Re: Workqueues in Ada

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Wiktor Moskwa <wiktorDOTmoskwa@gmail.com> writes:

> The problem is that my poor implementation of workqueue is a 
> bottleneck. A task takes a unit of work from a queue, processes it 
> and puts it back at the and of the queue. Because the queue is 
> Ada.Containers.Doubly_Linked_Lists there are lots of memory
> allocations and deallocations - that's a performance problem.

I'm not clear on exactly what you're trying to do, but it sounds like
"units of work" get created and destroyed a lot less often than they get
added to and removed from the queue.  Consider using a doubly-linked
list with the links threaded through the units of work, so
adding/removing doesn't need to allocate/free anything -- just stir a
few pointers around.

> I consider making a circular list of nodes with "Next_To_Service"
> pointer going around as nodes are processed. I'll have to make
> sure that one node can be served by only one task at the time
> (probably by marking nodes as being served at the moment).
> New nodes joining and old leaving the system will be more
> difficult to implement correctly, I suppose.

I think the "marking nodes" thing is probably not a good idea.
It's complicated, and it doesn't scale well.

But if you have a max number of items in the queue at any time,
a circular buffer of pointers to units-of-work would do just fine.
The max number could be calculated at run time, and you could
even make it growable, if you wanted to (in which case "max" is a
misnomer.  ;-))

- Bob