Does anyone have a Fairly effective way of running a function repetitively in a precise and accurate number of milliseconds. I have tried to accomplish this by using the code below to try to run a function called wave() once a second for 30 seconds:
startTime = System.nanoTime();
wholeTime = System.nanoTime();
while (loop) {
if (startTime >= time2) {
startTime = System.nanoTime();
wave();
sec++;
}
if (sec == 30) {
loop = false;
endTime = System.nanoTime();
System.out.println(wholeTime - System.nanoTime());
}
}
}
This code did not work and am wondering why this code didn't work and if their is a better approach to the problem. Any ideas on how to improve fix the above code or other successful ways of accomplishing the problem are all welcome. Thank you for your help!
more simple:
long start=System.currentTimeMillis(); // Not very very accurate
while (System.currentTimeMillis()-start<30000)
{
wave();
// count something
}
You can use a Timer+TimerTask: https://docs.oracle.com/javase/7/docs/api/java/util/Timer.html
https://docs.oracle.com/javase/7/docs/api/java/util/TimerTask.html
http://bioportal.weizmann.ac.il/course/prog2/tutorial/essential/threads/timer.html
You may use Thread.sleep():
public static void main (String[] args) throws InterruptedException {
int count = 30;
long start = System.currentTimeMillis();
for(int i=0; i<count; i++) {
wave();
// how many milliseconds till the end of the second?
long sleep = start+(i+1)*1000-System.currentTimeMillis();
if(sleep > 0) // condition might be false if wave() runs longer than second
Thread.sleep(sleep);
}
}
Does anyone have a Fairly effective way of running a function repetitively in a precise and accurate number of milliseconds.
There is no way to do this kind of thing reliably and accurately in standard Java. The problem is that there is no way that you can guarantee that your thread will run when you want ti to run. For example:
your thread could be suspended to allow the GC to run
your thread could be preempted to allow another thread in your application to run
your thread could be suspended by the OS while it fetches pages by the JVM back from disk.
You can only get reliable behavior for this kind of code if you run on a hard realtime OS, and an realtime Java.
Note that this is not an issue with clock accuracy. The real problem is that the scheduler does not give you the kind of guarantees you need. For instance, none of the "sleep until X" functionality in a JVM can guarantee that your thread will wake up at time X exactly ... for any useful meaning of "exactly".
The other answers suggest various ways to do this, but beware that they are not (and cannot be) reliable and accurate in all circumstances .. or even on a typical machine running other things as well as your application.
Related
I want to print to the console every second. So far, I've been able to think of two ways to do this.
long start_time = System.currentTimeMillis();
while(true){
if ((System.currentTimeMillis() - start_time) >= 1000) {
System.out.println("One second passed");
start_time = System.currentTimeMillis();
}
}
And this
while(true){
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("One second passed");
}
Is any method safer or more reliable or more efficient than the other?
or maybe there are use cases for each?
Thanks
This is not as simple as it seems.
The problem with your solutions are that the first is very inefficient, and the second is liable to drift because sleep(1000) does not guarantee to sleep for exactly 1 second. (The javadoc says that sleep(1000) sleeps for at least one second.)
One possibility is to use ScheduledExecutorService.scheduleWithFixedDelay and compute the delay each time by looking at the difference between the millisecond clock and your start time (or end time). That will keep your messages in sync with real time (more or less).
Or maybe you can simplify that using scheduleAtFixedRate, because it looks like ScheduledThreadPoolExecutor will take care of the clock syncing problem.
Another possibility is to look for a 3rd-party library that implements a "cron-like" scheduler in Java.
I have been researching and I am struggling to actually choose the best option. I am using processing sketch that runs java code, and I want to start an animation in several computers( OS X and windows) at the same time. The basic idea is to send a OSC message to each computer and after they receive a message they will store the currentTime plus the timespan(let say after 10 second). And each computer track the currentTime and when it reach the intended Time they will start the animation. Now I cannot figure out which System should I use. System.currentTimeMillis() or System.nanoTime(); I already tested with two computers(both Systems) and it seems to work. Both computers are OS X but I never tried with a windows one and it seems for System.currentTimeMillis() can be a lag of 50ms. I'm really confuse in this matter. Someone can me explain or highlight.
thank in advance
Simultanous simulations on two or more computers is tricky for some reasons.
