Stopping and restarting a timer within its own callback - why does it then run the callback twice at a time, but only with fixedSpacing?

I want to start and stop a timer within the timer callback function*. I do this with code like the following script:

global timerRunCount
timerRunCount = 0;
% Try changing "fixedDelay" to "fixedSpacing".
timerObj = timer(ExecutionMode="fixedDelay", Period=1, StartDelay=1, TimerFcn=@timerFunction);
timerObj.start;

function timerFunction(tmr, ~)
    global timerRunCount
    timerRunCount = timerRunCount + 1;
    if isnan(tmr.InstantPeriod)
        disp("Iteration: " + timerRunCount + "; interval: N/A")
    else
        disp("Iteration: " + timerRunCount + "; interval: " + tmr.InstantPeriod)
    end
    switch timerRunCount
        case 5
            tmr.stop
            tmr.start
        case 10
            tmr.stop
            tmr.start
        case 15
            tmr.stop
            tmr.delete
    end
end

Result:

% Iteration: 1; interval: N/A
% Iteration: 2; interval: 1.001
% Iteration: 3; interval: 1.001
% Iteration: 4; interval: 1.001
% Iteration: 5; interval: 1.001
% Iteration: 6; interval: N/A
% Iteration: 7; interval: 1.002
% Iteration: 8; interval: 1.001
% Iteration: 9; interval: 1.001
% Iteration: 10; interval: 1.001
% Iteration: 11; interval: N/A
% Iteration: 12; interval: 1.001
% Iteration: 13; interval: 1.001
% Iteration: 14; interval: 1
% Iteration: 15; interval: 1.001

We see that it happily runs timerFunction() about every second. (I know that global variables are poor practice; it's just to keep this example simple).

Now see what happens when we change ExecutionMode to "fixedSpacing":

% Iteration: 1; interval: N/A
% Iteration: 2; interval: 1.046
% Iteration: 3; interval: 1.002
% Iteration: 4; interval: 1.002
% Iteration: 5; interval: 1.003
% Iteration: 6; interval: N/A
% Iteration: 7; interval: 0.004
% Iteration: 8; interval: 1
% Iteration: 9; interval: 0.003
% Iteration: 10; interval: 0.999
% Iteration: 11; interval: N/A
% Iteration: 12; interval: 0.001
% Iteration: 13; interval: 1.001
% Iteration: 14; interval: 0.002
% Iteration: 15; interval: 1.001

The first five iterations work as expected; but after we first start and stop the timer, it runs timerFunction() twice every time the timer triggers. We see this from the intervals alternating between ~1s and a few ms. In the third section, I have sometimes seen the timerFunction() run three times for every timer trigger (depending on how long the second stage runs for).

Can anyone please explain why a change to ExecutionMode causes this issue? By my understanding of the Matlab help, this should only change whether or not the duration of timerFunction() is included in the timer period. I don't actually need this issue fixed (the exact period of my timer isn't important), but I want to understand any strange subtleties of the timer object before basing my application on it.

* The reason is that I want to repeatedly run a function with a timer (checking a shared memory location for changes) and when it detects changes, switch to a different polling function, and when *that* detects changes, switch to yet another different polling function, etc. You have to stop a timer before you can change the period, and then restart it.

[David Szwer]

A related question, where someone noticed the same issue with ExecutionMode="fixedSpacing":

https://uk.mathworks.com/matlabcentral/answers/120702-prevent-unwanted-timer-callback-calls-in-the-execution-queue#comment_200715

[raym]

I tried fixedSpacing and fixedDelay,the problem still exist.

[raym]

https://www.mathworks.com/matlabcentral/answers/1914920-stop-and-re-start-timer-inside-timerfcn-cause-pseudo-multiple-parallel-duplicated-timer-in-fact-onl

Answer 1

[Sugandhi]

Hi David,

I work at MathWorks, and the relevant people are aware of the issue.

Regards,

Sugandhi.

[David Szwer]

That's good to know, thank you.

(I won't "Accept" this answer, until I hear about a workaround or a fix, but it's still very helpful).

[raym]

1. I eucountered the same problem today:
2. https://www.mathworks.com/matlabcentral/answers/1914920-stop-and-re-start-timer-inside-timerfcn-cause-pseudo-multiple-parallel-duplicated-timer-in-fact-onl