Minimum GOMAXPROCS should be 1 (#10)

We previously set the minimum GOMAXPROCS value to 2 as that was the
minimum allocation we expected internally. However, if the allocation is
actually smaller, we should use lower GOMAXPROCS.

This will also allow us to reduce the allocation size and use a more
appropriate GOMAXPROCS which will reduce the amount of CFS throttling.

internal/runtime/runtime.go

@@ -31,6 +31,3 @@ const (
 	// CPUQuotaMinUsed is return when CPU quota is larger than the min value
 	CPUQuotaMinUsed
 )
-
-// MinGOMAXPROCS defines the minimum value for GOMAXPROCS
-const MinGOMAXPROCS = 2

maxprocs/maxprocs.go

@@ -37,8 +37,9 @@ func currentMaxProcs() int {
 }
 
 type config struct {
-	printf func(string, ...interface{})
-	procs  func(int) (int, iruntime.CPUQuotaStatus, error)
+	printf        func(string, ...interface{})
+	procs         func(int) (int, iruntime.CPUQuotaStatus, error)
+	minGOMAXPROCS int
 }
 
 func (c *config) log(fmt string, args ...interface{}) {
@@ -60,6 +61,16 @@ func Logger(printf func(string, ...interface{})) Option {
 	})
 }
 
+// Min sets the minimum GOMAXPROCS value that will be used.
+// Any value below 1 is ignored.
+func Min(n int) Option {
+	return optionFunc(func(cfg *config) {
+		if n >= 1 {
+			cfg.minGOMAXPROCS = n
+		}
+	})
+}
+
 type optionFunc func(*config)
 
 func (of optionFunc) apply(cfg *config) { of(cfg) }
@@ -70,7 +81,10 @@ func (of optionFunc) apply(cfg *config) { of(cfg) }
 // Set is a no-op on non-Linux systems and in Linux environments without a
 // configured CPU quota.
 func Set(opts ...Option) (func(), error) {
-	cfg := &config{procs: iruntime.CPUQuotaToGOMAXPROCS}
+	cfg := &config{
+		procs:         iruntime.CPUQuotaToGOMAXPROCS,
+		minGOMAXPROCS: 1,
+	}
 	for _, o := range opts {
 		o.apply(cfg)
 	}
@@ -87,7 +101,7 @@ func Set(opts ...Option) (func(), error) {
 		return undoNoop, nil
 	}
 
-	maxProcs, status, err := cfg.procs(iruntime.MinGOMAXPROCS)
+	maxProcs, status, err := cfg.procs(cfg.minGOMAXPROCS)
 	if err != nil {
 		return undoNoop, err
 	}

maxprocs/maxprocs_test.go

@@ -138,15 +138,29 @@ func TestSet(t *testing.T) {
 		quotaOpt := stubProcs(func(min int) (int, iruntime.CPUQuotaStatus, error) {
 			return min, iruntime.CPUQuotaMinUsed, nil
 		})
-		undo, err := Set(logOpt, quotaOpt)
+		undo, err := Set(logOpt, quotaOpt, Min(5))
+		defer undo()
+		require.NoError(t, err, "Set failed")
+		assert.Equal(t, 5, currentMaxProcs(), "should use min allowed GOMAXPROCS")
+		assert.Contains(t, buf.String(), "using minimum allowed", "unexpected log output")
+	})
+
+	t.Run("Min unused", func(t *testing.T) {
+		buf, logOpt := testLogger()
+		quotaOpt := stubProcs(func(min int) (int, iruntime.CPUQuotaStatus, error) {
+			return min, iruntime.CPUQuotaMinUsed, nil
+		})
+		// Min(-1) should be ignored.
+		undo, err := Set(logOpt, quotaOpt, Min(5), Min(-1))
 		defer undo()
 		require.NoError(t, err, "Set failed")
-		assert.Equal(t, iruntime.MinGOMAXPROCS, currentMaxProcs(), "should use min allowed GOMAXPROCS")
+		assert.Equal(t, 5, currentMaxProcs(), "should use min allowed GOMAXPROCS")
 		assert.Contains(t, buf.String(), "using minimum allowed", "unexpected log output")
 	})
 
 	t.Run("QuotaUsed", func(t *testing.T) {
 		opt := stubProcs(func(min int) (int, iruntime.CPUQuotaStatus, error) {
+			assert.Equal(t, 1, min, "Default minimum value should be 1")
 			return 42, iruntime.CPUQuotaUsed, nil
 		})
 		undo, err := Set(opt)