclusive) nil maps
+}
+
+// clone returns a copy of m.
+// For concurrency, it counts as an update to m.
+func (m *goFileMap) clone() *goFileMap {
+	m.exclusive = [256]bool{} // original and copy are now nonexclusive
+	copy := *m
+	return &copy
+}
+
+// get returns the value for key k.
+func (m *goFileMap) get(k parseKey) *parseGoHandle {
+	return m.stripes[m.hash(k)][k]
+}
+
+// set updates the value for key k to v.
+func (m *goFileMap) set(k parseKey, v *parseGoHandle) {
+	m.unshare(k)[k] = v
+}
+
+// delete deletes the value for key k, if any.
+func (m *goFileMap) delete(k parseKey) {
+	// TODO(adonovan): opt?: skip unshare if k isn't present.
+	delete(m.unshare(k), k)
+}
+
+// forEachConcurrent calls f for each entry in the map.
+// Calls may be concurrent.
+// f must not modify m.
+func (m *goFileMap) forEachConcurrent(f func(parseKey, *parseGoHandle)) {
+	// Visit stripes in parallel chunks.
+	const p = 16 // concurrency level
+	var wg sync.WaitGroup
+	wg.Add(p)
+	for i := 0; i < p; i++ {
+		chunk := m.stripes[i*p : (i+1)*p]
+		go func() {
+			for _, stripe := range chunk {
+				for k, v := range stripe {
+					f(k, v)
+				}
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}
+
+// -- internal--
+
+// hash returns 8 bits from the key's file digest.
+func (m *goFileMap) hash(k parseKey) int {
+	h := k.file.Hash
+	if h == "" {
+		// Sadly the Hash isn't always a hash because cache.GetFile may
+		// successfully return a *fileHandle containing an error and no hash.
+		// Lump the duds together for now.
+		// TODO(adonovan): fix the underlying bug.
+		return 0
+	}
+	return unhex(h[0])<<4 | unhex(h[1])
+}
+
+// unhex returns the value of a valid hex digit.
+func unhex(b byte) int {
+	if '0' <= b && b <= '9' {
+		return int(b - '0')
+	}
+	return int(b) & ^0x20 - 'A' + 0xA // [a-fA-F]
+}
+
+// unshare makes k's stripe exclusive, allocating a copy if needed, and returns it.
+func (m *goFileMap) unshare(k parseKey) map[parseKey]*parseGoHandle {
+	i := m.hash(k)
+	if !m.exclusive[i] {
+		m.exclusive[i] = true
+
+		// Copy the map.
+		copy := make(map[parseKey]*parseGoHandle, len(m.stripes[i]))
+		for k, v := range m.stripes[i] {
+			copy[k] = v
+		}
+		m.stripes[i] = copy
+	}
+	return m.stripes[i]
+}

internal/lsp/source/view.go

@@ -532,7 +532,7 @@ type FileHandle interface {
 type FileIdentity struct {
 	URI span.URI
 
-	// Identifier represents a unique identifier for the file's content.
+	// Hash is a string of hex digits denoting the cryptographic digest of the file's content.
 	Hash string
 }
 

internal/memoize/memoize.go

@@ -234,19 +234,18 @@ func (s *Store) DebugOnlyIterate(f func(k, v interface{})) {
 	}
 }
 
-func (g *Generation) Inherit(hs ...*Handle) {
-	for _, h := range hs {
-		if atomic.LoadUint32(&g.destroyed) != 0 {
-			panic("inherit on generation " + g.name + " destroyed by " + g.destroyedBy)
-		}
-
-		h.mu.Lock()
-		if h.state == stateDestroyed {
-			panic(fmt.Sprintf("inheriting destroyed handle %#v (type %T) into generation %v", h.key, h.key, g.name))
-		}
-		h.generations[g] = struct{}{}
-		h.mu.Unlock()
+// Inherit makes h valid in generation g. It is concurrency-safe.
+func (g *Generation) Inherit(h *Handle) {
+	if atomic.LoadUint32(&g.destroyed) != 0 {
+		panic("inherit on generation " + g.name + " destroyed by " + g.destroyedBy)
 	}
+
+	h.mu.Lock()
+	if h.state == stateDestroyed {
+		panic(fmt.Sprintf("inheriting destroyed handle %#v (type %T) into generation %v", h.key, h.key, g.name))
+	}
+	h.generations[g] = struct{}{}
+	h.mu.Unlock()
 }
 
 // Cached returns the value associated with a handle.

internal/memoize/memoize_test.go

@@ -87,7 +87,8 @@ func TestCleanup(t *testing.T) {
 	expectGet(t, h1, g1, &v1)
 	expectGet(t, h2, g1, &v2)
 	g2 := s.Generation("g2")
-	g2.Inherit(h1, h2)
+	g2.Inherit(h1)
+	g2.Inherit(h2)
 
 	g1.Destroy("TestCleanup")
 	expectGet(t, h1, g2, &v1)

internal/span/uri.go

@@ -35,6 +35,10 @@ func (uri URI) Filename() string {
 }
 
 func filename(uri URI) (string, error) {
+	// This function is frequently called and its cost is
+	// dominated by the allocation of a net.URL.
+	// TODO(adonovan): opt: replace by a bespoke parseFileURI
+	// function that doesn't allocate.
 	if uri == "" {
 		return "", nil
 	}
@@ -80,6 +84,10 @@ func URIFromURI(s string) URI {
 	return URI(u.String())
 }
 
+// CompareURI performs a three-valued comparison of two URIs.
+// Lexically unequal URIs may compare equal if they are "file:" URIs
+// that share the same base name (ignoring case) and denote the same
+// file device/inode, according to stat(2).
 func CompareURI(a, b URI) int {
 	if equalURI(a, b) {
 		return 0