[RFC] Adding API for parallel block to task_arena to warm-up/retain/release worker threads

rfcs/proposed/parallel_block_for_task_arena/README.md

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+# Adding API for parallel block to task_arena to warm-up/retain/release worker threads
+
+## Introduction
+
+In oneTBB, there has never been an API that allows users to block worker threads within the arena.
+This design choice was made to preserve the composability of the application.<br>
+Before PR#1352, workers moved to the thread pool to sleep once there were no arenas with active
+demand. However, PR#1352 introduced a delayed leave behavior to the library that
+results in blocking threads for an _implementation-defined_ duration inside an arena
+if there is no active demand arcoss all arenas. This change significantly
+improved performance for various applications on high thread count systems.<br>
+The main idea is that usually, after one parallel computation ends,
+another will start after some time. The delayed leave behavior is a heuristic to utilize this,
+covering most cases within _implementation-defined_ duration.
+
+However, the new behavior is not the perfect match for all the scenarios:
+* The heuristic of delayed leave is unsuitable for the tasks that are submitted
+  in unpredictable pattern and/or duration.
+* If oneTBB is used in composable scenarios it is not behaving as
+  a good citizen consuming CPU resources.
+  * For example, if an application builds a pipeline where oneTBB is used for one stage
+    and OpenMP is used for a subsequent stage, there is a chance that oneTBB workers will
+    interfere with OpenMP threads. This interference might result in slight oversubscription,
+    which in turn might lead to underperformance.
+
+So there are two related problems but with different resolutions:
+* Completely disable new behavior for scenarios where the heuristic of delayed leave is unsuitable.
+* Optimize library behavior so customers can benefit from the heuristic of delayed leave but
+  make it possible to indicate that "it is the time to release threads".
+
+## Proposal
+
+Let's tackle these problems one by one.
+
+### Completely disable new behavior
+
+Let’s consider both “Delayed leave” and “Fast leave” as 2 different states in state machine.<br>
+* The "Delayed leave" heuristic benefits most of the workloads. Therefore, this is the 
+  default behavior for arena. 
+* Workloads that has rather negative performance impact from the heuristic of delayed leave
+  can create an arena in “Fast leave” state.
+
+<img src="completely_disable_new_behavior.png" width=800>
+
+There will be a question that we need to answer:
+* Do we see any value if arena potentially can transition from one to another state?
+
+To answer this question, the following scenarios should be considired:
+* What if different types of workloads are mixed in one application?
+* Different types of arenas can be used for different types of workloads.
+
+### When threads should leave?
+
+oneTBB itself can only guess when the ideal time to release threads from the arena is.
+Therefore, it does the best effort to preserve and enhance performance without completely
+messing composability guarantees (that is how delayed leave is implemented).
+
+As we already discussed, there are cases where it does not work perfectly,
+therefore customers that want to further optimize this
+aspect of oneTBB behavior should be able to do it.
+
+This problem can be considered from another angle. Essentially, if the user can indicate
+where parallel computation ends, they can also indicate where it starts.
+
+<img src="parallel_block_introduction.png" width=800>
+
+With this approach, the user not only releases threads when necessary but also specifies a
+programmable block where worker threads should expected new work coming regularly
+to the executing arena.
+
+Let’s add new state to the existing state machine. To represent "Parallel Block" state.
+
+> **_NOTE:_** The "Fast leave" state is colored Grey just for simplicity of the chart.
+              Let's assume that arena was created with the "Delayed leave". 
+              The logic demonstrated below is applicable to the "Fast leave" as well.
+
+<img src="parallel_block_state_initial.png" width=800>
+
+This state diagram leads to several questions. There are some of them:
+* What if there are multiple Parallel Blocks?
+* If “End of Parallel Block” leads back to “Delayed leave” how soon threads
+  will be released from arena?
+  * What if we indicated that threads should leave arena after the "Parallel Block"?
+  * What if we just indicated the end of the "Parallel Block"?
+
+The extended state machine aims to answer these questions.
+* The first call to the “Start of PB” will transition into the “Parallel Block” state.
