[concurrency-interest] BoundedPriorityBlockingQueue ?

Dhanji R. Prasanna dhanji at gmail.com
Tue Sep 12 22:41:25 EDT 2006

This is a very interesting discussion. I think separate queues for
different priorities are a better answer. I know that in short message
services (SMS) we tended even to use different applications based on
priorities (HA for billing messages for example).

The potential for a low priority take to slip past a high priority put
and cause unwanted delays is too great to entrust it to one
queue/threadpool. Some of it comes down to how you want to quantify
low and high priorities. If there are mission critical differences I
certainly would not trust them to the same threadpool.

On 9/13/06, David Holmes <dcholmes at optusnet.com.au> wrote:
> Hanson,
> The basic issue here is that you use a PriorityBlockingQueue to prioritize
> tasks, under the expectation that the queue will actually fill to some
> extent, so the relative importance of tasks is, well, important. You then
> place a bound on that queue and the result is you now have a second
> (unbounded) queue of threads trying to submit prioritized tasks. So then you
> wonder if those threads should be unblocked in order of priority of their
> tasks? But that way madness lies ... this isn't real-time priority
> preemptive systems programming. If you really think that the threads need to
> be queued in priority order too then make your priority queue bigger. If you
> size things out and you really must bound the size of the queue and you
> really expect lots of threads to block waiting to put to the queue and you
> really think the priority of the tasks those threads are submitting need to
> be accounted for, then you have a problem. Basically you would have to have
> each submitter check if its task is more important than the lowest priority
> one in the queue and try to swap the new task for the old one; and then keep
> retrying to submit the swapped out task. This is really, really messy. Throw
> in priority-aging to avoid starvation and things get even worse. :)
> Sometimes there is no complete solution - you have to define the boundaries
> where you take into account things like priority. Sometimes the answer is to
> have a completely different mechanism for things that are really important
> (most queueing systems (banks, airlines etc) don't use priority ordered
> queues, they use different queues for different "priorities" or allow
> bypassing of the queue altogether.
> Just my 2c.
> Cheers,
> David Holmes
> -----Original Message-----
> From: concurrency-interest-bounces at cs.oswego.edu
> [mailto:concurrency-interest-bounces at cs.oswego.edu]On
> Behalf Of Hanson Char
> Sent: Wednesday, 13 September 2006 6:20 AM
> To: rbalamohan at sonoasystems.com
> Cc: Brian Goetz; concurrency-interest; Kasper Nielsen
> Subject: Re: [concurrency-interest] BoundedPriorityBlockingQueue ?
> I see blocking on put on a BoundedPriorityBlockingQueue is not such a good
> idea.
> How about making the put method return a false boolean value upon queue
> full, or alternatively throwing something like a QueueFullException ?
> Would this be useful/sensible ?
> Hanson
> On 9/12/06, Brian Goetz <brian at quiotix.com> wrote:
> > The tricky part is if you want to guarantee that the thread offering the
> > element of highest priority is first to be unblocked.
> >
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