2008/7/2 Eliot Miranda eliot.miranda@gmail.com:
On Tue, Jul 1, 2008 at 5:05 PM, Igor Stasenko siguctua@gmail.com wrote:
2008/7/2 Eliot Miranda eliot.miranda@gmail.com:
This is the "mirrors" approach I alluded to. The right approach is to implement the debugger with a mirror between it and the system it looks at. There is a mirror for local debugging taht is fairly transparent. then there is a mirror for the renoter system which is much more involved because it has to use peek and poke to examine the memory space of the target process and it has to extract meta information from it (and in the presence of errors, e.g. the hard crash could have corrupted memory).
I have some ideas about it. First: nobody prevents me to have image containing separate object memories. Its only a question how to load and bootstrap it :)
Or, more specific: i think i will introduce the island model (or vats) from the start. So there will be a multiple islands (which can be possibly run in parallel native threads), which don't share anything, or sharing a protected memory region, writing to which is guarded by a lot of checks and rules :)
Since there is nothing outside, which dictates me to have: single heap, single GC, single object memory; it is possible to implement a system, which could have different memory regions governed by different memory managers and garbage collectors.
This assumes that an error in one island can't corrupt the heap of another island, not a safe assumption. You really need to use separate address spaces for isolation. If the mirrors are done right the tool side of things is the same anyway.
Then its coming closer to a point, when without speaking directly with hardware (without being restricted by OS) is the only reliable option. It would be much painless if things could operate on zero ring (kernel mode). But without it.. we have to live in user-mode memory model, provided by OS. And hence, should heed what OS dictates: use separate process to debug another process.
Things are really, can be a lot easier, if it could manage physical memory itself.. For instance, i feel headache each time when i thinking how to make Processes cheap. A straightforward approach is to use separate native thread for each Process instance, and let OS manage the stack. But this is far from cheap. A single native thread reserving at least two/three pages of physical memory for stack. I could make own stack, and use separate stack (not provided by OS by default), and can make own checks for stack space left. But then in case when you do foreign calls, things makes the whole idea not so attractive - foreign functions have no idea how large my stack is, and can't check it in same way is my code does.
So without hard hacking OS internals, the idea of using zeons of Processes scheduled between a limited numbers of native threads is barely possible.
Its shows, how inflexible and rigid operating systems can be, if you want to write in any other language than C :)
Then debugging looks much less scary than comparing to system crippled by VM written in C :)
Let me encourage you to think about this soon and work on it asap because it is a really important issue. If you don't have a good debugging environment up front your progress will be slow. No one has succeeded in bringing such a system for a Smalltalk-like language into production use yet (neither Klein nor TypedSmalltalk nor Exupery has made it to production use). Making it happen would be fantastic. You have the Spoon system to build upon.
Absolutely, a system without good means of debugging can't progress fast (and its mainly just a useless artifact for serious deployment). I'm concerned that amount of debug information to be added to method would hit hard the memory consumption.
You have to both provide a Smalltalk-level debugger and a machine code debugger which can display machine instructions, registers and memory. It would be nice to be able to reuse disassemblers et al from other components but you might find that Smalltalk implementations are quicker to write and easier to understand and use. But you will need them.
Heh.. right, but it would take ages :) A teamwork is what is needed. I looking for a people who may want to join crusade for the ultimate smalltalk self-sustaining system implementation :)
I hear you. But for me, first things first :)
best Eliot