On 12/30/11 9:52 AM, arrowdodger wrote:
Hi. First, let me put a little disclaimer:
I have absolutely no CS education and any degree in science, no idea on how
OS kernels and CPU schedulers are implemented and working. Moreover, i
haven't even know math at the level needed to talk about what i'm
proposing. What i'm going to propose may be just nonsence.
I've assumed that:
- Scheduler in FreeBSD is just a bunch of code, which implemens some
interface.
yes, though it is very complicated interface, and some of it is not
explicit.
- This implementation is self-contained - it doesn't call any other kernel
functions and do not depend on other state except itself.
That is not really true. There are all sorts of interactions. Not all
of them are
as simple as a call.
- OS kernel calls scheduler functions in some defined order.
The OS doesn't really call the scheduler in that way.
all sorts of threads of execution jump into the scheduler from all
sorts of places and
the internal state of the scheduler changes with these calls.
Sometimes those calls
never return, and sometimes they return a long time later.. As I
said, it's a very
complicated interface.
If these assumptions are true, it may be possible to compile scheduler
code as userland code and link it with sort-of driver, which would call
scheduler functions in same way as real kernel does. So we get a
statically-linked executable, which would emulate working kernel for the
scheduler.
yes that would certainly be true from some perspective. But it would be
quite a bit of work.
Now we will be using KLEE [1] - a virtual machine for symbolic execution.
It uses SAT solvers to reason about veriables values.
In our driver code we insert calls to klee_assert() after every call to
scheduler function to make KLEE dump current symbolic restrictions on
scheduler's internal state values. Finally, we mark all data, describing
scheduler state as symbolic and run program on KLEE.
As result, we get (i hope so) a set of all possible states in which
scheduler can ever be in form of KLEE test file (.ktest). A test is
represented by descriptions of what value each variable can have in the
current context. So, any of generated states is not intersecting with each
other.
It is a nice goal but I can't see it happening.. I think that the
scheduler is probably
a bit too complicated.. the automatic testing and verification woudl
be nice
but my head hurts thinking about it :-)
Now it's possible to concretize symbolic values for each test and save it
as normal executable. You may think of it as a model of how our scheduler
is functioning. Now we can symbolically execute these binaries again, but
for now marking as symbolic all "external" data from scheduler point of
view. This way we can track and debug scheduler decisions in any
circumstance.
In other words:
1. All possible scheduler states are being found.
2. Identical states are being thrown away. // done by KLEE
3. For each state model scheduler behavior for every input (and skipping
modellings, which yields same results).
I'm not sure if it can help to solve current ULE problems, but it should
really help debugging scheduler during development.
I don't think that the scheduler needs debugging so much as the
specification of it
needs thinking.. The scheduler has teh problem of trying to make
decisions about
what to do in the future from imperfect knowledge of the past, and no
knowledge of
the future.
What do you think? Does it make any sence, or i should just return under my
rock?
no, come out from your rock.. If you are interested in the
scheduler, by all means
go and read it and try and understand it, and then come back to us if
you do have ideas.
I don't think your idea is really bad but I think you underestimate
the difficulty of the task.
PS: Sorry for my english, i hope you understood what i've been trying to
say.
your english is fine.. what do you normally speak?
(and you are not really 6 years old, are you? :-)
[1] http://klee.llvm.org/
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