翻译于 2013/04/25 11:27
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Java performance has the reputation of being something of a Dark Art. Partly this is due to the sophistication of the platform, which makes it hard to reason about in many cases. However, there has historically also been a trend for Java performance techniques to consist of a body of folk wisdom rather than applied statistics and empirical reasoning. In this article, I hope to address some of the most egregious of these technical fairytales.
Of all the most outdated Java Performance fallacies, this is probably the most glaringly obvious.
Sure, back in the 90s and very early 2000s, Java could be slow at times.
However we have had over 10 years of improvements in virtual machine and JIT technology since thenand Java's overall performance is now screamingly fast.
In six separate web performance benchmarks, Java frameworks took 22 out of the 24 top-four positions.
The JVM's use of profiling to only optimize the commonly-used codepaths, but to optimize those heavily has paid off. JIT-compiled Java code is now as fast as C++ in a large (and growing) number of cases.
Despite this, the perception of Java as a slow platform persists, perhaps due to a negative historical bias from people who had experiences with early versions of the Java platform.
We suggest remaining objective and assessing up-to-date performance results before jumping to conclusions.
Consider the following short line of code:
MyObject obj = new MyObject();
To a Java developer, it seems obvious that this code must allocate an object and run the appropriate constructor.
From that we might begin to reason about performance boundaries. We know that there is some finite amount of work that must be going on, and so we can attempt to calculate performance impact based on our presumptions.
This is a cognitive bias that can trap us into thinking that we know, a priori, that any work will need to be done at all.
In actuality, both javac and the JIT compiler can optimize away dead code. In the case of the JIT compiler, code can even be optimized away speculatively, based on profiling data. In such cases the line of code won't run at all, and so it will have zero performance impact.
Furthermore, in some JVMs, such as JRockit, the JIT compiler can even decompose object operations so that allocations can be avoided even if the code path is not completely dead.
The moral of the story here is that context is significant when dealing with Java performance, and premature optimization can produce counter-intuitive results. For best results don’t attempt to optimize prematurely. Instead always build your code and use performance tuning techniques to locate and correct your performance hot spots.
MyObject obj = new MyObject();
As we saw above, reasoning about a small section of code is less accurate than analyzing overall application performance.
Nonetheless developers love to write microbenchmarks. The visceral pleasure that some people derive from tinkering with some low-level aspect of the platform seems to be endless.
Richard Feynman once said: "The first principle is that you must not fool yourself - and you are the easiest person to fool". Nowhere is this truer than when writing Java microbenchmarks.
Writing good microbenchmarks is profoundly difficult. The Java platform is sophisticated and complex, and many microbenchmarks only succeed in measuring transient effects, or other unintended aspects of the platform.
For example, a naively written microbenchmark will frequently end up measuring the timing subsystem or perhaps garbage collection rather than the effect it was trying to capture.
Only developers and teams that have a real need for should write microbenchmarks. These benchmarks should be published in their entirety (including source code), and should be reproducible and subject to peer review and deep scrutiny.
The Java platform's many optimizations imply that statistics of individual runs matters. A single benchmark must be run many times and the results aggregated to get a really reliable answer.
If you feel you must write microbenchmarks, then a good place to start is by reading the paper "Statistically Rigorous Java Performance Evaluation" by Georges, Buytaert, Eeckhout. Without proper treatment of the statistics, it is very easy to be misled.
There are well-developed tools and communities around them (for example, Google's Caliper) - if you absolutely must write microbenchmarks, then do not do so by yourself - you need the viewpoints and experience of your peers.
A very familiar cognitive fallacy among developers (and humans in general) is to assume that the parts of a system that they control are the important ones.
In Java performance, this manifests itself by Java developers believing that algorithmic quality is the dominant cause of performance problems. Developers think about code, so they have a natural bias towards thinking about their algorithms.
In practice, when dealing with a range of real-world performance problems, algorithm design was found to be the fundamental issue less than 10% of the time.
Instead, garbage collection, database access and misconfiguration were all much more likely to cause application slowness than algorithms.
Most applications deal with relatively small amounts of data, so that even major algorithmic inefficiencies don't often lead to severe performance problems. To be sure, we are acknowledging that the algorithms were suboptimal; nonetheless the amount of inefficiency they added was small relative to other, much more dominant performance effects from other parts of the application stack.
So our best advice is to use empirical, production data to uncover the true causes of performance problems. Measure; don't guess!
"Every problem in Computer Science can be solved by adding another level of indirection"
This programmer's aphorism, attributed to David Wheeler (and thanks to the Internet, to at least two other Computer Scientists), is surprisingly common, especially among web developers.
Often this fallacy arises due to analysis paralysis when faced with an existing, poorly understood architecture.
Rather than deal with an intimidating extant system, a developer will frequently choose to hide from it by sticking a cache in front and hoping for the best. Of course, this approach just complicates the overall architecture and makes the situation worse for the next developer who seeks to understand the status quo of production.
