Throttling CPU from within Java

问题

I have seen many questions in this (and others) forum with the same title, but none of them seemed to address exactly my problem. This is it: I have got a JVM that eats all the CPU on the machine that hosts it. I would like to throttle it, however I cannot rely on any throttling tool/technique external to Java as I cannot make assumptions as to where this Vm will be run. Thus, for instance, I cannot use processor affinity because if the VM runs on a Mac the OS won't make process affinity available.

What I would need is an indication as to whether means exist within Java to ensure the thread does not take the full CPU.

I would like to point straightaway that I cannot use techniques based on alternating process executions and pauses, as suggested in some forums, because the thread needs to generate values continuously.

Ideally I'd like some mean for, for instance, setting some VM or thread priority, or cap in some way the percentage of CPU consumed.

Any help would be much appreciated.

回答1:

What I would need is an indication as to whether means exist within Java to ensure the thread does not take the full CPU.

There is no way that I know of to do this within Java except for tuning your application to use less CPU.

You could put some Thread.sleep(...); calls in your calculation methods. A profiler would help with showing you the hot loops/methods/etc..
Forking fewer threads would also affect the CPU used. Moving to fixed sized thread-pools or lowering the number of threads in your pools.
It may not be CPU that is the problem but other resources. Watch your IO bandwidth for example. Slowing down your network or disk reads/writes might restore your server to proper operation.

From outside of the JVM you could use the ~unix nice command to affect the priority of the running JVM to not dominate the system. This will give it CPU if available but will let other applications get more of the CPU.

回答2:

I take it you want something more reliable than setting the threads' priorities?

If you want throttled execution of some code that is constantly generating values, you need to look into chunking up the work the thread(s) do, and coding in your own timer. For example, the java.util.Timer allows for scheduling execution at a fixed rate.

Any other technique will still consume as much CPU as is available (1 core per thread, assuming no locks preventing concurrent execution) when the scheduler doesn't have other tasks to prioritize ahead of yours.

回答3:

The detail is simply that you said "must generate values continuously", and if that, to the extreme, is true, then CPU saturation is actually the goal.

But, if you define "continuously" as X values per second, then there is room to work.

Because then you can run your process at 100% CPU, measure the number of values over time, and if you find that it's generates more values than necessary (more than X/sec), then you can now insert pauses in to the process as appropriate until the value rate reaches your desired goal.

The plan being to continually monitor and adjust the pauses to maintain your value rate over time. Then your process will take as much CPU as necessary to meet your values/sec goal.

Addenda:

If you have a benchmark of values/sec that you are happy with, then interjecting the sleeps will give "all the priority necessary" to the other applications, but still maintain your throughput. If, on the other hand, you don't have any solid requirement, that is the requirement is "run as fast as possible when nothing else is running, with no actual requirement for ANY results if some other process dominates the CPU", then that's truly a kernel issue of the host OS, and not something the JVM has any direct, portable mechanism to address.

On Unix systems, you have the nice(1) command to adjust process (not thread) priority, and Windows has their own mechanism. With these commands, you can knock the priority of your Java process to just above "idle" (the default "process" that always runs when nothing else is running). But it's platform specific, as this is an inherently platform specific problem. This may well be managed through platform specific startup scripts that launch your Java program (or even a Java launcher that detects the platform and "does the right thing" before executing your actual code).

Most systems will allow you to lower your own process priorities, but few will let you raise unless you're an admin/superuser or have whatever the appropriate role is for your host OS.

回答4:

Check to see if you have any "tight loops" in your code.

while (true) {
  if (object.checkSomething()) {
    ...
  }
}

If you do, then you are burning the CPU cycles on millions of checks that are probably not that time critical. The JVM will oblige (because it doesn't know if the check is "important" or not) and you'll get 100% CPU.

If you find such loops, rewrite them like so

while (true) {
  if (object.checkSomething()) {
    ...
  }
  try {
    Thread.sleep(100);
  } catch (InterruptedException e) {
    // purposefully do nothing
  }
}

and the sleeping will voluntarily release the CPU within the loop, preventing it from running too quickly (and checking the condition too many times).

回答5:

Really interesting thread. I found out Java does not provide means for doing what I want to do, and the only way to do this is from outside the JVM. I ended up using nice to alter the scheduling priority in my test (Linux) environment, will still need to find something similar for WIn-based OSs.

Everyone's intervention has been much appreciated.

来源：https://stackoverflow.com/questions/12302196/throttling-cpu-from-within-java

标签

java

multithreading

jvm

cpu

throttling