如何精确地以每秒几千的固定速率安排任务

Question

I currently use

 ScheduledFuture<?> schedulerFuture = scheduler.scheduleAtFixedRate(taskMaker, 0 , period, TimeUnit.MICROSECONDS);                
 ScheduledFuture<?> timerFuture = timer.schedule(timeAnalyser, time, TimeUnit.MICROSECONDS);

for executing many tasks per second, also asynchron in several threads.The problem is, that I get inaccuracies. For example, when I want to execute 3 Tasks in the first Thread in one second, 7 in the second Thread and 5 in the third Thread, sometimes i get 4, 7, 6 or 3, 6, 6. It is not that the processor cant make it when using bigger numbers, but the more tasks I want to execute per second the greater the inaccuracies become. Maybe it's because the timer (which stops the taskMaker ) is started not at the exactly same time as the taskMaker ?

Any changing suggestions? Is it the right way anyway to use the SchedulesExecutorService for executing tasks with such a high frequency?

Answer 1

The implementations of the various ExecutorService use a working queue, of which purpose is that as long a work job is available use the next available thread to execute it, if no Thread is available (is busy), than a new one is created (in the limit of the specified available threads, if this number is reached, than it will wait for a thread to become available), so you don't have any control over the threads ... and especially not on which tasks to be executed.

For such an Executor , you should consider implementing your own Executor, that might use a Executors.newSingleThreadScheduledExecutor() for your frequency (3, 7 and 5). eg:

ScheduledExecutorService exec1 = Executors.newSingleThreadScheduledExecutor();
ScheduledExecutorService exec2 = Executors.newSingleThreadScheduledExecutor();
ScheduledExecutorService exec3 = Executors.newSingleThreadScheduledExecutor();

exec1.scheduleAtFixedRate(task, 0, 1000 / 3, TimeUnit.MILLISECONDS);
exec2.scheduleAtFixedRate(task, 0, 1000 / 7, TimeUnit.MILLISECONDS);
exec2.scheduleAtFixedRate(task, 0, 1000/5, TimeUnit.MILLISECONDS);

Answer 2

From the javadoc

If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

I think you might want to manage the timings yourself using schedule rather than scheduleAtFixedRate (eg calculate the delay after one task executes and schedule the next task for that exact delay period).

Also note that unless you are doing lots of IO, you should keep the pool size to the number of processors on the server (eg Executors.newScheduledThreadPool(numberOfCpus) )