internal/cache: add robinHoodMap

Add `robinHoodMap` which is a hash map implemented using Robin Hood
hashing. `robinHoodMap` is specialized to use `key` and `*entry` as the
key and value for the map respectively. The memory for the backing array
is manually allocated. This lowers GC pressure by moving the allocated
memory out of the Go heap, and by moving all of `*entry` pointers out of
the Go heap.

Use `robinHoodMap` for both the `shard.blocks` and `shard.files` maps.

Author: petermattis

internal/cache/clockpro.go

@@ -19,7 +19,9 @@ package cache // import "github.com/cockroachdb/pebble/internal/cache"
 
 import (
 	"fmt"
+	"os"
 	"runtime"
+	"runtime/debug"
 	"sync"
 	"sync/atomic"
 
@@ -37,6 +39,17 @@ type key struct {
 	offset uint64
 }
 
+// file returns the "file key" for the receiver. This is the key used for the
+// shard.files map.
+func (k key) file() key {
+	k.offset = 0
+	return k
+}
+
+func (k key) String() string {
+	return fmt.Sprintf("%d/%d/%d", k.id, k.fileNum, k.offset)
+}
+
 // Handle provides a strong reference to an entry in the cache. The reference
 // does not pin the entry in the cache, but it does prevent the underlying byte
 // slice from being reused. When entry is non-nil, value is initialized to
@@ -107,8 +120,17 @@ type shard struct {
 	reservedSize int64
 	maxSize      int64
 	coldTarget   int64
-	blocks       map[key]*entry     // fileNum+offset -> block
-	files        map[fileKey]*entry // fileNum -> list of blocks
+	blocks       robinHoodMap // fileNum+offset -> block
+	files        robinHoodMap // fileNum -> list of blocks
+
+	// The blocks and files maps store values in manually managed memory that is
+	// invisible to the Go GC. This is fine for Value and entry objects that are
+	// stored in manually managed memory. Auto Values and the associated auto
+	// entries need to have a reference that the Go GC is aware of to prevent
+	// them from being reclaimed. The entries map provides this reference. When
+	// the "invariants" build tag is set, all Value and entry objects are Go
+	// allocated and the entries map will contain a reference to every entry.
+	entries map[*entry]struct{}
 
 	handHot  *entry
 	handCold *entry
@@ -121,7 +143,7 @@ type shard struct {
 
 func (c *shard) Get(id, fileNum, offset uint64) Handle {
 	c.mu.RLock()
-	e := c.blocks[key{fileKey{id, fileNum}, offset}]
+	e := c.blocks.Get(key{fileKey{id, fileNum}, offset})
 	var value *Value
 	if e != nil {
 		value = e.getValue()
@@ -160,7 +182,7 @@ func (c *shard) Set(id, fileNum, offset uint64, value *Value) Handle {
 	defer c.mu.Unlock()
 
 	k := key{fileKey{id, fileNum}, offset}
-	e := c.blocks[k]
+	e := c.blocks.Get(k)
 	if e != nil && e.manual != value.manual() {
 		panic(fmt.Sprintf("pebble: inconsistent caching of manual Value: entry=%t vs value=%t",
 			e.manual, value.manual()))
@@ -231,7 +253,7 @@ func (c *shard) Delete(id, fileNum, offset uint64) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	e := c.blocks[key{fileKey{id, fileNum}, offset}]
+	e := c.blocks.Get(key{fileKey{id, fileNum}, offset})
 	if e == nil {
 		return
 	}
@@ -243,7 +265,8 @@ func (c *shard) EvictFile(id, fileNum uint64) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	blocks := c.files[fileKey{id, fileNum}]
+	fkey := key{fileKey{id, fileNum}, 0}
+	blocks := c.files.Get(fkey)
 	if blocks == nil {
 		return
 	}
@@ -291,7 +314,12 @@ func (c *shard) metaAdd(key key, e *entry) bool {
 		return false
 	}
 
-	c.blocks[key] = e
+	c.blocks.Put(key, e)
+	if !e.managed {
+		// Go allocated entries need to be referenced from Go memory. The entries
+		// map provides that reference.
+		c.entries[e] = struct{}{}
+	}
 
 	if c.handHot == nil {
 		// first element
@@ -306,8 +334,9 @@ func (c *shard) metaAdd(key key, e *entry) bool {
 		c.handCold = c.handCold.prev()
 	}
 
-	if fileBlocks := c.files[key.fileKey]; fileBlocks == nil {
-		c.files[key.fileKey] = e
+	fkey := key.file()
+	if fileBlocks := c.files.Get(fkey); fileBlocks == nil {
+		c.files.Put(fkey, e)
 	} else {
 		fileBlocks.linkFile(e)
 	}
@@ -323,7 +352,12 @@ func (c *shard) metaDel(e *entry) {
 	}
 	e.setValue(nil)
 
