[#674] storage engine: Use per-shard worker pools for PUT operation

Make `StorageEngine` to use non-blocking worker pools with the same
(configurable) size for PUT operation. This allows you to switch to using
more free shards when overloading others, thereby more evenly distributing
the write load.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/engine/engine.go

@@ -4,6 +4,7 @@ import (
 	"sync"
 
 	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/shard"
+	"github.com/nspcc-dev/neofs-node/pkg/util"
 	"github.com/nspcc-dev/neofs-node/pkg/util/logger"
 	"go.uber.org/zap"
 )
@@ -15,6 +16,8 @@ type StorageEngine struct {
 	mtx *sync.RWMutex
 
 	shards map[string]*shard.Shard
+
+	shardPools map[string]util.WorkerPool
 }
 
 // Option represents StorageEngine's constructor option.
@@ -24,11 +27,15 @@ type cfg struct {
 	log *logger.Logger
 
 	metrics MetricRegister
+
+	shardPoolSize uint32
 }
 
 func defaultCfg() *cfg {
 	return &cfg{
 		log: zap.L(),
+
+		shardPoolSize: 20,
 	}
 }
 
@@ -41,9 +48,10 @@ func New(opts ...Option) *StorageEngine {
 	}
 
 	return &StorageEngine{
-		cfg:    c,
-		mtx:    new(sync.RWMutex),
-		shards: make(map[string]*shard.Shard),
+		cfg:        c,
+		mtx:        new(sync.RWMutex),
+		shards:     make(map[string]*shard.Shard),
+		shardPools: make(map[string]util.WorkerPool),
 	}
 }
 
@@ -59,3 +67,10 @@ func WithMetrics(v MetricRegister) Option {
 		c.metrics = v
 	}
 }
+
+// WithShardPoolSize returns option to specify size of worker pool for each shard.
+func WithShardPoolSize(sz uint32) Option {
+	return func(c *cfg) {
+		c.shardPoolSize = sz
+	}
+}