Provided by: libfabric-dev_1.17.0-3build2_amd64 
NAME
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_atomicvalid / fi_fetch_atomicvalid / fi_compare_atomicvalid / fi_query_atomic :
Indicates if a provider supports a specific atomic operation
SYNOPSIS
#include <rdma/fi_atomic.h>
ssize_t fi_atomic(struct fid_ep *ep, const void *buf,
size_t count, void *desc, fi_addr_t dest_addr,
uint64_t addr, uint64_t key,
enum fi_datatype datatype, enum fi_op op, void *context);
ssize_t fi_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
void **desc, size_t count, fi_addr_t dest_addr,
uint64_t addr, uint64_t key,
enum fi_datatype datatype, enum fi_op op, void *context);
ssize_t fi_atomicmsg(struct fid_ep *ep, const struct fi_msg_atomic *msg,
uint64_t flags);
ssize_t fi_inject_atomic(struct fid_ep *ep, const void *buf,
size_t count, fi_addr_t dest_addr,
uint64_t addr, uint64_t key,
enum fi_datatype datatype, enum fi_op op);
ssize_t fi_fetch_atomic(struct fid_ep *ep, const void *buf,
size_t count, void *desc, void *result, void *result_desc,
fi_addr_t dest_addr, uint64_t addr, uint64_t key,
enum fi_datatype datatype, enum fi_op op, void *context);
ssize_t fi_fetch_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
void **desc, size_t count, struct fi_ioc *resultv,
void **result_desc, size_t result_count, fi_addr_t dest_addr,
uint64_t addr, uint64_t key, enum fi_datatype datatype,
enum fi_op op, void *context);
ssize_t fi_fetch_atomicmsg(struct fid_ep *ep,
const struct fi_msg_atomic *msg, struct fi_ioc *resultv,
void **result_desc, size_t result_count, uint64_t flags);
ssize_t fi_compare_atomic(struct fid_ep *ep, const void *buf,
size_t count, void *desc, const void *compare,
void *compare_desc, void *result, void *result_desc,
fi_addr_t dest_addr, uint64_t addr, uint64_t key,
enum fi_datatype datatype, enum fi_op op, void *context);
size_t fi_compare_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
void **desc, size_t count, const struct fi_ioc *comparev,
void **compare_desc, size_t compare_count, struct fi_ioc *resultv,
void **result_desc, size_t result_count, fi_addr_t dest_addr,
uint64_t addr, uint64_t key, enum fi_datatype datatype,
enum fi_op op, void *context);
ssize_t fi_compare_atomicmsg(struct fid_ep *ep,
const struct fi_msg_atomic *msg, const struct fi_ioc *comparev,
void **compare_desc, size_t compare_count,
struct fi_ioc *resultv, void **result_desc, size_t result_count,
uint64_t flags);
int fi_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
enum fi_op op, size_t *count);
int fi_fetch_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
enum fi_op op, size_t *count);
int fi_compare_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
enum fi_op op, size_t *count);
int fi_query_atomic(struct fid_domain *domain,
enum fi_datatype datatype, enum fi_op op,
struct fi_atomic_attr *attr, uint64_t flags);
ARGUMENTS
ep Fabric endpoint on which to initiate atomic operation.
buf 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.
addr Address of remote memory to access.
key Protection key associated with the remote memory.
datatype
Datatype associated with atomic operands
op Atomic operation to perform
compare
Local compare buffer, containing comparison data.
result 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. See fi_mr(3).
dest_addr
Destination address for connectionless atomic operations. Ignored for connected
endpoints.
msg Message descriptor for atomic operations
flags Additional flags to apply for the atomic operation
context
User specified pointer to associate with the operation. This parameter is ignored
if the operation will not generate a successful completion, unless an op flag
specifies the context parameter be used for required input.
DESCRIPTION
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.
FI_INT8
Signed 8-bit integer.
FI_UINT8
Unsigned 8-bit integer.
FI_INT16
Signed 16-bit integer.
FI_UINT16
Unsigned 16-bit integer.
FI_INT32
Signed 32-bit integer.
FI_UINT32
Unsigned 32-bit integer.
FI_INT64
Signed 64-bit integer.
FI_UINT64
Unsigned 64-bit integer.
FI_INT128
Signed 128-bit integer.
FI_UINT128
Unsigned 128-bit integer.
FI_FLOAT
A single-precision floating point value (IEEE 754).
FI_DOUBLE
A double-precision floating point value (IEEE 754).
FI_FLOAT_COMPLEX
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.
FI_DOUBLE_COMPLEX
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.
