* Buffer writes in FdSink. This significantly reduces the number of

system calls / context switches when dumping a NAR and in the worker
  protocol.

src/libutil/serialise.cc

@@ -9,7 +9,30 @@ namespace nix {
 
 void FdSink::operator () (const unsigned char * data, unsigned int len)
 {
-    writeFull(fd, data, len);
+    if (!buffer) buffer = new unsigned char[bufSize];
+    
+    while (len) {
+        /* Optimisation: bypass the buffer if the data exceeds the
+           buffer size and there is no unflushed data. */
+        if (bufPos == 0 && len >= bufSize) {
+            writeFull(fd, data, len);
+            break;
+        }
+        /* Otherwise, copy the bytes to the buffer.  Flush the buffer
+           when it's full. */
+        size_t n = bufPos + len > bufSize ? bufSize - bufPos : len;
+        memcpy(buffer + bufPos, data, n);
+        data += n; bufPos += n; len -= n;
+        if (bufPos == bufSize) flush();
+    }
+}
+
+
+void FdSink::flush()
+{
+    if (fd == -1 || bufPos == 0) return;
+    writeFull(fd, buffer, bufPos);
+    bufPos = 0;
 }
 
 

src/libutil/serialise.hh

@@ -28,22 +28,29 @@ struct Source
 };
 
 
-/* A sink that writes data to a file descriptor. */
+/* A sink that writes data to a file descriptor (using a buffer). */
 struct FdSink : Sink
 {
     int fd;
+    unsigned int bufSize, bufPos;
+    unsigned char * buffer;
 
-    FdSink()
-    {
-        fd = -1;
-    }
+    FdSink() : fd(-1), bufSize(32 * 1024), bufPos(0), buffer(0) { }
     
-    FdSink(int fd) 
+    FdSink(int fd, unsigned int bufSize = 32 * 1024)
+        : fd(fd), bufSize(bufSize), bufPos(0), buffer(0)
     {
-        this->fd = fd;
+    }
+
+    ~FdSink()
+    {
+        flush();
+        if (buffer) delete[] buffer;
     }
     
     void operator () (const unsigned char * data, unsigned int len);
+
+    void flush();
 };