Status of AppCDS

[Jiangli Zhou]

Hi August,

Just to add some info in addition to David’s response. CDS is continuously being enhanced since JDK 8. 

Regarding how CDS works with AOT, the shared archive generated by CDS can coexist with the AOT library and used by JVM at runtime. Here is a very informative blog wrote by Matthew Gilliard, http://mjg123.github.io/2017/10/02/JVM-startup.html <http://mjg123.github.io/2017/10/02/JVM-startup.html>. It gives examples of how to use CDS and AOT to improve startup time.

Regards,
Jiangli


> On Oct 2, 2017, at 7:29 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> 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
>> ```