fi_atomic(3) Libfabric Programmer's Manual
fi_atomic - Remote atomic functions
- fi_atomic / fi_atomicv / fi_atomicmsg / fi_inject_atomic
- Initiates an atomic operation to remote memory
- fi_fetch_atomic / fi_fetch_atomicv / fi_fetch_atomicmsg
- Initiates an atomic operation to remote memory, retrieving the initial value.
- fi_compare_atomic / fi_compare_atomicv / fi_compare_atomicmsg
- Initiates an atomic compare-operation to remote memory, retrieving the initial value.
- fi_atomic_valid / fi_fetch_atomic_valid / fi_compare_atomic_valid
- Indicates if a provider supports a specific atomic operation
- Fabric endpoint on which to initiate atomic operation.
- Local data buffer that specifies first operand of atomic operation
- iov / comparev / resultv
- Vectored data buffer(s).
- count / compare_count / result_count
- Count of vectored data entries. The number of elements referenced, where each element is the indicated datatype.
- Address of remote memory to access.
- Protection key associated with the remote memory.
- Datatype associated with atomic operands
- Atomic operation to perform
- Local compare buffer, containing comparison data.
- Local data buffer to store initial value of remote buffer
- desc / compare_desc / result_desc
- Data descriptor associated with the local data buffer, local compare buffer, and local result buffer, respectively.
- Destination address for connectionless atomic operations. Ignored for connected endpoints.
- Message descriptor for atomic operations
- Additional flags to apply for the atomic operation
- User specified pointer to associate with the operation.
Atomic transfers are used to read and update data located in remote memory regions in an atomic fashion. Conceptually, they are similar to local atomic operations of a similar nature (e.g. atomic increment, compare and swap, etc.). Updates to remote data involve one of several operations on the data, and act on specific types of data, as listed below. As such, atomic transfers have knowledge of the format of the data being accessed. A single atomic function may operate across an array of data applying an atomic operation to each entry, but the atomicity of an operation is limited to a single datatype or entry.
Atomic Data Types
Atomic functions may operate on one of the following identified data types. A given atomic function may support any datatype, subject to provider implementation constraints.
- Signed 8-bit integer.
- Unsigned 8-bit integer.
- Signed 16-bit integer.
- Unsigned 16-bit integer.
- Signed 32-bit integer.
- Unsigned 32-bit integer.
- Signed 64-bit integer.
- Unsigned 64-bit integer.
- A single-precision floating point value (IEEE 754).
- A double-precision floating point value (IEEE 754).
- An ordered pair of single-precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.
- An ordered pair of double-precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.
- A double-extended precision floating point value (IEEE 754).
- An ordered pair of double-extended precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.
The following atomic operations are defined. An atomic operation often acts against a target value in the remote memory buffer and source value provided with the atomic function. It may also carry source data to replace the target value in compare and swap operations. A conceptual description of each operation is provided.
- Logical OR
- Logical AND
- Bitwise OR
- Bitwise AND
- Logical exclusive-OR (XOR)
- Bitwise exclusive-OR (XOR)
- Read data atomically
- Write data atomically
- Compare values and if equal swap with data
- Compare values and if not equal swap with data
- Compare values and if less than or equal swap with data
- Compare values and if less than swap with data
- Compare values and if greater than or equal swap with data
- Compare values and if greater than swap with data
- Swap masked bits with data
Base Atomic Functions
The base atomic functions – fi_atomic, fi_atomicv, fi_atomicmsg – are used to transmit data to a remote node, where the specified atomic operation is performed against the target data. The result of a base atomic function is stored at the remote memory region. The main difference between atomic functions are the number and type of parameters that they accept as input. Otherwise, they perform the same general function.
The call fi_atomic transfers the data contained in the user-specified data buffer to a remote node. For unconnected endpoints, the destination endpoint is specified through the dest_addr parameter. Unless the endpoint has been configured differently, the data buffer passed into fi_atomic must not be touched by the application until the fi_atomic call completes asynchronously. The target buffer of a base atomic operation must allow for remote read an/or write access, as appropriate.
The fi_atomicv call adds support for a scatter-gather list to fi_atomic. The fi_atomicv transfers the set of data buffers referenced by the ioc parameter to the remote node for processing.
The fi_inject_atomic call is an optimized version of fi_atomic. The fi_inject_atomic function behaves as if the FI_INJECT transfer flag were set, and FI_COMPLETION were not. That is, the data buffer is available for reuse immediately on returning from from fi_inject_atomic, and no completion event will be generated for this atomic. The completion event will be suppressed even if the endpoint has not been configured with FI_SELECTIVE_COMPLETION. See the flags discussion below for more details.
