Is System.nanoTime() completely useless?

问题:

As documented in the blog post Beware of System.nanoTime() in Java, on x86 systems, Java\'s System.nanoTime() returns the time value using a CPU specific counter. Now consider the following case I use to measure time of a call:

long time1= System.nanoTime();
foo();
long time2 = System.nanoTime();
long timeSpent = time2-time1;

Now in a multi-core system, it could be that after measuring time1, the thread is scheduled to a different processor whose counter is less than that of the previous CPU. Thus we could get a value in time2 which is less than time1. Thus we would get a negative value in timeSpent.

Considering this case, isn\'t it that System.nanotime is pretty much useless for now?

I know that changing the system time doesn\'t affect nanotime. That is not the problem I describe above. The problem is that each CPU will keep a different counter since it was turned on. This counter can be lower on the second CPU compared to the first CPU. Since the thread can be scheduled by the OS to the second CPU after getting time1, the value of timeSpent may be incorrect and even negative.

回答1:

That post is wrong, and nanoTime is safe. There\'s a comment on the post which links to a blog post by David Holmes, a realtime and concurrency guy at Sun. It says:

System.nanoTime() is implemented using the QueryPerformanceCounter/QueryPerformanceFrequency API [...] The default mechanism used by QPC is determined by the Hardware Abstraction layer(HAL) [...] This default changes not only across hardware but also across OS versions. For example Windows XP Service Pack 2 changed things to use the power management timer (PMTimer) rather than the processor timestamp-counter (TSC) due to problems with the TSC not being synchronized on different processors in SMP systems, and due the fact its frequency can vary (and hence its relationship to elapsed time) based on power-management settings.

So, on Windows, this was a problem up until WinXP SP2, but it isn\'t now.

I can\'t find a part II (or more) that talks about other platforms, but that article does include a remark that Linux has encountered and solved the same problem in the same way, with a link to the FAQ for clock_gettime(CLOCK_REALTIME), which says:

1. Is clock_gettime(CLOCK_REALTIME) consistent across all processors/cores? (Does arch matter? e.g. ppc, arm, x86, amd64, sparc).

It should or it\'s considered buggy.

However, on x86/x86_64, it is possible to see unsynced or variable freq TSCs cause time inconsistencies. 2.4 kernels really had no protection against this, and early 2.6 kernels didn\'t do too well here either. As of 2.6.18 and up the logic for detecting this is better and we\'ll usually fall back to a safe clocksource.

ppc always has a synced timebase, so that shouldn\'t be an issue.

So, if Holmes\'s link can be read as implying that nanoTime calls clock_gettime(CLOCK_REALTIME), then it\'s safe-ish as of kernel 2.6.18 on x86, and always on PowerPC (because IBM and Motorola, unlike Intel, actually know how to design microprocessors).

There\'s no mention of SPARC or Solaris, sadly. And of course, we have no idea what IBM JVMs do. But Sun JVMs on modern Windows and Linux get this right.

EDIT: This answer is based on the sources it cites. But i still worry that it might actually be completely wrong. Some more up-to-date information would be really valuable. I just came across to a link to a four year newer article about Linux\'s clocks which could be useful.

回答2:

I did a bit of searching and found that if one is being pedantic then yes it might be considered useless...in particular situations...it depends on how time sensitive your requirements are...

Check out this quote from the Java Sun site:

The real-time clock and System.nanoTime() are both based on the same system call and thus the same clock.

With Java RTS, all time-based APIs (for example, Timers, Periodic Threads, Deadline Monitoring, and so forth) are based on the high-resolution timer. And, together with real-time priorities, they can ensure that the appropriate code will be executed at the right time for real-time constraints. In contrast, ordinary Java SE APIs offer just a few methods capable of handling high-resolution times, with no guarantee of execution at a given time. Using System.nanoTime() between various points in the code to perform elapsed time measurements should always be accurate.

Java also has a caveat for the nanoTime() method:

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292.3 years (2⁶³ nanoseconds) will not accurately compute elapsed time due to numerical overflow.

It would seem that the only conclusion that can be drawn is that nanoTime() cannot be relied upon as an accurate value. As such, if you do not need to measure times that are mere nano seconds apart then this method is good enough even if the resulting returned value is negative. However, if you\'re needing higher precision, they appear to recommend that you use JAVA RTS.