First of all I would make sure, all connected computers synchronize their clocks with NTP. (See more https://en.wikipedia.org/wiki/Network_Time_Protocol)
Then the biggest difference is at most 50ms as far as I know.
Otherwise every approach will fail because of the differences of the clocks.
Second, clocks on different systems have different accuracy. I can recommend reading Alexey Shipilev's blog: https://shipilev.net/blog/2014/nanotrusting-nanotime/ . It is about the accuracy of clocks on machines in general.
Third you need to know that Linux has a round robin slide of 1ms and Windows about 10-15ms. Therefore Thread.sleep(...) will not work with smaller time spans reliable.
If you want to´work with smaller time spans you need to do a kind of "busy waiting" which is ugly but necessary:
public class SleepUtil {
public static final long MIN_PRECISION_IN_MICROS = 15L;
public static void main(String[] args) {
long before = System.nanoTime();
while (true) {
final long after = System.nanoTime();
long diff = (after - before) / 1000l;
before = after;
System.out.println(diff + " micros");
SleepUtil.sleepMicros(500);
}
}
private static void sleepMicros(int waitTimeInMicros) {
final long startTimeInNanos = System.nanoTime();
long elapsedTimeInMicros = 0L;
while (elapsedTimeInMicros < waitTimeInMicros - MIN_PRECISION_IN_MICROS) {
elapsedTimeInMicros = (System.nanoTime() - startTimeInNanos) / 1000L;
}
}
}
However, it will busy your cpu and not be always reliable (but most of the time).
Recently, I was writing a plugin using Java and found that retrieving an element(using get()) from a HashMap for the first time is very slow. Originally, I wanted to ask a question on that and found this (No answers though). With further experiments, however, I notice that this phenomenon happens on ArrayList and then all the methods.
Here is the code:
public class Test {
public static void main(String[] args) {
long startTime, stopTime;
// Method 1
System.out.println("Test 1:");
for (int i = 0; i < 20; ++i) {
startTime = System.nanoTime();
testMethod1();
stopTime = System.nanoTime();
System.out.println((stopTime - startTime) + "ns");
}
// Method 2
System.out.println("Test 2:");
for (int i = 0; i < 20; ++i) {
startTime = System.nanoTime();
testMethod2();
stopTime = System.nanoTime();
System.out.println((stopTime - startTime) + "ns");
}
}
public static void testMethod1() {
// Do nothing
}
public static void testMethod2() {
// Do nothing
}
}
Snippet: Test Snippet
The output would be like this:
Test 1:
2485ns
505ns
453ns
603ns
362ns
414ns
424ns
488ns
325ns
426ns
618ns
794ns
389ns
686ns
464ns
375ns
354ns
442ns
404ns
450ns
Test 2:
3248ns
700ns
538ns
531ns
351ns
444ns
321ns
424ns
523ns
488ns
487ns
491ns
551ns
497ns
480ns
465ns
477ns
453ns
727ns
504ns
I ran the code for a few times and the results are about the same. The first call would be even longer(>8000 ns) on my computer(Windows 8.1, Oracle Java 8u25).
Apparently, the first calls is usually slower than the following calls(Some calls may be longer in random cases).
Update:
I tried to learn some JMH, and write a test program
Code w/ sample output: Code
I don't know whether it's a proper benchmark(If the program has some problems, tell me), but I found that the first warm-up iterations spend more time(I use two warm-up iterations in case the warm-ups affect the results). And I think that the first warm-up should be the first call and is slower. So this phenomenon exists, if the test is proper.
So why does it happen?
You're calling System.nanoTime() inside a loop. Those calls are not free, so in addition to the time taken for an empty method you're actually measuring the time it takes to exit from nanotime call #1 and to enter nanotime call #2.
