parallel threads stalls until all thread batches are finished.

[Joe at bloow.edu]

On Wednesday, 23 August 2023 at 14:43:33 UTC, Sergey wrote:
> On Wednesday, 23 August 2023 at 13:03:36 UTC, Joe wrote:
>> I use
>>
>> foreach(s; taskPool.parallel(files, numParallel))
>> { L(s); } // L(s) represents the work to be done.
>
> If you make for example that L function return “ok” in case 
> file successfully downloaded, you can try to use TaskPool.amap.
>
> The other option - use std.concurrency probably.

I think I might know what is going on but not sure:

The tasks are split up in batches and each batch gets a thread. 
What happens then is some long task will block it's entire batch 
and there will be no re-balancing of the batches.


"A work unit is a set of consecutive elements of range to be 
processed by a worker thread between communication with any other 
thread. The number of elements processed per work unit is 
controlled by the workUnitSize parameter. "

So the question is how to rebalance these work units?

E.g., when a worker thread is done with it's batch it should look 
to help finish that batch rather than terminating and leaving all 
the work for the last thread.

This seems like a flaw in the design. E.g., if one happens to 
have n batches and every batch but one has tasks that finish 
instantly then essentially one has no parallelization.