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# PetscDeviceContextJoin

Converge a set of child contexts

## Synopsis

    #include <petscdevice.h>
    PetscErrorCode PetscDeviceContextJoin(PetscDeviceContext dctx, PetscInt n, PetscDeviceContextJoinMode joinMode, PetscDeviceContext **dsub)

Not Collective, Asynchronous

## Input Parameters

- dctx - A `PetscDeviceContext` to converge on

- n - The number of sub contexts to converge

- joinMode - The type of join to perform

- dsub - The sub contexts to converge

## Notes

If `PetscDeviceContextFork()` creates `n` edges from a source node which all depend on the source node, then this routine is the exact mirror. That is, it creates a node (represented in `dctx`) which receives `n` edges (and optionally destroys them) which is dependent on the completion of all incoming edges.

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_DESTROY`. All contexts in `dsub` will be destroyed by this routine. Thus all sub contexts must have been created with the `dctx` passed to this routine.

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_SYNC`. All sub contexts will additionally wait on `dctx` after converging. This has the effect of “synchronizing” the outgoing edges. Note the sync suffix does NOT refer to the host, i.e. this routine does NOT call `PetscDeviceSynchronize()`.

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_NO_SYNC`. `dctx` waits for all sub contexts but the sub contexts do not wait for one another or `dctx` afterwards.

## DAG representations

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_DESTROY`

      time ->

      -> dctx ---------/- |= CALL =| - dctx ->
      -> dsub[0] -----/
      ->  ... -------/
      -> dsub[n-1] -/

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_SYNC`

      time ->

      -> dctx ---------/- |= CALL =| -\----> dctx ------>
      -> dsub[0] -----/                \---> dsub[0] --->
      ->  ... -------/                  \--> ... ------->
      -> dsub[n-1] -/                    \-> dsub[n-1] ->

If `joinMode` is `PETSC_DEVICE_CONTEXT_JOIN_NO_SYNC`

      time ->

      -> dctx ----------/- |= CALL =| - dctx ->
      -> dsub[0] ------/----------------------->
      ->  ... --------/------------------------>
      -> dsub[n-1] --/------------------------->

## Asynchronous API Notes

This routine is explicitly marked as exhibiting asynchronous behavior. Asynchronous behavior implies that routines launching operations on (or associated with) a `PetscDeviceContext` may return to the caller before the operation has completed.

Sequential Consistency:

Operations using the same `PetscDeviceContext` which access objects or memory regions are ordered per the language specification.

Operations using separate `PetscDeviceContext`s which access the same object or memory region are strongly write-ordered. That is, the following operations:

- `write-write`

- `write-read`

- `read-write`

are strongly ordered. Formally:

Given an operation `A-B` (e.g. `A` = `write`, `B` = `read`) on an object or memory region `M` such that `A` “happens-before” `B`, where `A` uses `PetscDeviceContext` `X` and `B` uses `PetscDeviceContext` `Y`, then `B` shall not begin before `A` completes. This implies that any side-effects resulting from `A` are also observed by `B`.

Note the omission of `read-read`; there is no implied ordering between separate `PetscDeviceContext`s for consecutive reads.

Operations using separate `PetscDeviceContext`s which access separate objects or memory regions may execute in an arbitrary order and offer no guarantee of sequential consistency.

Memory Consistency:

If this routine modifies the participating object(s) then – unless otherwise stated – the contents of any externally held references to internal data structures should be considered to be in an undefined state. A well-defined state can only be restored by re-acquiring these references through the appropriate API or by calling `PetscDeviceContextSynchronize()`.

Unless otherwise stated, exceptions to this rule are:

- References returned by the routine itself. If a routine returns a pointer, the value of the top-most pointer is guaranteed to always be valid. For example, given a routine which asynchronously allocates memory and returns a pointer to the memory, the value of said pointer is immediately valid but dereferencing the pointer may not be.

- References to structures. If a routine returns a `PetscFoo`, or array thereof then the objects themselves are always valid (though their member variables `PetscFoo->data` may not be).

## See Also

`PetscDeviceContextFork()`, `PetscDeviceContextForkWithStreamType()`, `PetscDeviceContextSynchronize()`, `PetscDeviceContextJoinMode`

## Level

beginner

## Location

src/sys/objects/device/interface/dcontext.cxx

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Index of all manual pages