To make things worse, you're doing that on windows where nanotime performs worse compared to other platforms.
Regarding JMH: I don't think it's much help in this situation. It's designed to measure by averaging many iterations, to avoid dead code elimination, account for JIT warmup, avoid ordering dependence, ... and afaik it simply uses nanotime under the hood too.
Its design goals pretty much aim for the opposite of what you're trying to measure.
You are measuring something. But that something might be several cache misses, nanotime call overhead, some JVM internals (class loading? some kind of lazy initialization in the interpreter?), ... probably a combination thereof.
The point is that your measurement can't really be taken at face value. Even if there is a certain cost for calling a method for the first time, the time you're measuring only provides an upper bound for that.
This kind of behaviour is often caused by the compiler or RE. It starts to optimize the execution after the first iteration. Additionally class loading can have an effect (I guess this is not the case in your example code as all classes are loaded in the first loop latest).
See this thread for a similar problem.
Please keep in mind this kind of behaviour is often dependent on the environment/OS it's running on.
Say, for example, I want to run the following program
double x = 15.6
System.out.println(x);
But I wanted to repeat the program until a certain time has elapsed, such as the following:
do{
double x = 15.6
System.out.println(x);
}while (current time is earlier than 12.00pm)
Even though the example is completely hypothetical, how would I make that do while loop so that the program would keep running over and over again until a certain time, say 3pm, or9.30pm.
If this is not possible, is there any way I can simulate this, by running the program every so many seconds, until that time has been reached?
a) You usually don't need the code to actually run until a time has come - you wouldn't have any control over the amount of times the code executed this way. Regular code has to sleep sometimes, to give control to OS and other processes so that they don't clog the system with 100% CPU load. As such, actually running the code constantly is a wrong approach to 99% of the possible problems related to timings. Please describe the exact problem you want to solve using this code.
b) For those 99% of problems, use a Timer instance. Simple as that. https://docs.oracle.com/javase/7/docs/api/java/util/Timer.html - schedule the task to run e.g. 1000 times a second, and check the time in each event, terminating the Timer instance when the time threshold has been exceeded.
For example, this code above will give you continuous execution of Do something part, every 1 second, until 16.11.2014 20:00 GMT. By changing delayMs you can easily achieve higher/lower time granularity. If you expect your code to be run more often than 1000/sec, you should probably use JNI anyway, since Java timers/clocks are known to have <10ms granularity on some (older) platforms, see How can I measure time with microsecond precision in Java? etc.
Timer timer = new Timer();
int delayMs = 1000; // check time every one second
long timeToStop;
try {
timeToStop = new SimpleDateFormat( "DD.MM.YYYY HH:mm" ).parse( "16.11.2014 20:00" ).getTime(); // GMT time, needs to be offset by TZ
} catch (ParseException ex) {
throw new RuntimeException( ex );
}
timer.scheduleAtFixedRate( new TimerTask() {
#Override
public void run() {
if ( System.currentTimeMillis() < timeToStop ) {
System.out.println( "Do something every " + delayMs + " milliseconds" );
} else {
timer.cancel();
}
}
}, 0, delayMs );
or you can use e.g. ExecutorService service = Executors.newSingleThreadExecutor(); etc. - but it's virtually impossible to give you a good way to solve your problem without explicitly knowing what the problem is.
Something like this
//get a Date object for the time to stop, then get milliseconds
long timeToStop = new SimpleDateFormat("DD:MM:HH:mm").parse("16:11:12:00").getTime();
//get milliseconds now, and compare to milliseconds from before
do {
//do stuff
} while(System.currentTimeMillis() < timeToStop)
I am doing a past exam paper of Java, I am confused about one question listed below:
What would happen when a thread executes the following statement in its run() method? (Choose all that apply.)
sleep(500);
A. It is going to stop execution, and start executing exactly 500 milliseconds later.
B. It is going to stop execution, and start executing again not earlier than 500 milliseconds later.
C. It is going to result in a compiler error because you cannot call the sleep(…) method inside the run() method.
D. It is going to result in a compiler error because the sleep(…) method does not take any argument.
I select A,B. but the key answer is only B, does there exist any circumstances that A could also happen? Could anyone please clarify that for me? Many thanks.
