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关于jvm:Try-to-Avoid-XX-UseGCLogFileRotation

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Try to Avoid -XX:+UseGCLogFileRotation

Source：http://link.zhihu.com/?target=https%3A//dzone.com/articles/try-to-avoid-xxusegclogfilerotation

Developers take advantage of the JVM argument -XX:+UseGCLogFileRotation to rotate GC log files.

开发人员利用 JVM 参数 -XX:+UseGCLogFileRotation 来旋转 GC 日志文件。

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:/home/GCEASY/gc.log -
XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=20M"

As shown above, the JVM will rotate the GC log file whenever its size reaches 20MB. It will generate up to five files, with extensions gc.log.0,  gc.log.1, gc.log.2, gc.log.3, and gc.log.4.

如上所示的配置会产生 5 个日志文件，并且每个日志文件有 20M。

Losing Old GC Logs

Suppose you configured  -XX:NumberOfGCLogFiles=5, then over a period of time, five GC log files will be created:

假如你配置了-XX:NumberOfGCLogFiles=5，那么在一段期间内，将创立五个 GC 日志文件。

  • gc.log.0 ← _oldest GC Log content_
  • gc.log.1
  • gc.log.2
  • gc.log.3
  • gc.log.4 ← _latest GC Log content_

The most recent GC log contents will be written to gc.log.4 and old GC log contents will be present in gc.log.0.

最新的 GC 日志内容将被写入gc.log.4，旧的 GC 日志内容将呈现在gc.log.0。

When the application starts to generate more GC logs than the configured  -XX:NumberOfGCLogFiles, in this case, five, then old GC log contents in gc.log.0 will be deleted. New GC events will be written to  gc.log.0. It means that you will end up not having all the generated GC logs. You will lose the visibility of all events.

当应用程序配置的 -XX:NumberOfGCLogFiles 产生更多的 GC 日志时（在本例中是 5 个），gc.log.0中的旧 GC 日志内容将被删除。新的 GC 事件将被写入gc.log.0。这意味着你最终将无奈取得所有生成的 GC 日志。你将失去所有事件的可见性。

Mixed-Up GC Logs

Suppose an application has created five GC log files, including:

假如一个应用程序创立了五个 GC 日志文件，包含：

  • gc.log.0
  • gc.log.1
  • gc.log.2
  • gc.log.3
  • gc.log.4

Then, let’s say you are restarting the application. Now, new GC logs will be written to gc.log.0 file and old GC log content will be present in gc.log.1, gc.log.2, gc.log.3, gc.log.4, etc.

而后，假如你正在重启应用程序。当初，新的 GC 日志将被写入 gc.log.0 文件，而旧的 GC 日志内容将呈现在 gc.log.1、gc.log.2、gc.log.3、gc.log.4 等等。

  • gc.log.0 ← GC log file content after restart
  • gc.log.1 ← GC log file content before restart
  • gc.log.2 ← GC log file content before restart
  • gc.log.3 ← GC log file content before restart
  • gc.log.4 ← GC log file content before restart

So, your new GC log contents get mixed up with old GC logs. Thus, to mitigate this problem, you might have to move all the old GC logs to a different folder before you restart the application.

所以你的新 GC 日志和旧的 GC 日志会混合在一起，为了缓解这个问题，你可能要在重启应用程序之前把所有旧的 GC 日志移到一个不同的文件夹里。

Forwarding GC Logs to a Central Location（将 GC 日志转发到一个核心地位）

In this approach, the current active file to which GC logs are written is marked with the extension  .current. For example, if GC events are currently written to the file gc.log.3, it would be named as: gc.log.3.current.

在这种办法中，以后写入 GC 日志的流动文件被标记为扩展名.current。例如，如果 GC 事件以后被写入文件gc.log.3，它将被命名为。gc.log.3.current。

If you want to forward GC logs from each server to a central location, then most DevOps engineers use  rsyslog. However, this file naming convention poses a significant challenge to use rsyslog, as described in this blog.

如果你想把每台服务器的 GC 日志转发到一个核心地位，那么大多数 DevOps 工程师会应用 rsyslog。然而，这种文件命名常规给应用rsyslog 带来了微小的挑战，正如[本博客所述]（http://www.planetcobalt.net/s…）。

Tooling

Now, to analyze the GC log file using the GC tools such as (GCeasy, GCViewer, etc.), you will have to upload multiple GC log files instead of just one single GC Log file.

当初，为了应用 GC 工具剖析 GC 日志文件，如（GCeasy, GCViewer 等），你将不得不上传多个 GC 日志文件，而不是只有一个 GC 日志文件。

Recommended Solution

We can suffix the GC log file with the time stamp at which the JVM was restarted, then the GC Log file locations will become unique. Then, new GC logs will not override the old GC logs. It can be achieved by suffixing %t to the GC log file name, as shown below:

咱们能够在 GC 日志文件后缀上 JVM 重启的工夫戳，那么 GC 日志文件的地位将变得举世无双。而后，新的 GC 日志就不会笼罩旧的 GC 日志了。这能够通过在 GC 日志文件名后缀 %t 来实现，如下所示。

“-XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:/home/GCEASY/gc-%t.log”

 %t suffixes timestamp to the GC log file in the format:  YYYY-MM-DD_HH-MM-SS. So, the generated GC log file name will start to look like: gc-2019-01-29_20-41-47.log.

 %t后缀为 GC 日志文件的工夫戳，格局为:  yyyy-mm-dd_hh-mm-ss。因而，生成的 GC 日志文件名将开始看起来像：gc-2019-01-29_20-41-47.log.

This simple solution addresses all the shortcomings of -XX:+UseGCLogFileRotation.

这个简略的解决方案解决了 -XX:+UseGCLogFileRotation 的所有毛病。

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