Ubuntu Manpage: fi_cntr - Completion and event counter operations

Provided by: libfabric-dev_1.17.0-3build2_amd64

NAME

       fi_cntr - Completion and event counter operations

       fi_cntr_open / fi_close
              Allocate/free a counter

       fi_cntr_read
              Read the current value of a counter

       fi_cntr_readerr
              Reads the number of operations which have completed in error.

       fi_cntr_add
              Increment a counter by a specified value

       fi_cntr_set
              Set a counter to a specified value

       fi_cntr_wait
              Wait for a counter to be greater or equal to a threshold value

SYNOPSIS

              #include <rdma/fi_domain.h>

              int fi_cntr_open(struct fid_domain *domain, struct fi_cntr_attr *attr,
                  struct fid_cntr **cntr, void *context);

              int fi_close(struct fid *cntr);

              uint64_t fi_cntr_read(struct fid_cntr *cntr);

              uint64_t fi_cntr_readerr(struct fid_cntr *cntr);

              int fi_cntr_add(struct fid_cntr *cntr, uint64_t value);

              int fi_cntr_adderr(struct fid_cntr *cntr, uint64_t value);

              int fi_cntr_set(struct fid_cntr *cntr, uint64_t value);

              int fi_cntr_seterr(struct fid_cntr *cntr, uint64_t value);

              int fi_cntr_wait(struct fid_cntr *cntr, uint64_t threshold,
                  int timeout);

ARGUMENTS

       domain Fabric domain

       cntr   Fabric counter

       attr   Counter attributes

       context
              User specified context associated with the counter

       value  Value to increment or set counter

       threshold
              Value to compare counter against

       timeout
              Time in milliseconds to wait.  A negative value indicates infinite timeout.

DESCRIPTION

Counters record the number of requested operations that have completed. Counters can provide a light-
weight completion mechanism by allowing the suppression of CQ completion entries. They are useful for
applications that only need to know the number of requests that have completed, and not details about
each request. For example, counters may be useful for implementing credit based flow control or tracking
the number of remote processes that have responded to a request.

Counters typically only count successful completions. However, if an operation completes in error, it
may increment an associated error value. That is, a counter actually stores two distinct values, with
error completions updating an error specific value.

Counters are updated following the completion event semantics defined in fi_cq(3). The timing of the up‐
date is based on the type of transfer and any specified operation flags.

fi_cntr_open
fi_cntr_open allocates a new fabric counter. The properties and behavior of the counter are defined by
struct fi_cntr_attr.

struct fi_cntr_attr {
enum fi_cntr_events events; /* type of events to count */
enum fi_wait_obj wait_obj; /* requested wait object */
struct fid_wait *wait_set; /* optional wait set */
uint64_t flags; /* operation flags */
};

events A counter captures different types of events. The specific type which is to counted are one of
the following:

- FI_CNTR_EVENTS_COMP
The counter increments for every successful completion that occurs on an associated bound end‐
point. The type of completions – sends and/or receives – which are counted may be restricted us‐
ing control flags when binding the counter and the endpoint. Counters increment on all successful
completions, separately from whether the operation generates an entry in an event queue.

wait_obj
Counters may be associated with a specific wait object. Wait objects allow applications to block
until the wait object is signaled, indicating that a counter has reached a specific threshold.
Users may use fi_control to retrieve the underlying wait object associated with a counter, in or‐
der to use it in other system calls. The following values may be used to specify the type of wait
object associated with a counter: FI_WAIT_NONE, FI_WAIT_UNSPEC, FI_WAIT_SET, FI_WAIT_FD,
FI_WAIT_MUTEX_COND, and FI_WAIT_YIELD. The default is FI_WAIT_NONE.

- FI_WAIT_NONE
Used to indicate that the user will not block (wait) for events on the counter.

- FI_WAIT_UNSPEC
Specifies that the user will only wait on the counter using fabric interface calls, such as fi_cn‐
tr_wait. In this case, the underlying provider may select the most appropriate or highest per‐
forming wait object available, including custom wait mechanisms. Applications that select
FI_WAIT_UNSPEC are not guaranteed to retrieve the underlying wait object.

- FI_WAIT_SET
Indicates that the event counter should use a wait set object to wait for events. If specified,
the wait_set field must reference an existing wait set object.

