Any feedback here..? On Wed, Apr 5, 2017 at 7:43 PM, Flavio Pompermaier <pomperma...@okkam.it> wrote:
> Hi to all, > I had a very long but useful chat with Fabian and I understood a lot of > concepts that was not clear at all to me. We started from the Flink runtime > documentation page (https://ci.apache.org/projects/flink/flink-docs- > release-1.2/concepts/runtime.html) but > I discovered that the terminology is very inconsistent and misleading > along the page... > > For example, one of the very first sentences is : > "Flink chains operator subtasks together into tasks. Each task is executed > by one thread." > What I first understood was that every operator can be executed only by a > single thread in all the cluster....probably it should be better "one > thread per task slot" (at least). > Moreover, if I'm not wrong, a Task Slot can execute only 1 subtask (aka > parallel instance) of each task and there's no limit to the number of > subtasks per slot (and this is not highlighted at all in that document). > The only constraint is that they should belong to different tasks (right?). > > If there's a google doc version of that page I could try to rewrite it > down in order to make it easier to understand some parts...however I still > have some more questions: > > 1. Is it correct that a single Task Slot can execute only a single > subtask of each task and that this task is executed by a single thread > within the slot)? > 2. If it so: > 1. why at that page there's written "By default, Flink allows > subtasks to share slots even if they are subtasks of different tasks, so > long as they are from the same job"? It seems that it is more common to > run > multiple subtasks of the same task (in a slot) than executing different > substasks of different tasks, although this is still permitted...from > what > I understood a slot cannot run multiple subtask of the same task at all! > 2. and why this constraint? Is there any good reason for that? A > subtask is mapped to 1 thread in the TaskManager, so why a TM with 2 > slots > can run 2 subtasks of the same task (in the same JVM) while a TM with 1 > slot cannot (while it can execute an arbitrary number of subtasks of > different tasks)? > 3. It it is not so, there's no images representing such a situation in > that page... > 4. Isn't dangerous to allow (potentially) an unlimited number of > threads per TM slot?? > > Cheers, > Flavio > >