-	delete(c.blocks, e.key)
+	c.blocks.Delete(e.key)
+	if !e.managed {
+		// Go allocated entries need to be referenced from Go memory. The entries
+		// map provides that reference.
+		delete(c.entries, e)
+	}
 
 	if e == c.handHot {
 		c.handHot = c.handHot.prev()
@@ -342,10 +376,11 @@ func (c *shard) metaDel(e *entry) {
 		c.handTest = nil
 	}
 
+	fkey := e.key.file()
 	if next := e.unlinkFile(); e == next {
-		delete(c.files, e.key.fileKey)
+		c.files.Delete(fkey)
 	} else {
-		c.files[e.key.fileKey] = next
+		c.files.Put(fkey, next)
 	}
 
 	c.metaCheck(e)
@@ -354,21 +389,28 @@ func (c *shard) metaDel(e *entry) {
 // Check that the specified entry is not referenced by the cache.
 func (c *shard) metaCheck(e *entry) {
 	if invariants.Enabled {
-		for _, t := range c.blocks {
-			if e == t {
-				panic("not reached")
-			}
+		if _, ok := c.entries[e]; ok {
+			fmt.Fprintf(os.Stderr, "%p: %s unexpectedly found in entries map\n%s",
+				e, e.key, debug.Stack())
+			os.Exit(1)
 		}
-		for _, t := range c.files {
-			if e == t {
-				panic("not reached")
-			}
+		if c.blocks.findByValue(e) != nil {
+			fmt.Fprintf(os.Stderr, "%p: %s unexpectedly found in blocks map\n%s\n%s",
+				e, e.key, &c.blocks, debug.Stack())
+			os.Exit(1)
+		}
+		if c.files.findByValue(e) != nil {
+			fmt.Fprintf(os.Stderr, "%p: %s unexpectedly found in files map\n%s\n%s",
+				e, e.key, &c.files, debug.Stack())
+			os.Exit(1)
 		}
 		// NB: c.hand{Hot,Cold,Test} are pointers into a single linked list. We
 		// only have to traverse one of them to check all of them.
 		for t := c.handHot.next(); t != c.handHot; t = t.next() {
 			if e == t {
-				panic("not reached")
+				fmt.Fprintf(os.Stderr, "%p: %s unexpectedly found in blocks list\n%s",
+					e, e.key, debug.Stack())
+				os.Exit(1)
 			}
 		}
 	}
@@ -553,6 +595,8 @@ func clearCache(obj interface{}) {
 		s.mu.Lock()
 		s.maxSize = 0
 		s.evict()
+		s.blocks.free()
+		s.files.free()
 		s.mu.Unlock()
 	}
 }
@@ -567,9 +611,10 @@ func newShards(size int64, shards int) *Cache {
 		c.shards[i] = shard{
 			maxSize:    size / int64(len(c.shards)),
 			coldTarget: size / int64(len(c.shards)),
-			blocks:     make(map[key]*entry),
-			files:      make(map[fileKey]*entry),
+			entries:    make(map[*entry]struct{}),
 		}
+		c.shards[i].blocks.init(16)
+		c.shards[i].files.init(16)
 	}
 	// TODO(peter): This finalizer is used to clear the cache when the Cache
 	// itself is GC'd. Investigate making this explicit, and then changing the
@@ -703,7 +748,7 @@ func (c *Cache) Metrics() Metrics {
 	for i := range c.shards {
 		s := &c.shards[i]
 		s.mu.RLock()
-		m.Count += int64(len(s.blocks))
+		m.Count += int64(s.blocks.Count())
 		m.Size += s.sizeHot + s.sizeCold
 		s.mu.RUnlock()
 		m.Hits += atomic.LoadInt64(&s.hits)

internal/cache/entry.go

@@ -59,9 +59,13 @@ type entry struct {
 	}
 	size  int64
 	ptype entryType
-	// Is the memory for the entry manually managed? A manually managed entry can
-	// only store manually managed values (Value.manual() is true).
+	// Can the entry hold a manual Value? Only a manually managed entry can store
+	// manually managed values (Value.manual() is true).
 	manual bool
+	// Was the entry allocated using the Go allocator or the manual
+	// allocator. This can differ from the setting of the manual field due when
+	// the "invariants" build tag is set.
+	managed bool
 	// referenced is atomically set to indicate that this entry has been accessed
 	// since the last time one of the clock hands swept it.
 	referenced int32
@@ -78,11 +82,12 @@ func newEntry(s *shard, key key, size int64, manual bool) *entry {
 		e = &entry{}
 	}
 	*e = entry{
-		key:    key,
-		size:   size,
-		ptype:  etCold,
-		manual: manual,
-		shard:  s,
+		key:     key,
+		size:    size,
+		ptype:   etCold,
+		manual:  manual,
+		managed: e.managed,
+		shard:   s,
 	}
 	e.blockLink.next = e
 	e.blockLink.prev = e

internal/cache/entry_normal.go

@@ -24,6 +24,7 @@ func entryAllocNew() *entry {
 	a := entryAllocPool.Get().(*entryAllocCache)
 	e := a.alloc()
 	entryAllocPool.Put(a)
+	e.managed = true
 	return e
 }