PetscDeviceContextForkWithStreamType#

Create a set of dependent child contexts from a parent context with a prescribed PetscStreamType

Synopsis#

#include <petscdevice.h> 
PetscErrorCode PetscDeviceContextForkWithStreamType(PetscDeviceContext dctx, PetscStreamType stype, PetscInt n, PetscDeviceContext **dsub)

Not Collective, Asynchronous

Input Parameters#

dctx - The parent PetscDeviceContext
stype - The prescribed PetscStreamType
n - The number of children to create

Output Parameter#

dsub - The created child context(s)

Notes#

This routine creates n edges of a DAG from a source node which are causally dependent on the source node. This causal dependency is established as-if by calling PetscDeviceContextWaitForContext() on every child.

dsub is allocated by this routine and has its lifetime bounded by dctx. That is, dctx expects to free dsub (via PetscDeviceContextJoin()) before it itself is destroyed.

This routine only accounts for work queued on dctx up until calling this routine, any subsequent work enqueued on dctx has no effect on dsub.

The PetscStreamType of dctx does not have to equal stype. In fact, it is often the case that they are different. This is useful in cases where a routine can locally exploit stream parallelism without needing to worry about what stream type the incoming PetscDeviceContext carries.

DAG representation#

  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 PetscDeviceContexts 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 PetscDeviceContexts for consecutive reads.

Operations using separate PetscDeviceContexts 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).

Level#

intermediate

Location#

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

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