The Ruby VM wonkiness continued at RubyConf 2008 - Day 1. Next up we heard the "State of Rubinius" address on the status of the anointed heir to the Ruby federation. Evan Phoenix is a fun guy, and it proved to be a both informative and enjoyable session.Evan says Rubinius is "meta-circular-ish", and slow, but getting faster. They have a new VM that is written in C++, which is a sharp tool, and as such, it is possible to poke your eye out. However, he says it allows them to better model work. The three main advantages are:
- type safety
- organization
- architecture
What is type safety, may ask many Rubyists. Well, sometimes a duck actually IS a duck. The Rubinius code just feels better with OO nature of C++, as opposed to just plain C. Also, it is possible to make the VM class hierarchy that mirrors the Ruby class hierarchy.
Exporting Methods - a day in the life of a primitive operation
- things you cannot do in Ruby
- small example
class String : Object {
Fixnum* size();
}
class String
def size
Ruby.primitive :string_size
end
end
There was interesting question about why flatten out ala :string_size aka no namespacing. Evan did not seem to give a definitive answer...
Method Dispatch - From resolution to execution
Resolution
- going from receiver and method name to actual method
- three mechanisms
- hierarchical lookup
- global cache
- inline cache
Execution
- every method provide an execute function pointer
- each primitive is an executor
- Ruby methods use executor specialization
Ruby Methods
- Specialized executors based on argument info
- Very simple fast version for majority of cases
- Slower fallback case for all cases
Critical Path
- Populate message object
- Call rep over trampoline
- Fill in more of message object
- Call method executor
Execution/Framing
- Method context objects store info about each method
- Chained together using refs (spaghetti stack)
- MethodContext creation speed is crucial
- Default contexts are allocated in special section of memory
- Normal execution follows simple stack pattern
- We can exploit to allow for fast creation and deallocation
Calling to C
Can you imaging life without ruby gems that use C extensions? For example:
- hpricot
- mongrel
- mysql
- sqlite
Not a problem. A simple recompile == usable in Rubinius
Tricky
- GC interaction requires indirection
- Uses 'quantum leap" stack jumping
- Allows for semi-graceful recovery from extension segfaults
Performance
Rubinius is doing well on micro-benchmarks
- method dispatch
- GC
- OK on macro-benchmarks
- Poorly on mega-benchmarks
Solutions
- LLVM
- JIT
- AOT
- Improve algorithmic efficiency
Rubinius is concentrating on compatibility right now, but will then address performance. Just like Koichi-san said, it is a curve of improvements.
Evan ended up with a nice quote from Woody Allen - "If you're not failing every now and again, it's a sign you're not doing anything very innovative."
