Need Information for memory sharing with AppCDS

[Jiangli Zhou]

Hi,

On Sun, Mar 8, 2020 at 11:56 PM 向伟(识月) <shiyue.xw at alibaba-inc.com> wrote:
>
> Hi Ioi,
>
> Thanks a lot for your information.
> I have the question about the usage in the document:
> For common class path prefix:
> ---------------------------------------
>     Run the Hello and Hi applications with the same shared archive:
> java -XX:SharedArchiveFile=common.jsa -cp common.jar:hello.jar:hi.jar Hello
> java -XX:SharedArchiveFile=common.jsa -cp common.jar:hello.jar:hi.jar Hi
>
> If a class with name ABC is both in hello.jar and hi.jar, the content is different.
> With the class path -cp common.jar:hello.jar:hi.jar, the second run "java .. Hi" can't load the correct class ABC in hi.jar.
>

For the above usage, I'd recommend to only archive the classes from
common.jar to create a common.jsa. The common.jsa can be used for both
applications for sharing. As described in the earlier email, the
follow two conditions must be true to achieve sharing:

- Both processes must be running with the same JVM binary
- Both processes must be memory mapping using the same copy of common.jsa.

If each container uses a different copy of the common.jsa, then there
is no memory sharing.

> Is there any solution for this usage?

I'd recommend to not include ABC in the common.jsa in this case.

Hope that helps.

Best,
Jiangli
>
> Thanks,
> Wei Xiang
> ------------------------------------------------------------------
> 发件人：Ioi Lam <ioi.lam at oracle.com>
> 发送时间：2020年3月9日(星期一) 13:03
> 收件人：hotspot-runtime-dev <hotspot-runtime-dev at openjdk.java.net>
> 主　题：Re: Need Information for memory sharing with AppCDS
>
> On 3/8/20 8:51 PM, 向伟(识月) wrote:
> > Hi,
> >
> > We am trying to take advantage of the characteristics of memory sharing with AppCDS, and we expected that the
> > JVM internal packages and common packages can be shared with AppCDS.
> >
> > In the user scenario, there are some common base packages for different Java processes with "-cp common_path"
> > Process A: -cp common_path:pathA
> > Process B: -cp common_path:pathA:pathB
> > Process C: -cp common_path:pathC
> >
> > I check the code in JDK11, and it uses SharedPathsMiscInfo to do the consistency check for class path.
> > The below usage can work in JDK11
> > Dump stage:
> > -cp common_path
> >
> > Runtime stage:
> > -cp  common_path:new_path
> >
> > But I find that SharedPathsMiscInfo has been removed in the latest JDK code.
> >
> > I need the information:
> > 1. the new flow to do the consistency check.
> > 2. constraint condition about the memory sharing among the different Java processes.
> >
> > Is there any user guide to use the memory sharing feature for AppCDS?
>
> Hi Wei,
>
> The topic of sharing the same CDS archive between different apps that
> have the same classpath prefix can be found here:
>
> https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html#application-class-data-sharing
>
> See the section "Sharing a Shared Archive Across Multiple Application
> Processes".
>
> SharedPathsMiscInfo was removed in this RFE in JDK 14:
> https://bugs.openjdk.java.net/browse/JDK-8227370
>
> However, we still enforce the same set of consistency checks as previous
> JDK versions.
>
> Please let me know if you have further questions.
>
> Thanks
> - Ioi
>
> > Thanks,
> > Wei Xiang
>