Java Performance Q3_QDwAAQBAJ (100%)

[github-actions[bot]]

Congrats on starting Java Performance by Scott Oaks, I hope you enjoy it! It has an average of unknown/5 stars and 0 ratings on Google Books.

Book details (JSON)

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When you're finished with reading this book, just close this issue and I'll mark it as completed. Best of luck! 👍

[ryanlevell]

When you can't find the right book, use the title + Google API id, e.g.:

https://www.googleapis.com/books/v1/volumes?q=intitle:java%20performance%20Q3_QDwAAQBAJ

[ryanlevell]

Introduction

no optimization in the world is going to make the array-based code as fast as a different version that uses a hash map. A good algorithm is the most important thing when it comes to fast performance.

[ryanlevell]

Profiling Tools

Sampling Profiler: Least amount of overhead by firing periodically. Before Java 8 the stack trace could only be obtained during safe points, e.g. blocked by sync lock or waiting for IO. A new interface AsyncGetCallTrace was introduced in Java 8 and where async profilers come from. Now the profiler can get the stack trace at any time.

Some tools like async-profiler can measure native code as well.

Instrumented Profiler: Alters bytecode and gives more beneficial information at the cost of being very intrusive. It is recommended to use a sampling profiler to narrow the search to a package or class, then use the instrumented profiler on that single package/class.

jvisualvm has a built-in profiler that will show very different results. This is because it measure blocking that does not consume CPU time. Most profilers do no report method that are blocked. This is because they may not be optimizable, e.g. waiting for waiting for a network call to complete. Measuring blocked methods can often be toggled in most profilers.

[ryanlevell]

Common Compiler Flags

-XX:ReservedCodeCacheSize=N

This is the maximum cache size.

On 64-bit machines with sufficient memory, setting the value too high is unlikely to have a practical effect on the application.

-XX:+PrintCompilation

Prints every line that is compiled that has information about what has been compiled.

timestamp compilation_id attributes (tiered_level) method_name deopt

Attributes

% The compilation is OSR. On Stack Replacement is when the compiled code replaces code that is still running. s The method is synchronized. ! The method has an exception handler. b Compilation occurred in blocking mode. n Compilation occurred for a wrapper to a native method.

The printed lines may also contain:

• made not entrant Happens 1. When code optimization assumption is incorrect (e.g., due to an interface variable changing to a different instance) and 2. when code compiled by C1 has now been compiled to C2.
• made zombie When all the objects referenced by compiled code that is now marked made not entrant gas been garbage collected, it can be marked made zombie and the code is removed from the code cache.

Tier Level

0 Interpreted code 1 Simple C1 compiled code 2 Limited C1 compiled code 3 Full C1 compiled code 4 C2 compiled code

C1 is a faster compiler. C2 is slower but makes better optimizations. Modern JVMs use both called tiered compilation.

-XX:CICompilerCount=N

The number of compiler threads. 1/3 of the threads for C1, the rest for C2.

Note: There is no need to modify this flag with modern versions of Java. For Docker, one consequence of early versions of Java 8 is that it would use the machine's full specs rather than what is provided for the Docker instance to calculate JVM parameters. This may be one place you will see this flag used.