[#1510] placement: Cache container nodes

In case we have lots of objects in a single container,
`GetContainerNodes` invoked indirectly by a policer can be seen in
pprof.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/services/object_manager/placement/netmap.go

@@ -3,7 +3,9 @@ package placement
 import (
 	"crypto/sha256"
 	"fmt"
+	"sync"
 
+	"github.com/hashicorp/golang-lru/simplelru"
 	"github.com/nspcc-dev/neofs-node/pkg/core/netmap"
 	cid "github.com/nspcc-dev/neofs-sdk-go/container/id"
 	netmapSDK "github.com/nspcc-dev/neofs-sdk-go/netmap"
@@ -12,6 +14,12 @@ import (
 
 type netMapBuilder struct {
 	nmSrc netmap.Source
+	// mtx protects lastNm and containerCache fields.
+	mtx    sync.Mutex
+	lastNm *netmapSDK.Netmap
+	// containerCache caches container nodes by ID. It is used to skip `GetContainerNodes` invocation if
+	// neither netmap nor container has changed.
+	containerCache simplelru.LRUCache
 }
 
 type netMapSrc struct {
@@ -20,15 +28,22 @@ type netMapSrc struct {
 	nm *netmapSDK.Netmap
 }
 
+// defaultContainerCacheSize is the default size for the container cache.
+const defaultContainerCacheSize = 10
+
 func NewNetworkMapBuilder(nm *netmapSDK.Netmap) Builder {
+	cache, _ := simplelru.NewLRU(defaultContainerCacheSize, nil) // no error
 	return &netMapBuilder{
 		nmSrc:          &netMapSrc{nm: nm},
+		containerCache: cache,
 	}
 }
 
 func NewNetworkMapSourceBuilder(nmSrc netmap.Source) Builder {
+	cache, _ := simplelru.NewLRU(defaultContainerCacheSize, nil) // no error
 	return &netMapBuilder{
 		nmSrc:          nmSrc,
+		containerCache: cache,
 	}
 }
 
@@ -45,11 +60,28 @@ func (b *netMapBuilder) BuildPlacement(cnr cid.ID, obj *oid.ID, p *netmapSDK.Pla
 	binCnr := make([]byte, sha256.Size)
 	cnr.Encode(binCnr)
 
+	b.mtx.Lock()
+	if nm == b.lastNm {
+		raw, ok := b.containerCache.Get(string(binCnr))
+		b.mtx.Unlock()
+		if ok {
+			cn := raw.(netmapSDK.ContainerNodes)
+			return BuildObjectPlacement(nm, cn, obj)
+		}
+	} else {
+		b.containerCache.Purge()
+		b.mtx.Unlock()
+	}
+
 	cn, err := nm.GetContainerNodes(p, binCnr)
 	if err != nil {
 		return nil, fmt.Errorf("could not get container nodes: %w", err)
 	}
 
+	b.mtx.Lock()
+	b.containerCache.Add(string(binCnr), cn)
+	b.mtx.Unlock()
+
 	return BuildObjectPlacement(nm, cn, obj)
 }