fix: give each translation worker its share of the CPUs

Translate runs NumCPU/2 sentence workers and each of them started
GOMAXPROCS goroutines for the logit projection of every step: on 16
cores, 128 goroutines fighting for 16 cores. The model now caps the
goroutines of a step (SetThreads, called once after Load) so that
workers x threads stays within the machine.

internal/marian/decode.go

@@ -212,6 +212,9 @@ func (m *Model) decodeStep(b *beam, crossK, crossV [][]float32, srcLen int, s *d
 func (m *Model) logProbs(hidden, out [][]float32) [][]float32 {
 	d, vocab := m.cfg.DModel, m.cfg.VocabSize
 	workers := runtime.GOMAXPROCS(0)
+	if m.threads > 0 {
+		workers = min(workers, m.threads)
+	}
 	chunk := (vocab + workers - 1) / workers
 	var wg sync.WaitGroup
 	for lo := 0; lo < vocab; lo += chunk {

internal/marian/model.go

@@ -60,10 +60,12 @@ type decoderLayer struct {
 	finalLayerNorm      layerNorm
 }
 
-// Model is a loaded translation model. It is read-only once loaded:
-// Translate may be called from several goroutines at once.
+// Model is a loaded translation model. It is read-only once loaded (but
+// for SetThreads, right after Load): Translate may be called from several
+// goroutines at once.
 type Model struct {
 	cfg        config
+	threads    int       // goroutines a decoding step may start (0: one per CPU)
 	embed      []float32 // vocab × d, shared by encoder, decoder and output
 	encPos     []float32 // positions × d
 	decPos     []float32
@@ -221,6 +223,13 @@ func (m *Model) bind(t map[string]tensor) error {
 	return missing
 }
 
+// SetThreads caps how many goroutines each decoding step starts (0: one
+// per CPU, the default). When several sentences are translated at once,
+// give each its share of the CPUs, or every step of every sentence starts
+// a team of goroutines and they oversubscribe the machine. It must be
+// called before the first Translate.
+func (m *Model) SetThreads(n int) { m.threads = n }
+
 // maxSourceTokens leaves room for the end-of-sentence token.
 func (m *Model) maxSourceTokens() int { return m.cfg.MaxPositions - 1 }