Initialize ELG with the number of affinitized cores

The MultiThreadedEventLoopGroup that runs the HTTPServer is initialized with all the available cores. On Linux, it could be instead initialised to the number of affinitized cores. If the number of affinitized cores is lesser than the `System.coreCount`, initializing the MultiThreadedEventLoopGroup with the later comes with a performance penalty.

Package.swift

@@ -36,9 +36,12 @@ let package = Package(
     targets: [
         // Targets are the basic building blocks of a package. A target can define a module or a test suite.
         // Targets can depend on other targets in this package, and on products in packages which this package depends on.
+        .target(
+            name: "CLinuxHelpers",
+            dependencies: []),
         .target(
             name: "KituraNet",
-            dependencies: ["NIO", "NIOFoundationCompat", "NIOHTTP1", "NIOOpenSSL", "SSLService", "LoggerAPI", "NIOWebSocket"]),
+            dependencies: ["NIO", "NIOFoundationCompat", "NIOHTTP1", "NIOOpenSSL", "SSLService", "LoggerAPI", "NIOWebSocket", "CLinuxHelpers"]),
         .testTarget(
             name: "KituraNetTests",
             dependencies: ["KituraNet"]),

Sources/CLinuxHelpers/ProcessorAffinity.c

@@ -0,0 +1,25 @@
+#ifdef __linux__
+
+#define _GNU_SOURCE
+#include <sched.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int linux_sched_getaffinity() {
+    cpu_set_t mask;
+    long nproc, i, count = 0;
+
+    if (sched_getaffinity(0, sizeof(cpu_set_t), &mask) == -1) {
+        return -1;
+    } else {
+        nproc = sysconf(_SC_NPROCESSORS_ONLN);
+        for (i = 0; i < nproc; i++) {
+            if(CPU_ISSET(i, &mask))
+                count += 1;
+        }
+        return count;
+    }
+}
+
+#endif

Sources/CLinuxHelpers/include/ProcessorAffinity.h

@@ -0,0 +1,3 @@
+#if __linux__
+int linux_sched_getaffinity();
+#endif

Sources/KituraNet/HTTP/HTTPServer.swift

@@ -21,6 +21,7 @@ import NIOOpenSSL
 import SSLService
 import LoggerAPI
 import NIOWebSocket
+import CLinuxHelpers
 
 /// An HTTP server that listens for connections on a socket.
 public class HTTPServer : Server {
@@ -66,11 +67,18 @@ public class HTTPServer : Server {
     private let maxPendingConnections = 100
 
     /// The event loop group on which the HTTP handler runs
-    let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
+    private let eventLoopGroup: MultiThreadedEventLoopGroup
 
     private var ctx: ChannelHandlerContext?
 
-    public init() { }
+    public init() {
+#if os(Linux)
+        let numberOfCores = Int(linux_sched_getaffinity())
+        self.eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: numberOfCores > 0 ? numberOfCores : System.coreCount)
+#else
+        self.eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
+#endif
+    }
 
     /// SSL cert configs for handling client requests
     public var sslConfig: SSLService.Configuration? {