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COUNTER(9)             FreeBSD Kernel Developer's Manual            COUNTER(9)

NAME
     counter - SMP-friendly kernel counter implementation

SYNOPSIS
     #include <sys/types.h>
     #include <sys/systm.h>
     #include <sys/counter.h>

     counter_u64_t
     counter_u64_alloc(int wait);

     void
     counter_u64_free(counter_u64_t c);

     void
     counter_u64_add(counter_u64_t c, int64_t v);

     void
     counter_enter();

     void
     counter_exit();

     void
     counter_u64_add_protected(counter_u64_t c, int64_t v);

     uint64_t
     counter_u64_fetch(counter_u64_t c);

     void
     counter_u64_zero(counter_u64_t c);

     #include <sys/sysctl.h>

     SYSCTL_COUNTER_U64(parent, nbr, name, access, ptr, descr);

     SYSCTL_ADD_COUNTER_U64(ctx, parent, nbr, name, access, ptr, descr);

     SYSCTL_COUNTER_U64_ARRAY(parent, nbr, name, access, ptr, len, descr);

     SYSCTL_ADD_COUNTER_U64_ARRAY(ctx, parent, nbr, name, access, ptr, len,
         descr);

DESCRIPTION
     counter is a generic facility to create counters that can be utilized for
     any purpose (such as collecting statistical data).  A counter is
     guaranteed to be lossless when several kernel threads do simultaneous
     updates.  However, counter does not block the calling thread, also no
     atomic(9) operations are used for the update, therefore the counters can
     be used in any non-interrupt context.  Moreover, counter has special
     optimisations for SMP environments, making counter update faster than
     simple arithmetic on the global variable.  Thus counter is considered
     suitable for accounting in the performance-critical code paths.

     counter_u64_alloc(wait)
             Allocate a new 64-bit unsigned counter.  The wait argument is the
             malloc(9) wait flag, should be either M_NOWAIT or M_WAITOK.  If
             M_NOWAIT is specified the operation may fail.

     counter_u64_free(c)
             Free the previously allocated counter c.

     counter_u64_add(c, v)
             Add v to c.  The KPI does not guarantee any protection from
             wraparound.

     counter_enter()
             Enter mode that would allow to safely update several counters via
             counter_u64_add_protected().  On some machines this expands to
             critical(9) section, while on other is a nop.  See IMPLEMENTATION
             DETAILS.

     counter_exit()
             Exit mode for updating several counters.

     counter_u64_add_protected(c, v)
             Same as counter_u64_add(), but should be preceded by
             counter_enter().

     counter_u64_fetch(c)
             Take a snapshot of counter c.  The data obtained is not
             guaranteed to reflect the real cumulative value for any moment.

     counter_u64_zero(c)
             Clear the counter c and set it to zero.

     SYSCTL_COUNTER_U64(parent, nbr, name, access, ptr, descr)
             Declare a static sysctl oid that would represent a counter.  The
             ptr argument should be a pointer to allocated counter_u64_t.  A
             read of the oid returns value obtained through
             counter_u64_fetch().  Any write to the oid zeroes it.

     SYSCTL_ADD_COUNTER_U64(ctx, parent, nbr, name, access, ptr, descr)
             Create a sysctl oid that would represent a counter.  The ptr
             argument should be a pointer to allocated counter_u64_t.  A read
             of the oid returns value obtained through counter_u64_fetch().
             Any write to the oid zeroes it.

     SYSCTL_COUNTER_U64_ARRAY(parent, nbr, name, access, ptr, len, descr)
             Declare a static sysctl oid that would represent an array of
             counter.  The ptr argument should be a pointer to allocated array
             of counter_u64_t's.  The len argument should specify number of
             elements in the array.  A read of the oid returns len-sized array
             of uint64_t values  obtained through counter_u64_fetch().  Any
             write to the oid zeroes all array elements.

     SYSCTL_ADD_COUNTER_U64_ARRAY(ctx, parent, nbr, name, access, ptr, len,
             descr)
             Create a sysctl oid that would represent an array of counter.
             The ptr argument should be a pointer to allocated array of
             counter_u64_t's.  The len argument should specify number of
             elements in the array.  A read of the oid returns len-sized array
             of uint64_t values obtained through counter_u64_fetch().  Any
             write to the oid zeroes all array elements.

IMPLEMENTATION DETAILS
     On all architectures counter is implemented using per-CPU data fields
     that are specially aligned in memory, to avoid inter-CPU bus traffic due
     to shared use of the variables between CPUs.  These are allocated using
     UMA_ZONE_PCPU uma(9) zone.  The update operation only touches the field
     that is private to current CPU.  Fetch operation loops through all per-
     CPU fields and obtains a snapshot sum of all fields.

     On amd64 a counter update is implemented as a single instruction without
     lock semantics, operating on the private data for the current CPU, which
     is safe against preemption and interrupts.

     On i386 architecture, when machine supports the cmpxchg8 instruction,
     this instruction is used.  The multi-instruction sequence provides the
     same guarantees as the amd64 single-instruction implementation.

     On some architectures updating a counter require a critical(9) section.

EXAMPLES
     The following example creates a static counter array exported to
     userspace through a sysctl:

           #define MY_SIZE 8
           static counter_u64_t array[MY_SIZE];
           SYSCTL_COUNTER_U64_ARRAY(_debug, OID_AUTO, counter_array, CTLFLAG_RW,
               &array[0], MY_SIZE, "Test counter array");

SEE ALSO
     atomic(9), critical(9), locking(9), malloc(9), sysctl(9), uma(9)

HISTORY
     The counter facility first appeared in FreeBSD 10.0.

AUTHORS
     The counter facility was written by Gleb Smirnoff and Konstantin
     Belousov.

FreeBSD 11.1-RELEASE-p4         March 14, 2016         FreeBSD 11.1-RELEASE-p4
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