Status of AppCDS

[David Holmes]

On 4/10/2017 6:16 AM, August Nagro wrote:
>  > you have to create the shared archive if not using a JRE installer.
> 
> I've tried `java -Xshare:dump` on a Windows and Mac machine with JRE 9 
> installed with the oracle installer. In both cases I had to run the 
> following to generate the archive:
> ```
> sudo touch /lib/[arch]/server/classes.jsa
> sudo chmod 777 /lib/[arch]/server/classes.jsa
> java -Xshare:dump
> ```

Of course you have to have permission to write the shared archive into 
the default location. That's why you would normally do this as part of 
deloying the JRE on your machine (eg as part of the installer process on 
Windows).

There is also an option to specify a different archive location using 
-XX:+UnlockDiagnosticVMoptions -XX:SharedArchiveFile=xxx

David
-----

>  > 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:
> 
> That's great; I was just going off the docs for `-XX:+UseAppCDS` on the 
> man page for java.
> 
>  > *CDS has been constantly improved over the last few releases 
> andcontinues to be improved for the next versions of the platform.
> 
> Good to here!
> 
> Thanks,
> 
> - August
> 
> On Mon, Oct 2, 2017 at 9:30 PM David Holmes <david.holmes at oracle.com 
> <mailto: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
>      > ```
>      >
>