New 'cpu' directive; now uses all cores by default (if needed)

config/config.go

@@ -65,6 +65,7 @@ type Config struct {
 	TLS        TLSConfig
 	Middleware []middleware.Middleware
 	Startup    []func() error
+	MaxCPU     int
 }
 
 // Address returns the host:port of c as a string.

config/directives.go

 package config
 
-import "os"
+import (
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+)
 
 // dirFunc is a type of parsing function which processes
 // a particular directive and populates the config.
@@ -64,5 +69,45 @@ func init() {
 			p.cfg.TLS = tls
 			return nil
 		},
+		"cpu": func(p *parser) error {
+			sysCores := runtime.NumCPU()
+
+			if !p.nextArg() {
+				return p.argErr()
+			}
+			strNum := p.tkn()
+
+			setCPU := func(val int) {
+				if val < 1 {
+					val = 1
+				}
+				if val > sysCores {
+					val = sysCores
+				}
+				if val > p.cfg.MaxCPU {
+					p.cfg.MaxCPU = val
+				}
+			}
+
+			if strings.HasSuffix(strNum, "%") {
+				// Percent
+				var percent float32
+				pctStr := strNum[:len(strNum)-1]
+				pctInt, err := strconv.Atoi(pctStr)
+				if err != nil || pctInt < 1 || pctInt > 100 {
+					return p.err("Parse", "Invalid number '"+strNum+"' (must be a positive percentage between 1 and 100)")
+				}
+				percent = float32(pctInt) / 100
+				setCPU(int(float32(sysCores) * percent))
+			} else {
+				// Number
+				num, err := strconv.Atoi(strNum)
+				if err != nil || num < 0 {
+					return p.err("Parse", "Invalid number '"+strNum+"' (requires positive integer or percent)")
+				}
+				setCPU(num)
+			}
+			return nil
+		},
 	}
 }

server/server.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"log"
 	"net/http"
+	"runtime"
 
 	"github.com/mholt/caddy/config"
 	"github.com/mholt/caddy/middleware"
@@ -36,6 +37,11 @@ func New(conf config.Config) (*Server, error) {
 		return nil, errors.New("Address " + addr + " is already in use")
 	}
 
+	// Use all CPUs (if needed) by default
+	if conf.MaxCPU == 0 {
+		conf.MaxCPU = runtime.NumCPU()
+	}
+
 	// Initialize
 	s := new(Server)
 	s.config = conf
@@ -53,6 +59,10 @@ func (s *Server) Serve() error {
 		return err
 	}
 
+	if s.config.MaxCPU > 0 {
+		runtime.GOMAXPROCS(s.config.MaxCPU)
+	}
+
 	if s.config.TLS.Enabled {
 		return http.ListenAndServeTLS(s.config.Address(), s.config.TLS.Certificate, s.config.TLS.Key, s)
 	} else {