+* The last call to the “End of PB” will transition back to the “Delayed leave” state
+  or into the "One-time Fast leave" if it is indicated that threads should leave sooner.
+* Concurrent or nested calls to the “Start of PB” or the “End of PB”
+  increment/decrement reference counter.
+
+<img src="parallel_block_state_final.png" width=800>
+
+Let's consider the semantics that an API for explicit parallel blocks can provide:
+* Start of a parallel block:
+  * Indicates the point from which the scheduler can use a hint and keep threads in the arena
+    for longer.
+  * Serves as a warm-up hint to the scheduler:
+    * Allows worker threads to be available by the time real computation starts.
+* "Parallel block" itself:
+  * Scheduler can implement different policies to retain threads in the arena.
+  * The semantics for retaining threads is a hint to the scheduler;
+    thus, no real guarantee is provided. The scheduler can ignore the hint and
+    move threads to another arena or to sleep if conditions are met.
+* End of a parallel block:
+  * Indicates the point from which the scheduler may drop a hint and
+    no longer retain threads in the arena.
+  * Indicates that arena should enter the “One-time Fast leave” thus workers can leave sooner.
+    * If work was submitted immediately after the end of the parallel block,
+      the default arena behavior with regard to "workers leave" policy is restored.
+    * If the default "workers leave" policy was the "Fast leave", the result is NOP.
+
+
+### Proposed API
+
+```cpp
+class task_arena {
+    enum class workers_leave : /* unspecified type */ {
+        fast = /* unspecifed */,
+        delayed = /* unspecifed */
+    };
+
+    task_arena(int max_concurrency = automatic, unsigned reserved_for_masters = 1,
+               priority a_priority = priority::normal,
+               workers_leave a_workers_leave = workers_leave::delayed);
+
+    task_arena(const constraints& constraints_, unsigned reserved_for_masters = 1,
+               priority a_priority = priority::normal,
+               workers_leave a_workers_leave = workers_leave::delayed);
+
+    void initialize(int max_concurrency, unsigned reserved_for_masters = 1,
+                    priority a_priority = priority::normal,
+                    workers_leave a_workers_leave = workers_leave::delayed);
+
+    void initialize(constraints a_constraints, unsigned reserved_for_masters = 1,
+                    priority a_priority = priority::normal,
+                    workers_leave a_workers_leave = workers_leave::delayed);
+
+    void start_parallel_block();
+    void end_parallel_block(bool set_one_time_fast_leave = false);
+
+    class scoped_parallel_block {
+        scoped_parallel_block(task_arena& ta, bool set_one_time_fast_leave = false);
+    };
+};
+
+namespace this_task_arena {
+    void start_parallel_block();
+    void end_parallel_block(bool set_one_time_fast_leave = false);
+}
+```
+
+By the contract, users should indicate the end of _parallel block_ for each
+previous start of _parallel block_.<br>
+Let's introduce RAII scoped object that will help to manage the contract.
+
+If the end of the parallel block is not indicated by the user, it will be done automatically when
+the last public reference is removed from the arena (i.e., task_arena is destroyed or a thread
+is joined for an implicit arena). This ensures correctness is
+preserved (threads will not be retained forever).
+
+## Considerations
+
+The alternative approaches were also considered.<br>
+We can express this state machine as complete graph and provide low-level interface that
+will give control over state transition.
+
+<img src="alternative_proposal.png" width=600>
+
+We considered this approach too low-level. Plus, it leaves a question: "How to manage concurrent changes of the state?".
+
+The retaining of worker threads should be implemented with care because
+it might introduce performance problems if:
+* Threads cannot migrate to another arena because they are
+  retained in the current arena.
+* Compute resources are not homogeneous, e.g., the CPU is hybrid.
+  Heavier involvement of less performant core types might result in artificial work
+  imbalance in the arena.
+
+
+## Open Questions in Design
+
+Some open questions that remain:
+* Are the suggested APIs sufficient?
+* Are there additional use cases that should be considered that we missed in our analysis?
+* Do we see any value if arena potentially can transition from one to another state?
+  * What if different types of workloads are mixed in one application?
+  * What if there concurrent calls to this API?