[concurrency-interest] Awaiting a set of tasks on an ExecutorService

Nathan and Ila Reynolds nathanila at gmail.com
Fri Aug 18 17:22:57 EDT 2017

What about CountedCompleter?  The master thread registers a callback via 
CountedCompleter.onCompletion().  Call 
CountedCompleter.addToPendingCount() before a new task is enqueued. When 
a task is completed, call CountedCompleter.tryComplete().

The master thread should call addToPendingCount(1) when the stream of 
tasks is first started.  The master thread should call tryComplete() 
when the end of the stream is reached.  Otherwise, onCompletion() might 
be called prematurely because the master thread is too slow at adding tasks.

If CountedCompleter does not work, consider Phaser.bulkRegister() and 


On 8/18/2017 2:54 PM, Shevek wrote:
> I can't use CountDownLatch because of the streamy nature of the 
> source. I have no idea how many tasks there are going to be up front. 
> I read an incoming stream, it turns out there's about 50 million in 
> it, and this is going to go up by the usual orders of magnitude.
> I could use a List of CountDownLatches each with ... but then I 
> rapidly get into "Someone cleverer than me MUST have solved this 
> before" territory.
> I can do my own tricks with an AtomicLong and wait()/notify(), but I 
> have to make sure the master thread calls get() on the relevant Future 
> each time a job finishes, otherwise there isn't a happens-before 
> relationship with the master thread, so I'd need the job to put its 
> own Future onto a Deque, or something...?
> The tasks all return Void, but it's nice to collect the first few 
> exceptions (currently there are about 750,000 total exceptions thrown).
> S.
> On 08/18/2017 01:48 PM, Josh Humphries wrote:
>> I think the easiest thing would be to decorate each task to call 
>> "latch.countDown()" on a CountDownLatch that is initialized with the 
>> total number of tasks. After they are all submitted, the code that 
>> wants to wait for them to finish would simply await the latch. This 
>> decouples completion of all tasks from the actual ExecutorService 
>> that is running them, so you can share the same ExecutorService for 
>> multiple, even overlapping, sets of tasks.
>> ----
>> *Josh Humphries*
>> jhump at bluegosling.com <mailto:jhump at bluegosling.com>
>> On Fri, Aug 18, 2017 at 4:42 PM, Shevek <shevek at anarres.org 
>> <mailto:shevek at anarres.org>> wrote:
>>     Hi,
>>     I need to build a complicated object on a multiprocessor system. The
>>     target object uses synchronization to guarantee thread-safe access.
>>     However, the individual tasks which build elements of the target
>>     object are expensive, so I farm them out to an ExecutorService. The
>>     question is:
>>     What is the recommended way to submit a stream of tasks to an
>>     ExecutorService, and then at a point, wait until they're all done.
>>     * I don't know the list of tasks up front. I read them iteratively
>>     from a set of files or events.
>>     * A big collection of Future(s) isn't realistic, there are just too
>>     many tasks.
>>     * Reaping completed futures on submission means that I end up with
>>     exceptions in weird places; I really need to gather max 100
>>     suppressed exceptions, and discard the rest.
>>     * ForkJoinPool has invoke() but I think for a million-task job, I
>>     still end up with a huge list of futures. This also assumes I ignore
>>     the note about synchronization.
>>     * CompletionService allows me to wait for _any_ of the submitted
>>     tasks, but not for _all_ of them.
>>     * Bonus points for sharing a single ExecutorService and having
>>     "sets" of tasks which can be independently awaited. This starts to
>>     sound very like FJP, except for the blocking/I/O and the stream-ness
>>     of my task set.
>>     Mostly, dear list, I'm expecting a one-liner from some API class
>>     that I've missed because this is NOT an original request, I just
>>     can't find a standard solution.
>>     Thank you.
>>     S.
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