FI_LONG_DOUBLE
A double-extended precision floating point value (IEEE 754). Note that the size of
a long double and number of bits used for precision is compiler, platform, and/or
provider specific. Developers that use long double should ensure that libfabric is
built using a long double format that is compatible with their application, and
that format is supported by the provider. The mechanism used for this validation
is currently beyond the scope of the libfabric API.
FI_LONG_DOUBLE_COMPLEX
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.
Atomic Operations
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.
FI_MIN Minimum
if (buf[i] < addr[i])
addr[i] = buf[i]
FI_MAX Maximum
if (buf[i] > addr[i])
addr[i] = buf[i]
FI_SUM Sum
addr[i] = addr[i] + buf[i]
FI_PROD
Product
addr[i] = addr[i] * buf[i]
FI_LOR Logical OR
addr[i] = (addr[i] || buf[i])
FI_LAND
Logical AND
addr[i] = (addr[i] && buf[i])
FI_BOR Bitwise OR
addr[i] = addr[i] | buf[i]
FI_BAND
Bitwise AND
addr[i] = addr[i] & buf[i]
FI_LXOR
Logical exclusive-OR (XOR)
addr[i] = ((addr[i] && !buf[i]) || (!addr[i] && buf[i]))
FI_BXOR
Bitwise exclusive-OR (XOR)
addr[i] = addr[i] ^ buf[i]
FI_ATOMIC_READ
Read data atomically
result[i] = addr[i]
FI_ATOMIC_WRITE
Write data atomically
addr[i] = buf[i]
FI_CSWAP
Compare values and if equal swap with data
if (compare[i] == addr[i])
addr[i] = buf[i]
FI_CSWAP_NE
Compare values and if not equal swap with data
if (compare[i] != addr[i])
addr[i] = buf[i]
FI_CSWAP_LE
Compare values and if less than or equal swap with data
if (compare[i] <= addr[i])
addr[i] = buf[i]
FI_CSWAP_LT
Compare values and if less than swap with data
if (compare[i] < addr[i])
addr[i] = buf[i]
FI_CSWAP_GE
Compare values and if greater than or equal swap with data
if (compare[i] >= addr[i])
addr[i] = buf[i]
FI_CSWAP_GT
Compare values and if greater than swap with data
if (compare[i] > addr[i])
addr[i] = buf[i]
FI_MSWAP
Swap masked bits with data
addr[i] = (buf[i] & compare[i]) | (addr[i] & ~compare[i])
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 connectionless 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 requested
message size that can be used with fi_inject_atomic is limited by inject_size.
The fi_atomicmsg call supports atomic functions over both connected and connectionless
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.
struct fi_msg_atomic {
const struct fi_ioc *msg_iov; /* local scatter-gather array */
void **desc; /* local access descriptors */
size_t iov_count;/* # elements in ioc */
const void *addr; /* optional endpoint address */
const struct fi_rma_ioc *rma_iov; /* remote SGL */
size_t rma_iov_count;/* # elements in remote SGL */
enum fi_datatype datatype; /* operand datatype */
enum fi_op op; /* atomic operation */
void *context; /* user-defined context */
uint64_t data; /* optional data */
};
struct fi_ioc {
void *addr; /* local address */
size_t count; /* # target operands */
};
struct fi_rma_ioc {
uint64_t addr; /* target address */
size_t count; /* # target operands */
uint64_t key; /* access key */
};
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.
Fetch-Atomic Functions
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.
For FI_ATOMIC_READ operations, the source buffer operand (e.g. fi_fetch_atomic buf
parameter) is ignored and may be NULL. The results are written into the result buffer.
Compare-Atomic Functions
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.
Query Atomic Attributes
The fi_query_atomic call acts as an enhanced atomic valid operation (see the atomic valid
function definitions above). It is provided, in part, for future extensibility. The
query operation reports which atomic operations are supported by the domain, for suitably
configured endpoints.
The behavior of fi_query_atomic is adjusted based on the flags parameter. If flags is 0,
then the operation reports the supported atomic attributes for base atomic operations,
similar to fi_atomicvalid for endpoints. If flags has the FI_FETCH_ATOMIC bit set, the
operation behaves similar to fi_fetch_atomicvalid. Similarly, the flag bit
FI_COMPARE_ATOMIC results in query acting as fi_compare_atomicvalid. The FI_FETCH_ATOMIC
and FI_COMPARE_ATOMIC bits may not both be set.