Large, sprawling architectures are written one line, and one subsystem at a time. However, in many cases simpler, refactored architectures are more performant - and they are almost always easier to understand.
So when you are evaluating whether caching is really necessary, plan to collect basic usage statistics (miss rate, hit rate, etc.) to prove that the caching layer is actually adding value.
A fact of life of the Java platform is that all application threads must periodically stop to allow Garbage Collection to run. This is sometimes brandished as a serious weakness, even in the absence of any real evidence.
Empirical studies have shown that human beings cannot normally perceive changes in numeric data (e.g. price movements) occurring more frequently than once every 200ms.
Consequently for applications that have a human as their primary user, a useful rule of thumb is that Stop-The-World (STW) pause of 200ms or under is usually of no concern. Some applications (e.g. streaming video) need lower GC jitter than this, but many GUI applications will not.
There are a minority of applications (such as low-latency trading, or mechanical control systems) for which a 200ms pause is unacceptable. Unless your application is in that minority it is unlikely your users will perceive any impact from the garbage collector.
It is also worth mentioning that in any system where there are more application threads than physical cores, the operating system scheduler will have to intervene to time-slice access to the CPUs. Stop-The-World sounds scary, but in practice, every application (whether JVM or not) has to deal with contended access to scarce compute resources.
Without measurement, it isn't clear that the JVM's approach has any meaningful additional impact on application performance.
In summary, determine whether pause times are actually affecting your application by turning on GC logs. Analyze the logs (either by hand, or with scripting or a tool) to determine the pause times. Then decide whether these really pose a problem for your application domain. Most importantly, ask yourself a most poignant question: have any users actually complained?
因此对以人类作为首要用户的应用，一条有用的经验就是200毫秒或低于200毫秒的 Stop-The-World (STW)停顿通常无需考虑。有些应用（例如视频流）需要比这个更低的GC波动，但是很多GUI应用不是的。
One common response to the feeling that Stop-The-World pauses are somehow bad is for application groups to invent their own memory management techniques within the Java heap. Often this boils down to implementing an object pooling (or even full-blown reference-counting) approach and requiring any code using the domain objects to participate.
This technique is almost always misguided. It often has its roots in the distant past, where object allocation was expensive and mutability was deemed inconsequential. The world is very different now.
Modern hardware is incredibly efficient at allocation; the bandwidth to memory is at least 2 to 3GB on recent desktop or server hardware. This is a big number; outside of specialist use cases it is not that easy to make real applications saturate that much bandwidth.
Object pooling is generally difficult to implement correctly (especially when there are multiple threads at work) and has several negative requirements that render it a poor choice for general use:
In summary, object pooling should only be used when GC pauses are unacceptable, and intelligent attempts at tuning and refactoring have been unable to reduce pauses to an acceptable level.
By default, the Oracle JDK will use a parallel, stop-the-world collector for collecting the old generation.
An alternative choice is Concurrent-Mark-Sweep (CMS). This allows application threads to continue running throughout most of the GC cycle, but it comes at a price, and with quite a few caveats.
Allowing application threads to run alongside GC threads invariably results in application threads mutating the object graph in a way that would affect the liveness of objects. This has to be cleaned up after the fact, and so CMS actually has two (usually very short) STW phases.
This has several consequences:
Depending on the application circumstances these prices may be worth paying or they may not. But there’s no such thing as a free lunch. The CMS collector is a remarkable piece of engineering, but it is not a panacea.
So before concluding that CMS is your correct GC strategy, you should first determine that STW pauses from Parallel Old are unacceptable and can't be tuned. And finally, (and I can’t stress this enough), be sure that all metrics are obtained on a production-equivalent system.
Oracle JDK默认使用一个并行的，全部停止（stop-the-world STW)垃圾收集器来收集老年代的垃圾。
When an application is in trouble and GC is suspected, many application groups will respond by just increasing the heap size. Under some circumstances, this can produce quick wins and allow time for a more considered fix. However, without a full understanding of the causes of the performance problem, this strategy can actually make matters worse.
Consider a badly coded application that is producing too many domain objects (with a typical lifespan of say two to three seconds). If the allocation rate is high enough, garbage collections could occur so rapidly that the domain objects are promoted into the tenured (old) generation. Once in tenured, the domain objects die almost immediately, but they would not be collected until the next full collection.
If this application has its heap size increased, then all we're really doing is adding space for relatively short-lived domain objects to propagate into and die. This can make the length of Stop-The-World pauses worse for no benefit to the application.
Understanding the dynamics of object allocation and lifetime before changing heap size or tuning other parameters is essential. Acting without measuring can make matters worse. The tenuring distribution information from the garbage collector is especially important here.
如果这个应用增加其堆内存，那么我们能做的是增加空间，为了存放那些相对短期存在，然后消逝的领域对象。这会使得 Stop-The-World 的时间更长，对应用毫无益处。