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Saturday, January 02, 2010

Don't Yield to pressure?

or: does Thread.yield have its place in todays Java programs?

I was profiling a rather old legacy codebase (since the first rule of performance optimization is "profile it" with the close second of "have clear goals in mind" - but that's an other post) and - after optimizing the first few hotspots, Thread.yield appeared at the top of the most timely methods. I was intrigued, since I didn't use yield since I wrote "cooperative multitasking" programs for Windows 3.1 in VB 3 (and I'm not a big Python programmer either). So I scoured the 'net for Information on why/when you should use Thread.yield, but came up with relative few pieces of information:

  • I found indications that some old Linux kernels behaved poorly on single processor machines if Thread.yield wasn't used (as in: one thread consuming all the CPU)
  • There were discussion about using Thread.sleep(0) vs. Thread.yield (apparently there is a difference regarding the treatment of remaining time-quantum by the scheduler)
  • ... this was pretty much it ...

So I've decided to do some micro-benchmarks. They consisted of a producer and a consumer thread, connected by an unbounded queue (a LinkedBlockingQueue to be more exact) and I measure the number of items produced / consumer in 10 seconds. The first set of measurements were performed on a dual-core machine, while the second set in a VM to simulate a single-CPU system (it's kind of ironic that one has to perform simulation to evaluate single-core systems). This isn't meant to be a performance, evaluation, thus all the numbers are normalized to the produced/direct number.

2 CPUs 1 CPU
Direct 1 1 1 0.06
Thread.yield() 0.12 0.12 0.04 0.04
Thread.sleep(0) 0.1 0.1 0.04 0.04
Priority - 1 0.81 0.81 1 0.05

(My) conclusions:

  • These days there is no need for "helping" the OS scheduler out. Both of the proposed methods (yield and sleep) reduced the throughput of the system considerably.
  • Speaking of throughput: make a decision about the (performance) numbers your system should achieve. This includes both throughput and delay. Concrete (and realistic!) numbers. "As good as possible" is not a number! Neither is "better than the current". Then profile and optimize it until the numbers are achieved and no further.
  • In the case of a single CPU system there was a big imbalance between the speed of the producer and consumer which Thread.yield (or Thread.sleep) seemed to solve. Consider however, that this "solution" comes at the price of (almost) two orders of magnitude reduction in the throughput. A much better solution would be (in case you are in the rare situation of single CPU - maybe you're on a VPS, or you have multiple CPUs, but the number of threads far outweigh the number of CPUs) to use a bounder queue. This way the producer gets slowed down (by blocking repeatedly) if it produces faster than the consumer can consume. Then again, you need consider if this is acceptable for your application. Maybe the "overflow" situation is rare and it is more important to handle each element (and there are enough resources for it in the long run) than response speed. You have to know your application and its priorities. There is no way around it.

Finally I would like to leave you with the following short (~30 min) presentation about performance optimization on the JVM: Making every millisecond count! JVM performance tuning in the real-world.


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