If the FI_TAGGED bit is set, the provider will indicate if it supports atomic operations
to tagged receive buffers. The FI_TAGGED bit may be used by itself, or in conjunction
with the FI_FETCH_ATOMIC and FI_COMPARE_ATOMIC flags.
The output of fi_query_atomic is struct fi_atomic_attr:
struct fi_atomic_attr {
size_t count;
size_t size;
};
The count attribute field is as defined for the atomic valid calls. The size field
indicates the size in bytes of the atomic datatype. The size field is useful for
datatypes that may differ in sizes based on the platform or compiler, such FI_LONG_DOUBLE.
Completions
Completed atomic operations are reported to the initiator of the request through an
associated completion queue or counter. Any user provided context specified with the
request will be returned as part of any completion event written to a CQ. See fi_cq for
completion event details.
Any results returned to the initiator as part of an atomic operation will be available
prior to a completion event being generated. This will be true even if the requested
completion semantic provides a weaker guarantee. That is, atomic fetch operations have
FI_DELIVERY_COMPLETE semantics. Completions generated for other types of atomic
operations indicate that it is safe to re-use the source data buffers.
Any updates to data at the target of an atomic operation will be visible to agents (CPU
processes, NICs, and other devices) on the target node prior to one of the following
occurring. If the atomic operation generates a completion event or updates a completion
counter at the target endpoint, the results will be available prior to the completion
notification. After processing a completion for the atomic, if the initiator submits a
transfer between the same endpoints that generates a completion at the target, the results
will be available prior to the subsequent transfer’s event. Or, if a fenced data transfer
from the initiator follows the atomic request, the results will be available prior to a
completion at the target for the fenced transfer.
The correctness of atomic operations on a target memory region is guaranteed only when
performed by a single actor for a given window of time. An actor is defined as a single
libfabric domain (identified by the domain name, and not an open instance of that domain),
a coherent CPU complex, or other device (e.g. GPU) capable of performing atomic operations
on the target memory. The results of atomic operations performed by multiple actors
simultaneously are undefined. For example, issuing CPU based atomic operations to a
target region concurrently being updated by NIC based atomics may leave the region’s data
in an unknown state. The results of a first actor’s atomic operations must be visible to
a second actor prior to the second actor issuing its own atomics.
FLAGS
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.
FI_COMPLETION
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.
FI_MORE
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.
FI_INJECT
Indicates that the control of constant data buffers should be returned to the 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. Constant data
buffers refers to any data buffer or iovec used by the atomic APIs that are marked
as `const'. Non-constant or output buffers are unaffected by this flag and may be
accessed by the provider at anytime until the operation has completed. This flag
can only be used with messages smaller than inject_size.
FI_FENCE
Applies to transmits. Indicates that the requested operation, also known as the
fenced operation, and any operation posted after the fenced operation will be
deferred until all previous operations targeting the same peer endpoint have
completed. Operations posted after the fencing will see and/or replace the results
of any operations initiated prior to the fenced operation.
The ordering of operations starting at the posting of the fenced operation (inclusive) to
the posting of a subsequent fenced operation (exclusive) is controlled by the endpoint’s
ordering semantics.
FI_TAGGED
Specifies that the target of the atomic operation is a tagged receive buffer
instead of an RMA buffer. When a tagged buffer is the target memory region, the
addr parameter is used as a 0-based byte offset into the tagged buffer, with the
key parameter specifying the tag.
RETURN VALUE
Returns 0 on success. On error, a negative value corresponding to fabric errno is
returned. Fabric errno values are defined in rdma/fi_errno.h.
ERRORS
-FI_EAGAIN
See fi_msg(3) for a detailed description of handling FI_EAGAIN.
-FI_EOPNOTSUPP
The requested atomic operation is not supported on this endpoint.
-FI_EMSGSIZE
The number of atomic operations in a single request exceeds that supported by the
underlying provider.
NOTES
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.
fi_atomic(ep, buf, count, NULL, dest_addr, addr, key,
FI_UINT64, FI_ATOMIC_WRITE, context)
{
for (i = 1; i < count; i ++)
ATOMIC_WRITE_U64(((uint64_t *) addr)[i],
((uint64_t *) buf)[i]);
}
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.
The ordering of atomic operations carried as part of different request messages is subject
to the message and data ordering definitions assigned to the transmitting and receiving
endpoints. Both message and data ordering are required if the results of two atomic
operations to the same memory buffers are to reflect the second operation acting on the
results of the first. See fi_endpoint(3) for further details and message size
restrictions.
SEE ALSO
fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_cq(3), fi_rma(3)
AUTHORS
OpenFabrics.