Status of AppCDS

[David Holmes]

On 3/10/2017 12:29 PM, David Holmes wrote:
> Moving to hotspot-runtime-dev and bcc'ing the discuss list.
> 
> Hello Augusto,

Apologies - August.

David

> 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
>> ```
>>