Apparently there is something in Sun's JVM called "Class Data Sharing", which involves preloading the rt.jar classes and writing a memory image of them down to disk, which is later mmaped as read-only and thus able to be shared memory between running VMs, with a view to saving memory across a system and (more to the point) reducing start-up time for systems running `java -client`. The referenced pages mention that Sun's "installer" builds classes.jsa on the target machine at installation, but it describes the (undocumented) option to build it yourself as necessary, # java -client -Xshare:dump It writes classes.jsa to the JRE's system directory, so you'll need to be root. My own local testing showed a small but noticeable startup speed improvement. Perhaps others could try this and report. AfC
There's no Client VM for amd64 for example.
Nice idea though, we just need to come up with nice USE flag name + description (the long description in metadata.xml would probably help here). The flag would be obviously masked on amd64 because nelchael is right.
For what it's worth, by all appearances the thing just (politely) fails if the VM doesn't support it (Try `java -server -Xshare:dump` to see what I mean); could just include it in the ebuild; if it works it works, if not, no biggie. Not so sure you need a whole USE flag for it. Incidentally, more testing has shown the startup benefit is definitely significant, and the simple fact of page sharing between VMs is exciting. If you want to see it using the shared data, try: $ java -client -verbose:class -Xshare:off HelloWorld (ie, the normal state of affairs) vs $ java -client -verbose:class -Xshare:on HelloWorld (force it, and compare what it says when loading classes) AfC P.S. I've always forced my build and launcher systems to specify -client; most people thought I was weird for it as it was the "default"; interestingly on my current system the `java` front end has decided that -server is the default.
(In reply to comment #3) > Not so sure you need a whole USE flag for it. we would have to be able to support the default option, as we don't know the side effects. Plus we could always enable the flag by default and then ppl could disable it if they really wanted to. > > Incidentally, more testing has shown the startup benefit is definitely > significant, and the simple fact of page sharing between VMs is exciting. > Numbers, we need numbers (well, I want numbers) Poor me and my amd64 vms :(
For reference, here is some doco's http://java.sun.com/javase/6/docs/technotes/guides/vm/class-data-sharing.html
(In reply to comment #5) > For reference, here are some docs That is the URL I put in the bug's URL field :) AfC
Turns out that Sun's 1.6 JRE also supports Class Data Sharing for it's server VM. (as I found out when I ran `java -Xshare:dump` and forgot to supply `-client` onyl to have it succeed and write: /opt/sun-jdk-1.6.0.04/jre/lib/i386/server/classes.jsa Running again as `java -client -Xshare:dump` resulted in: /opt/sun-jdk-1.6.0.04/jre/lib/i386/client/classes.jsa being created. Huh. AfC
I just noticed the emul-linux-x86-java:1.5 ebuilds do this, and a bit of search revealed that we already did it in 1.5 JDK's (client only) until it disappeared for some unknown reason in 1.5.0.09...
(In reply to comment #7) > Turns out that Sun's 1.6 JRE also supports Class Data Sharing for it's server > VM. (as I found out when I ran `java -Xshare:dump` and forgot to supply > `-client` onyl to have it succeed and write: Not on amd64 :( gehinom ~ # GENTOO_VM=sun-jre-bin-1.6 java -Xshare:dump Error occurred during initialization of VM Dumping a shared archive is not supported on the Server JVM. gehinom ~ # GENTOO_VM=sun-jre-bin-1.6 java -client -Xshare:dump Error occurred during initialization of VM Dumping a shared archive is not supported on the Server JVM. But we can add it for x86.
OK, it's added in sun-jdk, sun-jre-bin (for x86) and the emul-linux package (amd64). In 1.6 for both client and server, in 1.5 for client only. JRE's already had it for client VM because the construct.sh script creates it - just added to 1.6 JRE's for server VM. Moved it from construct.sh to the ebuild.
