A picture is worth...
A picture is worth...
JamVM and OpenJDK - Progess Update
Porting JamVM to use the OpenJDK/IcedTea class-library has been on my TODO list for quite a while. It appears on my FOSDEM slides for 2008 and again in 2009. It doesn't this year, as I didn't do a talk. Maybe that's why I've finally got round to doing it. I told Twisti I'd get "Hello World" running in 3 months. I've missed by just over a week, mainly because I only started 2 months ago.
root@traken:/usr/local/jamvm/bin# java -Xbootclasspath:/usr/lib/jvm/java-6-openjdk/jre/lib/rt.jar -Dsun.boot.library.path=/usr/lib/jvm/java-6-openjdk/jre/lib/amd64 -showversion hello
java version "1.6.0_0"
OpenJDK Runtime Environment (IcedTea6 1.6.1) (6b16-1.6.1-3ubuntu1)
JamVM (build 1.5.5-openjdk, inline-threaded interpreter)
hoping that if the existing Linux file systems developers could just talk to each other, we could come up with a way to improve funding for Linux file systems development. I don’t know that it helped, but if it did, I consider this to be my most important contribution to Linux file systems.
Netifera uses JamVM as remote probeNetifera is a very interesting 100% Java-based Open Source network security platform. The next version will include a probe, which is a "deployable software agent that makes it possible to run all netifera platform tools remotely".
JamVM on Beagle Board
I've been intending to get a Beagle Board for quite a while now, to replace a Neo1973 as the development platform for JamVM on ARM. The Neo has an ARM920T core, and I've been particularly wanting to test JamVMs code-copying JIT on a more modern implementation (the Beagle Board has a Cortex A8). But I've been waiting for the Rev C board, as this has working EHCI USB host, and 256MB RAM (previous revisions had 128MB). The board arrived 2 weeks ago, but I haven't had much time to set it up until now...
So far, the results are quite encouraging. SciMark 2.0 shows a 55% speed improvement:
While jBYTEmark shows an 82% improvement on Integer tests:
The option -Xcodestats prints out the size of the JIT code-cache when the VM exits. This shows 77K was used after running SciMark 2.0, and 178K after jBYTEmark. This is using the code-profiling introduced in JamVM 1.5.2.
I read with interest Jeroen's recent post on his blog about his experiences running JRuby 1.2.0RC2 on IKVM. I know I don't run enough "real-world" tests on JamVM, so I finally got around to trying to run it on JamVM over the weekend. Then wished I hadn't. I secretly hoped it would "just work", but it almost immediately segv-ed. This was the first of 5 problems.
The segv was relatively easy to find and fix. A regression introduced in JamVM 1.5.1, when I added unloader objects to unload JNI libraries after the classloader which loaded them is garbage-collected. This was itself a fix for library unloading, which used to be done within GC, but broke if the JNI_OnUnload function called back into Java.
Basically, I didn't take into account libraries which have a JNI_OnUnload function being loaded by the boot classloader (the NULL loader). This is pointless, as the boot classloader is never collected, and therefore no library loaded by it can be unloaded. However, the fix was simple - just ignore them.
The next problem was with MemoryManagerMXBean. I spent some time implementing native support for ThreadMXBean in JamVM 1.4.4 as part of the thread re-work, but never got round to implementing the full set, as nothing much seemed to use them. For now, a simple implementation which just returns "no memory managers" appears to be sufficient.
After that there was a problem with annotations, where an AnnotationTypeMismatchException was being thrown. This took some time to track down because I had to remember how annotations worked! It ended up being a mismatch between an annotation array value and the method return type. When parsing the annotation, the array values can be any one of a number of types, so in JamVM an Object array is created (when the array is created, the elements haven't yet been parsed so the type isn't known). But the method return value is the specific type, in this case String. Luckily, a similar problem has been found with the implementation in gcj, so I was able to adapt the fix into JamVMs version of sun.reflect.annotation.AnnotationInvocationHandler.
Problem number 4 was with VMClassLoader.getPackage(). The default GNU Classpath implementation relies on a META-INF/INDEX.LIST file existing in the first Jar in the bootclasspath. JRuby uses Constantine, which uses the package name to load an appropriate constant class. As the Constantine Jar is added to the bootclasspath, even if the INDEX.LIST existed, it wouldn't have any package information for it. A quick and dirty implementation of VMClassLoader.getBootPackages() which doesn't need INDEX.LIST fixed this.