I select A,B. but the key answer is only B, does there exist any circumstances that A could also happen? Could anyone please clarify that for me?
Yes, depending on your application, you certainly might get 500ms of sleep time and not a nanosecond more.
However, the reason why B is the better answer is that there are no guarantees about when any thread will be run again. You could have an application with a large number of CPU bound threads. Even though the slept thread is now able to be run, it might not get any cycles for a significant period of time. The precise sleep time also depends highly on the particulars of the OS thread scheduler and clock accuracy. Your application also may also have to compete with other applications on the same system which may delay its continued execution.
For example, this following program on my extremely fast 8xi7 CPU Macbook Pro shows a max-sleep of 604ms:
public class MaxSleep {
public static void main(String[] args) throws Exception {
final AtomicLong maxSleep = new AtomicLong(0);
ExecutorService threadPool = Executors.newCachedThreadPool();
// fork 1000 threads
for (int i = 0; i < 1000; i++) {
threadPool.submit(new Runnable() {
#Override
public void run() {
for (int i = 0; i < 10; i++) {
long total = 0;
// spin doing something that eats CPU
for (int j = 0; j < 10000000; j++) {
total += j;
}
// this IO is the real time sink though
System.out.println("total = " + total);
try {
long before = System.currentTimeMillis();
Thread.sleep(500);
long diff = System.currentTimeMillis() - before;
// update the max value
while (true) {
long max = maxSleep.get();
if (diff <= max) {
break;
}
if (maxSleep.compareAndSet(max, diff)) {
break;
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
}
threadPool.shutdown();
threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
System.out.println("max sleep ms = " + maxSleep);
}
}
JVM cannot guarantee exactly 500 ms but it will start on or after ~500 ms as it will need to start its 'engine' back considering no other threads are blocking any resources which may delay a bit.
Read: Inside the Hotspot VM: Clocks, Timers and Scheduling Events
Edit: As Gray pointed out in the comment - the scheduling with other threads also a factor, swapping from one to another may cost some time.
According to Javadoc:-
Sleep()
Causes the currently executing thread to sleep (temporarily cease
execution) for the specified number of milliseconds, subject to the
precision and accuracy of system timers and schedulers. The thread
does not lose ownership of any monitors.
So it may be ~500ms
B. It is going to stop execution, and start executing again not earlier than 500 milliseconds later.
Looks more prominent.
As you know that there are two very closely-related states in Thread : Running and Runnable.
Running : means currently executing thread. Its execution is currently going on.
Runnable : means thread is ready to get processed or executed. But is waiting to be picked up by Thread Schedular. Now this thread schedular, as per its own wish/defined algorithm depending upon the JVM(for example, slicing algorithm) will pick up one of the available/Runnable threads to process them.
So whenever you call sleep method, it only guarantees that the execution of the code by the thread which it ran on, is paused for the specified milliseconds in the argument(e.g. 300ms in threadRef.sleep(300);). As soon as the time defined goes by, it comes back to Runnable state(means back to Available State to be picked up by Thread Schedular).
Therefore, there is no guarantee that your remaining code will start getting executed immediately after sleep method completion.
These sleep times are not guaranteed to be precise, because they are limited by the facilities provided by the underlying OS. Option B: Not earlier than 500 is more correct.
You cannot select A and B as they are opposite to each other, the main difference: exactly 500 milliseconds later and not earlier than 500 milliseconds later
first mean exactly what it means (500 milliseconds only), second means that it can sleep 501 or 502 or even 50000000000000
next question - why B is true, it is not so simply question, you need to understand what is the different between hard-realtime and soft-realtime, and explaining of all reasons is quite offtopic, so simply answer - because of many technical reasons java cannot guarantee hard real time execution of your code, this is why it stated that sleep will finish not earlier than ...
you can read about thread scheduling, priorities, garbage collection, preemptive multitasking - all these are related to this