是什么让JNI呼叫变慢？ [英] What makes JNI calls slow?

问题描述

我知道在Java中进行JNI调用时跨越边界很慢。

I know that 'crossing boundaries' when making a JNI call in Java is slow.

但是我想知道是什么这会让它变慢吗？
在进行JNI调用时，底层的jvm实现做了什么让它变得如此之慢？

However I want to know what is it that makes it slow? What does the underlying jvm implementation do when making a JNI call that makes it so slow?

推荐答案

首先，它是值得注意的是，通过慢，我们谈论的事情可能需要几十纳秒。对于简单的原生方法，在2010年，我在Windows桌面上平均测量了40秒的呼叫，在我的Mac桌面上测量了11 ns。除非您正在进行许多调用，否则您将不会注意到。

First, it's worth noting that by "slow," we're talking about something that can take tens of nanoseconds. For trivial native methods, in 2010 I measured calls at an average 40 ns on my Windows desktop, and 11 ns on my Mac desktop. Unless you're making many calls, you're not going to notice.

也就是说，调用本机方法可以慢于比进行普通的Java方法调用。原因包括：

That said, calling a native method can be slower than making a normal Java method call. Causes include:

• JVM不会内联本机方法。它们也不会为这个特定的机器进行即时编译 - 它们已经编译好了。


• 可以复制Java数组以便在本机代码中进行访问，然后将其复制回来。成本可以是阵列大小的线性。我测量了100,000个阵列的JNI 复制，在我的Windows桌面上平均约为75微秒，在Mac上平均为82微秒。幸运的是，可以通过 GetPrimitiveArrayCritical获取直接访问权限或 NewDirectByteBuffer 。


• 如果方法是传递一个对象，或者需要进行回调，那么本机方法可能会自己调用JVM。从本机代码访问Java字段，方法和类型需要类似于反射的东西。签名在字符串中指定并从JVM查询。这很慢和容易出错。


• Java字符串是对象，具有长度并且是编码的。访问或创建字符串可能需要O（n）副本。

• Native methods will not be inlined by the JVM. Nor will they be just-in-time compiled for this specific machine -- they're already compiled.
• A Java array may be copied for access in native code, and later copied back. The cost can be linear in the size of the array. I measured JNI copying of a 100,000 array to average about 75 microseconds on my Windows desktop, and 82 microseconds on Mac. Fortunately, direct access may be obtained via GetPrimitiveArrayCritical or NewDirectByteBuffer.
• If the method is passed an object, or needs to make a callback, then the native method will likely be making its own calls to the JVM. Accessing Java fields, methods and types from the native code requires something similar to reflection. Signatures are specified in strings and queried from the JVM. This is both slow and error-prone.
• Java Strings are objects, have length and are encoded. Accessing or creating a string may require an O(n) copy.

可以在Java中找到一些可能过时的讨论。 平台性能：策略和策略，2000年，Steve Wilson和Jeff Kesselman，9.2：检查JNI成本部分。大约三分之一的时间这个页面，由@Philip在下面的评论中提供。

Some additional discussion, possibly dated, can be found in "Java¿ Platform Performance: Strategies and Tactics", 2000, by Steve Wilson and Jeff Kesselman, in section "9.2: Examining JNI costs". It's about a third of the way down this page, provided in the comment by @Philip below.

2009年IBM developerWorks论文使用Java Native Interface的最佳实践提供了一些避免使用JNI的性能缺陷的建议。

The 2009 IBM developerWorks paper "Best practices for using the Java Native Interface" provides some suggestions on avoiding performance pitfalls with JNI.

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