The fi_atomicmsg call supports atomic functions over both connected and unconnected endpoints, with the ability to control the atomic operation per call through the use of flags. The fi_atomicmsg function takes a struct fi_msg_atomic as input.
The following list of atomic operations are usable with base atomic operations: FI_MIN, FI_MAX, FI_SUM, FI_PROD, FI_LOR, FI_LAND, FI_BOR, FI_BAND, FI_LXOR, FI_BXOR, and FI_ATOMIC_WRITE.
The fetch atomic functions – fi_fetch_atomic, fi_fetch_atomicv, and fi_fetch atomicmsg – behave similar to the equivalent base atomic function. The difference between the fetch and base atomic calls are the fetch atomic routines return the initial value that was stored at the target to the user. The initial value is read into the user provided result buffer. The target buffer of fetch-atomic operations must be enabled for remote read access.
The following list of atomic operations are usable with fetch atomic operations: FI_MIN, FI_MAX, FI_SUM, FI_PROD, FI_LOR, FI_LAND, FI_BOR, FI_BAND, FI_LXOR, FI_BXOR, FI_ATOMIC_READ, and FI_ATOMIC_WRITE.
The compare atomic functions – fi_compare_atomic, fi_compare_atomicv, and fi_compare atomicmsg – are used for operations that require comparing the target data against a value before performing a swap operation. The compare atomic functions support: FI_CSWAP, FI_CSWAP_NE, FI_CSWAP_LE, FI_CSWAP_LT, FI_CSWAP_GE, FI_CSWAP_GT, and FI_MSWAP.
Atomic Valid Functions
The atomic valid functions – fi_atomicvalid, fi_fetch_atomicvalid, and fi_compare_atomicvalid –indicate which operations the local provider supports. Needed operations not supported by the provider must be emulated by the application. Each valid call corresponds to a set of atomic functions. fi_atomicvalid checks whether a provider supports a specific base atomic operation for a given datatype and operation. fi_fetch_atomicvalid indicates if a provider supports a specific fetch-atomic operation for a given datatype and operation. And fi_compare_atomicvalid checks if a provider supports a specified compare-atomic operation for a given datatype and operation.
If an operation is supported, an atomic valid call will return 0, along with a count of atomic data units that a single function call will operate on.
Completed atomic operations are reported to the user through one or more event collectors associated with the endpoint. Users provide context which are associated with each operation, and is returned to the user as part of the event completion. See fi_cq for completion event details.
Updates to the target buffer of an atomic operation are visible to processes running on the target system either after a completion has been generated, or after the completion of an operation initiated after the atomic call with a fencing operation occurring in between. For example, the target process may be notified by the initiator sending a message after the atomic call completes, or sending a fenced message immediately after initiating the atomic operation.
The fi_atomicmsg, fi_fetch_atomicmsg, and fi_compare_atomicmsg calls allow the user to specify flags which can change the default data transfer operation. Flags specified with atomic message operations override most flags previously configured with the endpoint, except where noted (see fi_control). The following list of flags are usable with atomic message calls.
- Indicates that a completion entry should be generated for the specified operation. The endpoint must be bound to a completion queue with FI_SELECTIVE_COMPLETION that corresponds to the specified operation, or this flag is ignored.
- Indicates that the user has additional requests that will immediately be posted after the current call returns. Use of this flag may improve performance by enabling the provider to optimize its access to the fabric hardware.
- Indicates that the outbound non-const data buffers (buf and compare parameters) should be returned to user immediately after the call returns, even if the operation is handled asynchronously. This may require that the underlying provider implementation copy the data into a local buffer and transfer out of that buffer. The use of output result buffers are not affected by this flag.
- Indicates that the requested operation, also known as the fenced operation, be deferred until all previous operations targeting the same target endpoint have completed.
Returns 0 on success. On error, a negative value corresponding to fabric
errno is returned. Fabric errno values are defined in
- The requested atomic operation is not supported on this endpoint.
- The number of atomic operations in a single request exceeds that supported by the underlying provider.
Atomic operations operate on an array of values of a specific data type. Atomicity is only guaranteed for each data type operation, not across the entire array. The following pseudo-code demonstrates this operation for 64-bit unsigned atomic write. ATOMIC_WRITE_U64 is a platform dependent macro that atomically writes 8 bytes to an aligned memory location.
The number of array elements to operate on is specified through a count parameter. This must be between 1 and the maximum returned through the relevant valid operation, inclusive. The requested operation and data type must also be valid for the given provider.