A JVM Does That?

Google Tech Talk (more info below)
March 29, 2011

Presented by Cliff Click, Azul Systems.

ABSTRACT

Just what the heck is a JVM *supposed* to do? JVMs already provide a host of services. The 'J' part definitely slants the service selection and the 'V' part means that underneath the illusion there's a lot of really cruddy stuff. The success of these illusions has led to the real popularity of JVMs. In particular, JVMs are probably the most popular way to distribute ready-to-use GC technology to the masses, and the 2nd most popular way to distribute ready-to-use compilation technology (just behind "gcc" I'm guessing).

Just what are these illusions? The illusion that bytecodes are fast and have a reasonable cost model : (machine code generation, profiling, JIT'ing). The illusion that you can quickly change the program at any time (dynamic class loading, deoptimization, re-JIT'ing). The illusion of infinite memory (Garbage Collection). The illusion of a consistent threading and memory model (the JMM, volatiles, locks). The illusion of quick time access (i.e. Intel's "rdtsc" is mostly useless for time). The illusion that these other illusions all work on all sorts of machines from cell phones to 1000-cpu mainframes (it's kinda sorta mostly true).

But these are not enough! The People cry out for more illusions! The illusion of an Infinite Stack (tail recursion), the illusion that running-code-is-data (closures), the illusion that Integers are as cheap as 'ints' (autoboxing optimizations), or that BigIntegers are as cheap as 'ints' (tagged ints), the illusion that memory supports atomic update (software transactional memory), or the illusion that with enough hard work other language implementation experts can make a JVM run utterly different languages (invokedynamic). Meanwhile, JVM engineers labor under the illusion that they can maintain this giant piggy pile of code, and maybe even expand the provided services without it collapsing under it's own weight.

Here's my take on what services could & should be provided by a JVM, what services probably belong to the next layer up (STMs, new concurrency models), and what services belong to the next layer down (fast time management, sane thread scheduling).

Category:

Science & Technology

Tags:

• google tech talk
• java
• jvm
• computer science

[tremgaming]

@fnyklr

#01:56 #03:41 #09:25 #25:09 #50:47

isnt that bad - at least still not drunk

apart from that: its a very interesting talk



[fnyklr]

drinking game: drink a shot every time he says jadajadajada...

[Compgenius999]

@tremgaming Here's an idea.

Drink a shot every time he says GC :D

[tohopes]

29:30 Good explanation of the problem with relying on fnalizers and garbage collection to release resources.

[cliffclick]

To the commenter about LRU behavior....

This was with the stock Sun (Oracle) JVM.

Nothing in the SoftRef spec claims to maintain any sort of last-ref time, and I don't believe Oracles implementation maintains this info either.

[MultiParsley]

The audio quality is a bit poor, actually hurt to listen to.

[occjjjusr]

I believe you may be wrong with respect to soft references. A good JVM can do much better than a shot in the dark to decide which references to throw out, because a last-reference time is maintained as a part of the reference, permitting a LRU algorithm to be applied by the jvm when deciding which references to throw out. What your implementation does is another question, but it is there.

There are still limitations but LRU avoids the basic sort of pathological cases you describe here.

[ChilapaOfTheAmazons]

The JVMs wouldn't be so bad if it weren't for the little dirt details that the vast majority of them is used to run code written by Java developers in love with design patterns and completely detached from reality.

Just saying...

[berlinbrown03]

How do get information on this stuff?

[blenderpanzi]

When he says that bytecodes have shortcomeings, does he mean stack based bytecode or bytecode in general?

[amynstevestachurski]

@tremgaming If you can take five shots in an hour and not be drunk, you might need to cut back a little.

[bormarek]

and standard java - sun(oracle) or ibm work's better.