Run the daemon worker on the same CPU as the client

On a system with multiple CPUs, running Nix operations through the
daemon is significantly slower than "direct" mode:

$ NIX_REMOTE= nix-instantiate '<nixos>' -A system
real    0m0.974s
user    0m0.875s
sys     0m0.088s

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m2.118s
user    0m1.463s
sys     0m0.218s

The main reason seems to be that the client and the worker get moved
to a different CPU after every call to the worker.  This patch adds a
hack to lock them to the same CPU.  With this, the overhead of going
through the daemon is very small:

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m1.074s
user    0m0.809s
sys     0m0.098s

src/libstore/build.cc

@@ -7,6 +7,7 @@
 #include "local-store.hh"
 #include "util.hh"
 #include "archive.hh"
+#include "affinity.hh"
 
 #include <map>
 #include <sstream>
@@ -366,6 +367,8 @@ void Goal::trace(const format & f)
 /* Common initialisation performed in child processes. */
 static void commonChildInit(Pipe & logPipe)
 {
+    restoreAffinity();
+
     /* Put the child in a separate session (and thus a separate
        process group) so that it has no controlling terminal (meaning
        that e.g. ssh cannot open /dev/tty) and it doesn't receive
@@ -568,6 +571,7 @@ static void runSetuidHelper(const string & command,
             args.push_back(0);
 
             restoreSIGPIPE();
+            restoreAffinity();
 
             execve(program.c_str(), (char * *) &args[0], 0);
             throw SysError(format("executing `%1%'") % program);

src/libstore/local-store.cc

@@ -5,6 +5,7 @@
 #include "pathlocks.hh"
 #include "worker-protocol.hh"
 #include "derivations.hh"
+#include "affinity.hh"
 
 #include <iostream>
 #include <algorithm>
@@ -1021,6 +1022,7 @@ void LocalStore::startSubstituter(const Path & substituter, RunningSubstituter &
 
     case 0: /* child */
         try {
+            restoreAffinity();
             if (dup2(toPipe.readSide, STDIN_FILENO) == -1)
                 throw SysError("dupping stdin");
             if (dup2(fromPipe.writeSide, STDOUT_FILENO) == -1)

src/libstore/remote-store.cc

@@ -3,6 +3,7 @@
 #include "remote-store.hh"
 #include "worker-protocol.hh"
 #include "archive.hh"
+#include "affinity.hh"
 #include "globals.hh"
 
 #include <sys/types.h>
@@ -15,7 +16,6 @@
 #include <unistd.h>
 #include <cstring>
 
-
 namespace nix {
 
 
@@ -71,8 +71,19 @@ void RemoteStore::openConnection(bool reserveSpace)
         if (GET_PROTOCOL_MAJOR(daemonVersion) != GET_PROTOCOL_MAJOR(PROTOCOL_VERSION))
             throw Error("Nix daemon protocol version not supported");
         writeInt(PROTOCOL_VERSION, to);
+
+        if (GET_PROTOCOL_MINOR(daemonVersion) >= 14) {
+            int cpu = lockToCurrentCPU();
+            if (cpu != -1) {
+                writeInt(1, to);
+                writeInt(cpu, to);
+            } else
+                writeInt(0, to);
+        }
+
         if (GET_PROTOCOL_MINOR(daemonVersion) >= 11)
             writeInt(reserveSpace, to);
+
         processStderr();
     }
     catch (Error & e) {

src/libstore/worker-protocol.hh

@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x10d
+#define PROTOCOL_VERSION 0x10e
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)