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);
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 pathes.
counter_u64_alloc
(how)
- 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.
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.
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.