- FI_WAIT_FD
Indicates that the counter should use a file descriptor as its wait mechanism. A file descriptor
wait object must be usable in select, poll, and epoll routines. However, a provider may signal an
FD wait object by marking it as readable, writable, or with an error.

- FI_WAIT_MUTEX_COND
Specifies that the counter should use a pthread mutex and cond variable as a wait object.

- FI_WAIT_YIELD
Indicates that the counter will wait without a wait object but instead yield on every wait. Al‐
lows usage of fi_cntr_wait through a spin.

wait_set
If wait_obj is FI_WAIT_SET, this field references a wait object to which the event counter should
attach. When an event is added to the event counter, the corresponding wait set will be signaled
if all necessary conditions are met. The use of a wait_set enables an optimized method of waiting
for events across multiple event counters. This field is ignored if wait_obj is not FI_WAIT_SET.

flags Flags are reserved for future use, and must be set to 0.

fi_close
The fi_close call releases all resources associated with a counter. When closing the counter, there must
be no opened endpoints, transmit contexts, receive contexts or memory regions associated with the
counter. If resources are still associated with the counter when attempting to close, the call will re‐
turn -FI_EBUSY.

fi_cntr_control
The fi_cntr_control call is used to access provider or implementation specific details of the counter.
Access to the counter should be serialized across all calls when fi_cntr_control is invoked, as it may
redirect the implementation of counter operations. The following control commands are usable with a
counter:

FI_GETOPSFLAG (uint64_t *)
Returns the current default operational flags associated with the counter.

FI_SETOPSFLAG (uint64_t *)
Modifies the current default operational flags associated with the counter.

FI_GETWAIT (void **)
This command allows the user to retrieve the low-level wait object associated with the counter.
The format of the wait-object is specified during counter creation, through the counter attribut‐
es. See fi_eq.3 for addition details using control with FI_GETWAIT.

fi_cntr_read
The fi_cntr_read call returns the current value of the counter.

fi_cntr_readerr
The read error call returns the number of operations that completed in error and were unable to update
the counter.

fi_cntr_add
This adds the user-specified value to the counter.

fi_cntr_adderr
This adds the user-specified value to the error value of the counter.

fi_cntr_set
This sets the counter to the specified value.

fi_cntr_seterr
This sets the error value of the counter to the specified value.

fi_cntr_wait
This call may be used to wait until the counter reaches the specified threshold, or until an error or
timeout occurs. Upon successful return from this call, the counter will be greater than or equal to the
input threshold value.

If an operation associated with the counter encounters an error, it will increment the error value asso‐
ciated with the counter. Any change in a counter’s error value will unblock any thread inside fi_cn‐
tr_wait.

If the call returns due to timeout, -FI_ETIMEDOUT will be returned. The error value associated with the
counter remains unchanged.

It is invalid for applications to call this function if the counter has been configured with a wait ob‐
ject of FI_WAIT_NONE or FI_WAIT_SET.

RETURN VALUES

       Returns 0 on success.  On error, a negative value corresponding to fabric errno is returned.

       fi_cntr_read / fi_cntr_readerr
              Returns the current value of the counter.

       Fabric errno values are defined in rdma/fi_errno.h.

NOTES

       In  order  to  support  a variety of counter implementations, updates made to counter values (e.g. fi_cn‐
       tr_set or fi_cntr_add) may not be immediately visible to counter read  operations  (i.e. fi_cntr_read  or
       fi_cntr_readerr).   A small, but undefined, delay may occur between the counter changing and the reported
       value being updated.  However, a final updated value will eventually be reflected  in  the  read  counter
       value.

       Additionally,  applications should ensure that the value of a counter is stable and not subject to change
       prior to calling fi_cntr_set or fi_cntr_seterr.  Otherwise, the resulting  value  of  the  counter  after
       fi_cntr_set  /  fi_cntr_seterr  is  undefined, as updates to the counter may be lost.  A counter value is
       considered stable if all previous updates using fi_cntr_set / fi_cntr_seterr and results of related oper‐
       ations are reflected in the observed value of the counter.

AUTHORS

       OpenFabrics.

Libfabric Programmer’s Manual                      2022-12-11                                         fi_cntr(3)