Re: [Sablevm-user] Technical Report

[John L. <je...@pi...>]

Hello Etienne

It's been a while since I spent time on my JIT and JVM, I've been working
on porting the JVM to run in the linux kernel, in preparation for an
experimental Operating System.

But I read your paper and would like to comment / ask questions. I found
many of the ideas very stimulating I would love to work with their
implementation at some stage (either in SableVM or my JVM / JIT).

My first comment is on the use of a threaded interpreter. As I understand
the bytecode array will no longer be one-byte instructions followed by
their operands, but instead will consist of 4 byte pointers to the
implementation for that instruction.

Isn't this terribly space inefficient? Wouldn't this require 64bits per
pointer on a 64-bit machine? I say this in light of the pressure put on
memory by the JIT and optimising process, this would make the pressure on
memory even worse.

You also say "The advantage of unmapping memory, relative to simply
freeing memory (using malloc/free) is that unmapped pages need not be
dumped to disk by the VMM system when they are to be ejected from RAM".

I don't really understand this line? Do you mean that instead of
malloc'ing a 5M heap (which will be regarded by the VMM as useful data),
you can selectively allocate the pieces being used by the heap?


John Leuner