New scheduling policies [Was: Re: [squeak-dev] Re: Suspending process fix]

Igor Stasenko siguctua at gmail.com
Wed Apr 29 17:59:04 UTC 2009


I got a new VM build, which is ready to be tested with new
scheduler(after i implement it).
I rewrote the external signaling stuff & interrupt checking.
Now its not signals any semaphores. Instead, i added a primitive which
explicitly fetching all pending signals to array and flushing pending
signals VM internal buffer. Then in interrupt checker i simply switch
active process to special 'interrupt process' (or scheduler process -
Andreas), if there any pending signals to handle.

What does it means for language side?
It means a very cool thing: you are no longer obliged to use
semaphores to respond to signals!
You can register any object in external objects table.
And new scheduler will simply do:

externalObjects := Smalltalk externalObjects.
signalIndexes do: [:index |
    (externalObjects at: i) handleExternalSignal.
]

so, as long as your registered object responds to
#handleExternalSignal, you are free to choose what to do in response
to signal.
Semaphores, of course will signal themselves.

After replacing scheduler with new model, the VM will no longer need
to know anything about semaphores. This is because any scheduling
related stuff will become 100% language-side specific.

So, that with new model, multiple primitives become obsolete:

primitiveYield
primitiveWait
primitiveSuspend
primitiveSignal
primitiveResume

instead of them there are two new primitives:

primitiveTransferToProcess
	"sets an ActiveProcess to new process,
	sets an InterruptedProcess to the process which was active
	set a ProcessAction to anAction object
	"

primitiveFetchPendingSignals
	"primitive, fill an array (first argument)with special objects
indexes, needed to be signaled.
	Returns a number of signals being filled.
	Or negative number indicating that array is not big enough to fetch
all signals at once.
	Primitive fails if first argument is not array.
	"

2009/4/29 Igor Stasenko <siguctua at gmail.com>:
> 2009/4/29 Andreas Raab <andreas.raab at gmx.de>:
>> Igor Stasenko wrote:
>>>
>>> I came to an idea , you might be interested in.
>>> As many of us know, some CPUs having a special mode - interrupt mode.
>>> What if we introduce the interrupt mode for scheduler?
>>
>> [... snip ...]
>>>
>>> Now i trying to imagine, how a basic stuff might look like(please
>>> correct me if its utterly wrong way ;), if we will be able to use
>>> interrupt mode.
>>
>> This is actually along similar lines of thought that I had when I was
>> thinking of how to get rid of the builtin VM scheduling behavior. The main
>> thought that I had was that the VM may have a "special" process - the
>> scheduler process (duh!) which it runs when it doesn't know what else to do.
>> The VM would then not directly schedule processes after semaphore signals
>> but rather put them onto a "ready" queue that can be read by the scheduler
>> process and switch to the scheduler process. The scheduler process decides
>> what to run next and resumes the process via a primitive. Whenever an
>> external signal comes in, the VM automatically activates the scheduler
>> process and the scheduler process then decides whether to resume the
>> previously running process or to switch to a different process.
>>
>> In a way this folds the timer process into the scheduler (which makes good
>> sense from my perspective because much of the work in the timer is stuff
>> that could be more effectively take place in the scheduler). The
>> implementation should be relatively straightforward - just add a scheduler
>> process and a ready list to the special objects, and wherever the VM would
>> normally process switch you just switch to the scheduler. Voila, there is
>> your user-manipulable scheduler ;-) And obviously, anything that is run out
>> of the scheduler process is by definition non-interruptable because there is
>> simply nothing to switch to!
>>
>
> Very nice indeed. That's even better that my first proposal.
> ProcessorScheduler>>schedulingProcessLoop
> [
>  self handlePendingSignalsAndActions.
>  activeProcess ifNil: [ self idle ] ifNotNil: [ self
> primitiveTransferControlTo: activeProcess].
> ]  repeat.
>
> and when any process, somehow stops running
> (suspend/wait/terminate/interrupted etc), VM will again switch to
> scheduler process loop.
>
> What is important in having it, that there is guarantee to be not
> preempted by anything. Simply by having this, many
> concurrency/scheduling related problems can be solved by language-side
> implementation, without fear of having gotchas from VM side.
>
> Also, VM doesn't needs to know details about priorities, suspending,
> etc etc..  - which means that we can simplify VM considerably and
> implement same parts on the language side, where everything is late
> bound :)
>
> As for moving to multi-cores.. yes, as Gulik suggests, its like adding
> a new dimension:
>  - local scheduler for each core
>  - single global scheduler for freezing everything
>
> This, of course, if we could afford running same object memory over
> multiple cores. Handling interpreter/object memory state(s) with
> multiple cores is not trivial thing.
>
> If we going to keep more isolated model (islands, hydra ) then we need
> no/minimal changes to scheduler - each scheduler serves own island and
> receives asynchronous signals from other collegues through shared
> queue.
>
>> Cheers,
>>  - Andreas
>>
>
>
> --
> Best regards,
> Igor Stasenko AKA sig.
>



-- 
Best regards,
Igor Stasenko AKA sig.



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