Finally, there was a problem with Class.getSimpleName(). The simple name is appended to the package name to locate the constant class. However, the GNU Classpath implementation of getSimpleName (which delegates to VMClass) is broken. Again, I took the fix from gcj.
After all this, jRuby runs!
rob@dougal:~$ jruby hello.rb
rob@dougal:~$ jirb --prompt simple
trap not supported or not allowed by this VM
>> include_class java.lang.System
=> "GNU Classpath"
A couple of people after my talk at FOSDEM asked me where the benchmarks were. I didn't have any partly because I didn't think there was time in the talk, partly because I didn't know what to benchmark against, and mostly because I hadn't had time to do any.
So, I've finally done some. I've benchmarked against the JDK template interpreter, because of the interest due to the recent work of Shark and Zero. The benchmarks are from SPECjvm2008. Not all of these run with GNU Classpath, and from the rest, I've selected benchmarks which should hopefully not be influenced by the different class libraries.
Note, these benchmarks are not compliant, and I am in no way publishing these results. For the record, the benchmarks were ran on an Athlon 64x2 3800+.
BUG Labs JVM saga end game
I've been following Bug Labs choice of JVM quite closely. After a series of comparisons between JamVM, CacaoVM and PhoneME they adopted PhoneME (initial test here and the follow-up). I blogged on the results of the first test, which were favourable to JamVM. However, for the second test, they sorted out the problems with running PhoneME's JIT, and the positions of JamVM and PhoneME reversed.
This was disheartening, but the results spoke for themselves. However, one odd fact is that the second test did not give any details of start-up time. JamVM clearly won this in the first test, and it's unlikely enabling PhoneME's JIT would have changed this.
So, I read with great interest the recent blog entry where they've got CacaoVM/GNU Classpath running on the BUG. It appears they will still ship with PhoneME, but CacaoVM/GNU Classpath will be an option for customers who require the Classpath exception.
So what? Well, I'd like an explanation why they seem so reluctant to use JamVM. From their own tests, JamVM came out on top for start-up, and came second in performance to PhoneME with JIT.
Perhaps they've finally cracked the performance problems with CacaoVM. But JamVM is not configured for top performance on ARM either (by default, the inlining interpreter is disabled on ARM).
Of course, there are many other advantages to JamVM on embedded systems besides start-up time. It has its own, compacting garbage-collector with full support for soft, weak and phantom references in addition to class unloading. CacaoVM relies on Boehm GC, exhibiting memory fragmentation issues, and it has no support for soft/weak/phantom references or class-unloading.
Things like this makes me very disheartened. As I've said before, it makes me wonder why I continue to work on JamVM at all. However, giving up will be a case of "cutting my nose off to spite my face".
If they've hit any problems with JamVM I'll be quite happy to work with them to fix them, but I've received no feedback or requests. Unfortunately, I have been unable to leave any comments on the blog entry itself. On enquiring with the webmaster, it appears that this is new software which is at an early stage. However, they've put this functionality at the top of their TODO list, and I can expect it in a day or two (thanks Brian).
To finish on a positive note, I've done quite a lot of work on JamVM over the last few months, including memory footprint and performance improvements over JamVM 1.5.1. Hopefully I'll make a new release before Christmas.
JamVM/GNU Classpath/iPhone roundup
It's a year since JamVM was ported to the iPhone/iPod Touch. A quick browse on Google shows up a couple of interesting things:
Lend me an ear while I call you a fool*
As the developer and maintainer of JamVM I get a regular stream of emails about licencing issues (2 so far this week). But this one left me speechless:
What is your intent for users of the JamVM code? Is it just the core of the VM that you have licensed using GPLv2, and so any changes to that core code or code linked with it must be provided as opensource? Since the class libraries come from Gnu Classpath, they are covered under the so-called 'classpath exception', and don't infect code that link with it, correct? Thus, is it allowed for a company to make a product using an unmodified JamVM as a standalone program that executes proprietary and unpublished Java code, without running afoul of GPLv2?While the question is clear, the use of the pejorative terms "infect" and "afoul" towards GPLed code immediately gets my back up. My instinct is simply to ignore it, but is there any more appropriate response?
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