Status of AppCDS

[David Holmes]

Moving to hotspot-runtime-dev and bcc'ing the discuss list.

Hello Augusto,

On 3/10/2017 7:25 AM, August Nagro wrote:
> Hello,
> 
> Both Class Data Sharing (CDS) [1] and AppCDS [2] are very interesting but
> seemingly neglected features of the Java Platform, offering the ability to
> reduce startup time.

AppCDS is under continual development and is not "neglected" at all.

> Class Data Sharing is the global cache stored in
> /lib/[arch]/server/classes.jsa, and circumvents long class-loading times by
> caching the JVM’s internal representation of system jars and memory-mapping
> them in during startup. The feature's documentation [1] has been updated
> for Java 9, but I have yet to see a JDK or JRE installation that can use it
> without manually generating the archive. In every case I've encountered,
> the file needs to first be created (and given appropriate access
> permissions) with admin access, and then generated by `java -Xshare:dump`.

Yes you have to create the shared archive if not using a JRE installer. 
But that can be done by whomever deploys the JRE in your environment. My 
understanding is that, due to the format of the archive, a given archive 
is only guaranteed to beusable on the machine on which it was created.

> AppCDS extends the same feature to user libraries. It works well, but has
> been experimental since (before?) JDK 8 and is commercial.

Not sure it is still considered "experimental" but it is still evolving. 
It is a commercial feature in Java 9 but will not be in the future:

http://mail.openjdk.java.net/pipermail/discuss/2017-September/004281.html

> 
> Java has been criticized for its slow startup times. Improvements would
> enable command line utilities, the realistic use of JShell .jsh scripts
> (currently takes >4 seconds for hello world on my Mac [3]), and other
> applications. I'm not sure how *CDS concerts with other OpenJDK efforts
> like AOT compilation, but it offers the advantage of architecture
> independency and the ability to be shared across multiple JVMs.

An archive can be shared across multiple JVMs on the same machine.

> Is *CDS doomed for deprecation, or will it be improved? Without work to CDS
> or changes to AppCDS, the current state is not very usable.

*CDS has been constantly improved over the last few releases and 
continues to be improved for the next versions of the platform.

Regards,
David Holmes

> Regards,
> 
> August Nagro
> 
> [1]: https://docs.oracle.com/javase/9/vm/class-data-sharing.htm
> [2]:
> http://docs.oracle.com/javase/9/tools/java.htm#JSWOR-GUID-31503FCE-93D0-4175-9B4F-F6A738B2F4C4
> 
> [3]:
> Executing command `jshell helloworld.jsh`, with file containing
> ```
> System.out.println("hello world");
> /exit
> ```
>