Thursday, October 28, 2010

Sunday, October 17, 2010

More on more uses for Iterables

Rúnar Bjarnason writes Scalaz Tutorial: Enumeration-Based I/O with Iteratees, an all-singing, all-dancing approach to composable iterators for I/O in Scala. This is a much fuller treatment of I/O iterators than my post, More uses for Iterables.

Wednesday, October 13, 2010

Java ain't got no rhythm, but can it keep time?

I need sub-millisecond timing in Java; I want to measure events in the low microsecond range. What to do? My options seem to be:

  1. Use System.currentTimeMillis() and average over long numbers of runs; many problems here.
  2. Use System.nanoTime() and time individual events. Great, but the Javadoc is very scary for multi-threaded code.
  3. Try out sun.misc.Perf.highResCounter() (many non-official references for this.)
  4. Write our own JNI call for gettimeofday().

After months of hitting my head I wised up and looked into the JDK6 C++ source code. Surprise!

The C++ source

Drilling down points me to os::javaTimeNanos() (nanoTime), os::elapsed_counter() (highResCounter), and os::javaTimeMillis() (currentTimeMillis). On Linux these are:


jlong os::javaTimeMillis() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "linux error");
  return jlong(time.tv_sec) * 1000  +  jlong(time.tv_usec / 1000);


jlong os::javaTimeNanos() {
  if (Linux::supports_monotonic_clock()) {
    struct timespec tp;
    int status = Linux::clock_gettime(CLOCK_MONOTONIC, &tp);
    assert(status == 0, "gettime error");
    jlong result = jlong(tp.tv_sec) * (1000 * 1000 * 1000) + jlong(tp.tv_nsec);
    return result;
  } else {
    timeval time;
    int status = gettimeofday(&time, NULL);
    assert(status != -1, "linux error");
    jlong usecs = jlong(time.tv_sec) * (1000 * 1000) + jlong(time.tv_usec);
    return 1000 * usecs;


jlong os::elapsed_counter() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  return jlong(time.tv_sec) * 1000 * 1000 + jlong(time.tv_usec) - initial_time_count;


void os::init(void) {
  char dummy;   /* used to get a guess on initial stack address */
//  first_hrtime = gethrtime();

  // With LinuxThreads the JavaMain thread pid (primordial thread)
  // is different than the pid of the java launcher thread.
  // So, on Linux, the launcher thread pid is passed to the VM
  // via the property.
  // Use this property instead of getpid() if it was correctly passed.
  // See bug 6351349.
  pid_t java_launcher_pid = (pid_t) Arguments::sun_java_launcher_pid();

  _initial_pid = (java_launcher_pid > 0) ? java_launcher_pid : getpid();

  clock_tics_per_sec = sysconf(_SC_CLK_TCK);



  if (Linux::page_size() == -1) {
    fatal1("os_linux.cpp: os::init: sysconf failed (%s)", strerror(errno));
  init_page_sizes((size_t) Linux::page_size());


  // main_thread points to the aboriginal thread
  Linux::_main_thread = pthread_self();

  initial_time_count = os::elapsed_counter();
  pthread_mutex_init(&dl_mutex, NULL);

(The net effect is to normalize to 0-time at JVM boot rather than the epoch.)

The solution

Just use System.nanoTime() and stop worrying so much. It is fine for microsecond timing—even across threads—just RTFM and ignore javadoc.

Sunday, October 10, 2010

Elena Yatzeck promotes the agile Army Leadership Field Manual

A positive, fascinating post from Elena Yatzeck on agile leadership as taught in the U.S. Army Leadership Field Manual. The only military members of teams immediate to me are presently serving overseas. Looks like I missed out.

Saturday, October 09, 2010

Larval Java

Very cool post from John Rose on larval objects in the VM. How are immutable objects really made in the JVM? How could it be improved? Read and find out.

Friday, October 08, 2010

Nice refactoring example from Bobby Johnson

Bobby Johnson walks us through a nice refactoring example. Its not rocket science or brain surgery, but good old-fashioned craftsman handling of code, great to show journeymen programmers not quite sure what code cleanup looks like.

Saturday, October 02, 2010

Come hear me at the Houston TechFest

Come here my talk at the Houston TechFest! I am speaking on Hands-free Coding: Code Generation with Annotation Processors on October 9, 2010 03:45 PM - 04:45 PM at the University of Houston University Center. See you there!

UPDATE: Unfortunately, I spent today laid up at home with a bad back, and was unable to present at TechFest. This is very disappointing. However, as promised, I'll still post sources in the next day or so.

Friday, October 01, 2010

Dick Management

Some Guy's Blog captures dick management perfectly: the difference between good and bad managers. I pursue the former and eschew the latter. You should, too.