Hi Christoph,

On Mon, Feb 1, 2021 at 8:29 AM Thiede, Christoph <
Christoph.Thiede at student.hpi.uni-potsdam.de> wrote:

> Hi all,
>
>
> in the context of the current Transcript debate [1], I am trying to figure
> out whether, and why Marcel's proposed implementation of <noContextSwitch>
> [2] could work.
>
>
> As a minimum example to test #valueNoContextSwitch, I tried the following:
>
>
> "Try to run this infinite loop unpreemptively."
>
> [
>
> [[false] whileFalse] valueNoContextSwitch.
>
> self inform: #done
>
> ] forkAt: Processor userBackgroundPriority.
>
>
> Now I would have expected my image (its UI process) to freeze while
> p2 is running unpreemptively.
>

RTFM :-)

An exact copy of BlockClosure>>value except that this version will not
preempt
the current process on block activation if a higher-priority process is
runnable.
Primitive. Essential.

i.e. this has an effect only on activating the block.  Once the block is
running preemption is still possible.  All that it guarantees is that no
preemption is possible from immediately before the send of valueNoContextSwitch
to the first send in the block activation.

> But - nothing happens, i.e., all other processes are still activated! I
> also exchanged valueNoContextSwitch with valueUninterruptably (which I
> implemented using primitive 123 analogously to <noContextSwitch>) with
> the same result.
>
>
> Here is my question: Am I misunderstanding the concept of
> unpreemptive/uninterruptable execution (are those two terms really
> synonyms?), or is unpreempted execution broken?
>

They're not synonyms, but neither of them apply to valueNoContextSwitch,
nor my proposed pragma which should perhaps be <noContextSwitchOnActivation>

Assuming the former is the case, what else would be a good and simple way
> to find out if a process is being run unpreemptively?
>
> And do we have any good documentation of this concept
> - self-documenting tests will be preferred? :-)
>

A test might have a lower priority process in some kind of loop that
invokes a block via valueNoContextSwitch, or calls a method marked
with <noContextSwitchOnActivation>.
This process would be run from a higher priority process that would loop
waiting for a short delay (1 microsecond?).  Ocne past its delay it would
preempt the lower priority process, sample its pc and fail if the pc was
the first pc of valueNoContextSwitch or a method marked with
<noContextSwitchOnActivation>.

A test with two such lower priority processes could be used to test
processPreemptionYields == false. If processPreemptionYields == true then
every preemption would cause the two lower priority processes to switch
order. If processPreemptionYields == false the second one would not run
until the first one yielded explicitly, or waited, etc.

Best,
>
> Christoph
>
>
> [1]
> http://forum.world.st/Transcript-error-when-forceUpdate-false-td5126397.html
> [2] http://forum.world.st/The-Inbox-Compiler-mt-454-mcz-td5126568.html
>
>
>

-- 
_,,,^..^,,,_
best, Eliot
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