How high are he memory costs of polymorphic inline caches?

[Remi Forax]

On 08/18/2014 12:01 PM, Raffaello Giulietti wrote:
> Starting with Java 7, the recommended way to implement dynamic 
> languages is to build upon invokedynamic (indy), method handles (MH) 
> and their combinators, all of which are intrinsically known to the 
> JVM. This intimacy allows the JVM to leverage many JIT optimizations 
> already at our service for ordinary Java code.
>
> A standard optimization is polymorphic inline caches (PIC). To support 
> PICs for dynamic languages on the JVM, best practices recommend to 
> build a tree-like structure at each active indy call site, where a 
> (very limited) cascade of guard-with-test (GWT) MHs represents the 
> spine of the cache.
>
> It seems to me, however, that all this requires a lot of objects for 
> each single indy site. Even granted that the JVM can beautifully 
> inline the code of the specialized MHs like GWT MHs and bound-MHs (for 
> the test), it still needs to maintain a large number of PIC trees. And 
> PIC trees cannot usually be shared, since each call site has its own 
> usage profile.
>
> So, the question is whether some of you has experience with large 
> scale projects written in a dynamic language implemented on the JVM, 
> that makes heavy use of indy and PICs. I'm curious about the memory 
> load for the PICs. I'm also interested whether the standard Oracle 
> server JVM satisfactorily keeps up with the load.
>
> For example, we have a large Smalltalk application with about 50'000 
> classes and about 600'000 methods. In Smalltalk, almost everything in 
> code is a method invocation, including operators like +, <=, etc. I 
> estimate some 5-10 millions method invocation sites. How many of them 
> are active during a typical execution, I couldn't tell. But if the 
> Smalltalk runtime were implemented on the JVM, PICs would quite 
> certainly represent a formidable share of the memory footprint.
>
> More generally, apart from toy examples, are there studies in 
> real-world usage of indy and PICs in large applications?
> Perhaps some figures from the JRuby folks, or better, their users' 
> applications would be interesting.
>
> Thanks for numbers
>
> Raffaello Giulietti

There are several Smalltalk implementations that already use 
invokedynamic, so you can ask their maintainers for data, on top of my 
head you have Rtalk by Mark Roos [1], Magic (smalltalk like), you can 
ask Duncan Mac Gregor [2], Redline Smalltalk by James Ladd [3] and 
Gravel Smalltalk by Wouter Gazendam [4].

I hope I've not forgotten somebody :)
Rémi


[1] http://www.drdobbs.com/jvm/rtalk-smalltalk-on-the-jvm/231500288
[2] http://www.infoq.com/news/2012/12/magik-jvm-port
[3] https://github.com/redline-smalltalk/redline-smalltalk
[4] https://github.com/gravel-st/gravel

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