Scheduling problems with tasks with the same priorities

[Meike Reichle]

Hi all!

We are using TaskJuggler for planning and tracking our projects.
Recently we've run into the behaviour (bug?) described here:
http://www.taskjuggler.org/FUDforum2/index.php?t=msg&rid=0&S=0165ad2584306999e8cbb7dc4c2181e5&th=3900&goto=9046#msg_9049

I realise that I can fix the error by adjusting the task's priority,
since the error is caused by two (or more) tasks with the same priority
competing for the remaining project time.
Nevertheless what I'd actually like to know is what task(s) the failing
task clashes with, so I can make an informed decision as to whether I
should prioritise the task in question above or below 500.

We are using TaskJuggler a lot for tracking past bookings and usually
have no priorities assigned to the individual tasks, so we keep running
into this problem.

I'd be grateful for any hints on how to deal with this behaviour. Maybe
it would even be worth a bugfix, at least to give it a less misleading
error message.

Best regards,
Meike

[Chris Schlaeger]

Hi Meike,

this thread is covering a number of related issues. The quickest way
to get some help is to post an example project and describe exactly
what problem you are seeing. And please tell us what version of TJ you
are using.

Chris

[Meike Reichle]

Hi Chris,

thank you for the quick reply!

> this thread is covering a number of related issues. The quickest way
> to get some help is to post an example project and describe exactly
> what problem you are seeing.

The problem I am experiencing is the one described in the post I linked
to and demonstrated in its attachment. I also attached a slightly more
reduced example to this mail.
My issue is that the tasks do actually fit into the project's run time,
however if all tasks have the same priority scheduling fails. If I set
the priority of the enclosing task to a value > 501 scheduling succeeds.

I am a bit puzzled by this and wondering if I am missing something or if
this is a bug in the scheduler.

> And please tell us what version of TJ you are using.

I am using TaskJuggler 2.4.1 on KDE 3.5.10 on Debian 5.0

Best,
Meike

[Chris Schlaeger]

> My issue is that the tasks do actually fit into the project's run time,
> however if all tasks have the same priority scheduling fails. If I set
> the priority of the enclosing task to a value > 501 scheduling succeeds.
>
> I am a bit puzzled by this and wondering if I am missing something or if
> this is a bug in the scheduler.

The TJ scheduler is using heuristics to determine the priority of
tasks if a group of tasks has the same priority. This heuristic cannot
detect that your three tasks have an implicit dependency because they
all compete for the same singular resource. Unfortunately, tasks with
a limit can have two very different meanings.

1. Work on this task whenever the resource is available, but don't
exceed X hours a week.
2. Spend X hours a week on this task and do whatever else otherwise.

In your case, task A fits into the 2nd class. The only way TJ can
figure out the difference between the 2 cases is when the user
specifies priorities to tell it. Class 1 has low priorites, class 2
tasks have high priorities. Long running tasks with limits are a
commonly used idiom in TJ projects. They should always have a priority
setting to tell TJ how to treat them.

If you want to know more about how the scheduler really works, I'd
recommend this thread:
http://www.taskjuggler.org/FUDforum2/index.php?t=msg&th=4106&start=0&rid=65&S=3cbae520f0195e4297bb962ad7190185

Chris

[Meike Reichle]

Thank you very much for the explanation! I see much clearer now :)

Best,
Meike