So to answer your question...no nanoTime() is not useless....its just not the most prudent method to use in every situation.

回答3:

No need to debate, just use the source. Here, SE 6 for Linux, make your own conclusions:

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;
  }
}

回答4:

Disclaimer: I am the developer of this library

You might like this better:

http://juliusdavies.ca/nanotime/

But it copies a DLL or Unix .so (shared object) file into the current user\'s home directory so that it can call JNI.

Some background information is on my site at:

http://juliusdavies.ca/posix_clocks/clock_realtime_linux_faq.html

回答5:

Linux corrects for discrepancies between CPUs, but Windows does not. I suggest you assume System.nanoTime() is only accurate to around 1 micro-second. A simple way to get a longer timing is to call foo() 1000 or more times and divide the time by 1000.

回答6:

Absolutely not useless. Timing aficionados correctly point out the multi-core problem, but in real-word applications it is often radically better than currentTimeMillis().

When calculating graphics positions in frame refreshes nanoTime() leads to MUCH smoother motion in my program.

And I only test on multi-core machines.

回答7:

I have seen a negative elapsed time reported from using System.nanoTime(). To be clear, the code in question is:

    long startNanos = System.nanoTime();

    Object returnValue = joinPoint.proceed();

    long elapsedNanos = System.nanoTime() - startNanos;

and variable \'elapsedNanos\' had a negative value. (I\'m positive that the intermediate call took less than 293 years as well, which is the overflow point for nanos stored in longs :)

This occurred using an IBM v1.5 JRE 64bit on IBM P690 (multi-core) hardware running AIX. I\'ve only seen this error occur once, so it seems extremely rare. I do not know the cause - is it a hardware-specific issue, a JVM defect - I don\'t know. I also don\'t know the implications for the accuracy of nanoTime() in general.

To answer the original question, I don\'t think nanoTime is useless - it provides sub-millisecond timing, but there is an actual (not just theoretical) risk of it being inaccurate which you need to take into account.

回答8:

I am linking to what essentially is the same discussion where Peter Lawrey is providing a good answer. Why I get a negative elapsed time using System.nanoTime()?

Many people mentioned that in Java System.nanoTime() could return negative time. I for apologize for repeating what other people already said.

1. nanoTime() is not a clock but CPU cycle counter.
2. Return value is divided by frequency to look like time.
3. CPU frequency may fluctuate.
4. When your thread is scheduled on another CPU, there is a chance of getting nanoTime() which results in a negative difference. That\'s logical. Counters across CPUs are not synchronized.
5. In many cases, you could get quite misleading results but you wouldn\'t be able to tell because delta is not negative. Think about it.
6. (unconfirmed) I think you may get a negative result even on the same CPU if instructions are reordered. To prevent that, you\'d have to invoke a memory barrier serializing your instructions.

It\'d be cool if System.nanoTime() returned coreID where it executed.

回答9:

This doesn\'t seem to be a problem on a Core 2 Duo running Windows XP and JRE 1.5.0_06.

In a test with three threads I don\'t see System.nanoTime() going backwards. The processors are both busy, and threads go to sleep occasionally to provoke moving threads around.

[EDIT] I would guess that it only happens on physically separate processors, i.e. that the counters are synchronized for multiple cores on the same die.

回答10:

Java is crossplatform, and nanoTime is platform-dependent. If you use Java - when don\'t use nanoTime. I found real bugs across different jvm implementations with this function.

回答11:

No, it\'s not... It just depends on your CPU, check High Precision Event Timer for how/why things are differently treated according to CPU.

Basically, read the source of your Java and check what your version does with the function, and if it works against the CPU you will be running it on.

IBM even suggests you use it for performance benchmarking (a 2008 post, but updated).

回答12:

The Java 5 documentation also recommends using this method for the same purpose.

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time.

Java 5 API Doc

回答13:

Also, System.currentTimeMillies() changes when you change your systems clock, while System.nanoTime() doesn\'t, so the latter is safer to measure durations.

回答14:

nanoTime is extremely insecure for timing. I tried it out on my basic primality testing algorithms and it gave answers which were literally one second apart for the same input. Don\'t use that ridiculous method. I need something that is more accurate and precise than get time millis, but not as bad as nanoTime.