The Java Virtual Machine provides a managed runtime environment for applications that have been compiled into bytecodes (but may not have necessarily been written in Java).  This offers numerous benefits to application developers and, often, improved performance over code statically compiled for a specific platform.  The JVM handles memory allocation and recovery automatically via the garbage collector (GC), reducing the potential for memory leaks.  Just-in-time (JIT) compilation provides the “write once, run anywhere” ability, eliminating the need to build separate binary versions of an application for each platform supported.

These advantages are not entirely without cost, though.  Although the overall speed of an application running on the JVM may be ultimately faster, there is a warm-up time required, as frequently used methods are compiled and optimized.  Each time an application is started the same profiling, analysis and compilation must be performed, even if the application is being used identically.

Azul Systems has, for many years, been working on ways to minimize the performance impact of these aspects of the JVM.  The Zing JVM uses the Falcon JIT compiler in place of the old C2 JIT and ReadyNow! technology to record a profile that can be used when restarting an application.

Azul’s Zulu build of OpenJDK now includes a similar set of technologies, which we call Checkpoint/Restore at Main (CRaM).

The idea of CRaM is to reduce the warm-up time of an application by performing a training run, which can then be used during a production run.  Training runs can be performed in three different ways:

  1. The application is used, as normal, and performs whatever functions are necessary.  The application terminates by exiting the main() method.  At this point, all data from the application run is recorded.  No changes are required to application code; adding the -Zcheckpoint JVM flag is all that is required.
  2. The application is used, as per scenario 1, above, but terminates by calling System.exit().  Again, no changes are required to the application code, but, in this case, the JVM flag, -Dcom.azul.System.exit.doCheckpointRestore=true must be used.
  3. In this scenario, the developer chooses a specific point in the application code where they would like to generate a checkpoint.  Changes to the application code are necessary; a call to the method, Dcom.azul.System.tryCheckpointRestore(), is placed where required.  This is useful for applications that do not terminate.  The call is ignored unless the -Zcheckpoint flag is specified for the JVM.  An additional flag, -XX:CRTrainingCount, is available to enable an application to process more than one transaction before recording the checkpoint.

The checkpoint is a sophisticated snapshot of the state of the application when it is created.  It includes the following information:

  • The JVMs internal representation of Java classes.  Each time an application starts, it needs to read the required classes and create its own representation of each class with initialized data.
  • The code that has been generated by the JVM JIT compilers, C1 and C2.  Because of the way this code is reused, it is necessary to turn off certain optimizations to enable the code to be reused in a production run.
  • Initialized system classes.  These are classes from the core class libraries and is independent of any application code.
  • Certain Java objects from the heap that are related to the startup of the application.

There are strict limitations to where a checkpoint can be used for a production run.  A checkpoint is closely tied to the platform used for the training run, and it includes very low-level information, such as memory pages from mapped system libraries like libc.  A checkpoint will not work if changes are made to system libraries, the JDK or the application code before a production run is performed.  Checkpoints should only be shared between machines running the same hardware and software stack.

To use a checkpoint for a production run, you would use a command-line like this:

java -Zrestore myAppClass <application arguments>

The previously stored checkpoint data will be used to minimize, as far as possible, the warm-up time associated with the application.  There are a couple of important points to note:

  • During the production run, code may be recompiled as a normal part of the JIT compilation process.  Unlike during the training run, all optimizations available to the JIT will be enabled.
  • The application needs to be started from the same directory where the training run was generated.  This is part of the state of the checkpoint.
  • No JVM command-line flags should be used.  A training run is tied to the command line flags used during its creation, and these are then automatically set during a production run.  Changing them could invalidate the information in the checkpoint.

Currently, the CRaM functionality is targeted at embedded applications, where the ability to run at optimum speed from startup is vital.  As such, the supported platforms for CRaM are Arm 32-bit processors only, running Linux with a kernel of 3.5, or higher and glibc version 2.13, or higher.  CRaM includes a utility, cr-compat-checker, that can be used to verify that a device meets these requirements.

To determine whether CRaM is appropriate for an application, it is crucial to understand how it changes the performance profile of the application.  CRaM is designed to reduce the time taken to get to the point where the checkpoint was generated.  Execution from that point on will be unchanged whether a checkpoint has been used or not.  When looking at the performance of a Java application, it can be divided into two parts: JVM startup time, i.e. time to get to the main() entry point; and time running from main().  When using CRaM, the time required to get to main() will be longer, but the time needed to get to where the checkpoint was created will be less.

To make this easier to understand, a diagram is useful:

As an example, consider a simple Spring Boot application.

Without using CRaM, the time to main() was 2 seconds and the time from entering main() to a fully initialized application, ready to process transactions was 31 seconds.  The time required, therefore, before a transaction could be processed was 33 seconds.

Having taken a checkpoint, using CRaM to start the application, time to main() was increased to 3 seconds.  However, the time from entering main() to fully initialized was only 18 seconds.  This reduces the time required before a transaction can be processed to only 21 seconds, which is substantially quicker.

As you can see, CRaM can make a significant difference in the effectiveness of applications that need to be ready to perform tasks as quickly as possible.  This is especially important in embedded applications, where resources are constrained, and devices may need to be restarted more frequently than a conventional server.

Azul is currently running beta trials of CRaM.  If you are interested in being part of this, please contact us for more information.

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