Test the build

A few benchmarks of varying size will be used throughout the tutorial. If you haven't already, set them up now. All of the following commands should be entered in repos/openjdk.

HelloWorld (simplest, will never trigger GC):
1. Copy the following code into a new Java file titled "HelloWorld.java" in mmtk-openjdk/repos/openjdk:
```
class HelloWorld {
   public static void main(String[] args) {
      System.out.println("Hello World!");
   }
}
```
2. Use the command ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/javac HelloWorld.java.
3. Then, run ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/java -XX:+UseThirdPartyHeap HelloWorld to run HelloWorld.
4. If your program printed out Hello World! as expected, then congratulations, you have MMTk working with OpenJDK!
The Computer Language Benchmarks Game fannkuchredux (micro benchmark, allocates a small amount of memory but - depending on heap size and the GC plan - may not trigger a collection):
1. Copy this code into a new file named "fannkuchredux.java" in mmtk-openjdk/repos/openjdk.
2. Use the command ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/javac fannkuchredux.java.
3. Then, run ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/java -XX:+UseThirdPartyHeap fannkuchredux to run fannkuchredux.
DaCapo benchmark suite (most complex, will likely trigger multiple collections):
1. Fetch using wget https://sourceforge.net/projects/dacapobench/files/9.12-bach-MR1/dacapo-9.12-MR1-bach.jar/download -O ./dacapo-9.12-MR1-bach.jar.
2. DaCapo contains a variety of benchmarks, but this tutorial will only be using lusearch. Run the lusearch benchmark using the command ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/java -XX:+UseThirdPartyHeap -Xms512M -Xmx512M -jar ./dacapo-9.12-MR1-bach.jar lusearch in repos/openjdk.

Rust Logs

By using one of the debug builds, you gain access to the Rust logs - a useful tool when testing a plan and observing the general behaviour of MMTk. There are two levels of trace that are useful when using MMTk - trace and debug. Generally, debug logs information about the slow paths (allocation through MMTk, rather than fast path allocation through the binding). trace includes all the information from debug, plus more information about both slow and fast paths and garbage collection activities. You can set which level to view the logs at by setting the environment variable RUST_LOG. For more information, see the env_logger crate documentation.

Working with different GC plans

You will be using multiple GC plans in this tutorial. You should familiarise yourself with how to do this now.

The OpenJDK build will always generate in mmtk-openjdk/repos/openjdk/build. From the same build, you can run different GC plans by using the environment variable MMTK_PLAN=[PlanName]. Generally you won't need multiple VM builds. However, if you do need to keep a build (for instance, to make quick performance comparisons), you can do the following: rename either the build folder or the folder generated within it (eg linux-x86_64-normal-server-$DEBUG_LEVEL).
1. Renaming the build folder is the safest method for this.
2. If you rename the internal folder, there is a possibility that the new build will generate incorrectly. If a build appears to generate strangely quickly, it probably generated badly.
3. A renamed build folder can be tested by changing the file path in commands as appropriate.
4. If you plan to completely overwrite a build, deleting the folder you are writing over will help prevent errors.

Try running your build with NoGC. Both HelloWorld and the fannkuchredux benchmark should run without issue. If you then run lusearch, it should fail when a collection is triggered. It is possible to increase the heap size enough that no collections will be triggered, but it is okay to let it fail for now. When we build using a proper GC, it will be able to pass. The messages and errors produced should look identical or nearly identical to the log below.

$ MMTK_PLAN=NoGC ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/java -XX:+UseThirdPartyHeap -Xms512M -Xmx512M -jar ./dacapo-9.12-MR1-bach.jar lusearch
Using scaled threading model. 24 processors detected, 24 threads used to drive the workload, in a possible range of [1,64]
Warning: User attempted a collection request, but it is not supported in NoGC. The request is ignored.
===== DaCapo 9.12-MR1 lusearch starting =====
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
thread '<unnamed>' panicked at 'internal error: entered unreachable code: GC triggered in nogc', /opt/rust/toolchains/nightly-2020-07-08-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/macros.rs:16:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
[2020-12-18T00:27:49Z INFO  mmtk::plan::global]   [POLL] nogc_space: Triggering collection
fatal runtime error: failed to initiate panic, error 5
Aborted (core dumped)

Try running your build with SemiSpace. lusearch should now pass, as garbage will be collected, and the smaller benchmarks should run the same as they did while using NoGC.
```
MMTK_PLAN=SemiSpace ./build/linux-x86_64-normal-server-$DEBUG_LEVEL/jdk/bin/java -XX:+UseThirdPartyHeap -Xms512M -Xmx512M -jar ./dacapo-9.12-MR1-bach.jar lusearch
```

MMTk User Guide

Test the build

Rust Logs

Working with different GC plans