有没有办法在不使用时降低Java堆？ [英] Is there a way to lower Java heap when not in use?

问题描述

我正在研究Java应用程序并努力优化其内存使用情况。据我所知，我正在遵循适当的垃圾收集指南。但是，似乎我的堆似乎处于最大大小，即使它不需要。

I'm working on a Java application at the moment and working to optimize its memory usage. I'm following the guidelines for proper garbage collection as far as I am aware. However, it seems that my heap seems to sit at its maximum size, even though it is not needed.

我的程序每小时运行一次资源密集型任务，当时计算机不被人使用。此任务使用了相当大的内存块，但在任务完成后立即释放所有内存。 NetBeans探查器显示内存使用情况如下所示：

My program runs a resource intensive task once an hour, when the computer is not in use by a person. This task uses a decent chunk of memory, but then frees it all immediately after the task completes. The NetBeans profiler reveals that memory usage looks like this:

我真的很想在不使用时将所有堆空间重新提供给操作系统。没有理由让我把它全部占用，而程序甚至不会在至少一个小时内做任何事情。

I'd really like to give all of that heap space back to the OS when not in use. There is no reason for me to hog it all while the program won't even be doing anything for at least another hour.

这可能吗？谢谢。

推荐答案

你可以玩 -XX：MaxHeapFreeRatio - 这是在GC缩小之前可用的堆的最大百分比（默认值为70）。也许将它设置得稍低（40或50？），然后使用 System.gc（）可能会花些时间来达到预期的行为？

You could perhaps play around with -XX:MaxHeapFreeRatio - this is the maximum percentage (default 70) of the heap that is free before the GC shrinks it. Perhaps setting it a bit lower (40 or 50?) and then using System.gc() might go some lengths to get you the desired behaviour?

没有办法强迫这种情况发生，但是，你可以尝试并鼓励JVM这样做，但你不能只是随心所欲地抽出记忆。虽然上面的内容可能会缩小堆，但内存不一定会直接返回给操作系统（尽管在最近的JVM实现中它也是如此。）

There's no way to force this to happen however, you can try and encourage the JVM to do so but you can't just yank memory away as and when you want to. And while the above may shrink the heap, that memory won't necessarily be handed straight back to the OS (though in recent implementations of the JVM it does.)

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