202 lines
5.4 KiB
Groff
202 lines
5.4 KiB
Groff
.\"-
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.\" Copyright (c) 2013 Gleb Smirnoff <glebius@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 7, 2014
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.Dt COUNTER 9
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.Os
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.Sh NAME
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.Nm counter
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.Nd "SMP-friendly kernel counter implementation"
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/systm.h
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.In sys/counter.h
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.Ft counter_u64_t
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.Fn counter_u64_alloc "int wait"
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.Ft void
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.Fn counter_u64_free "counter_u64_t c"
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.Ft void
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.Fn counter_u64_add "counter_u64_t c" "int64_t v"
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.Ft void
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.Fn counter_enter
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.Ft void
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.Fn counter_exit
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.Ft void
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.Fn counter_u64_add_protected "counter_u64_t c" "int64_t v"
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.Ft uint64_t
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.Fn counter_u64_fetch "counter_u64_t c"
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.Ft void
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.Fn counter_u64_zero "counter_u64_t c"
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.In sys/sysctl.h
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.Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
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.Fn SYSCTL_ADD_COUNTER_U64 ctx parent nbr name access ptr descr
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.Sh DESCRIPTION
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.Nm
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is a generic facility to create counters
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that can be utilized for any purpose (such as collecting statistical
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data).
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A
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.Nm
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is guaranteed to be lossless when several kernel threads do simultaneous
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updates.
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However,
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.Nm
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does not block the calling thread,
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also no
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.Xr atomic 9
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operations are used for the update, therefore the counters
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can be used in any non-interrupt context.
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Moreover,
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.Nm
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has special optimisations for SMP environments, making
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.Nm
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update faster than simple arithmetic on the global variable.
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Thus
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.Nm
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is considered suitable for accounting in the performance-critical
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code pathes.
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.Bl -tag -width indent
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.It Fn counter_u64_alloc wait
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Allocate a new 64-bit unsigned counter.
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The
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.Fa wait
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argument is the
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.Xr malloc 9
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wait flag, should be either
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.Va M_NOWAIT
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or
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.Va M_WAITOK .
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If
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.Va M_NOWAIT
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is specified the operation may fail.
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.It Fn counter_u64_free c
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Free the previously allocated counter
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.Fa c .
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.It Fn counter_u64_add c v
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Add
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.Fa v
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to
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.Fa c .
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The KPI does not guarantee any protection from wraparound.
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.It Fn counter_enter
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Enter mode that would allow to safely update several counters via
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.Fn counter_u64_add_protected .
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On some machines this expands to
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.Xr critical 9
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section, while on other is a nop.
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See
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.Sx IMPLEMENTATION DETAILS .
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.It Fn counter_exit
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Exit mode for updating several counters.
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.It Fn counter_u64_add_protected c v
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Same as
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.Fn counter_u64_add ,
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but should be preceded by
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.Fn counter_enter .
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.It Fn counter_u64_fetch c
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Take a snapshot of counter
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.Fa c .
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The data obtained is not guaranteed to reflect the real cumulative
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value for any moment.
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.It Fn counter_u64_zero c
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Clear the counter
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.Fa c
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and set it to zero.
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.It Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
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Declare a static
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.Xr sysctl
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oid that would represent a
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.Nm .
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The
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.Fa ptr
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argument should be a pointer to allocated
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.Vt counter_u64_t .
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A read of the oid returns value obtained through
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.Fn counter_u64_fetch .
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Any write to the oid zeroes it.
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.It Fn SYSCTL_ADD_COUNTER_U64 ctx parent nbr name access ptr descr
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Create a
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.Xr sysctl
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oid that would represent a
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.Nm .
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The
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.Fa ptr
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argument should be a pointer to allocated
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.Vt counter_u64_t .
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A read of the oid returns value obtained through
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.Fn counter_u64_fetch .
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Any write to the oid zeroes it.
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.El
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.Sh IMPLEMENTATION DETAILS
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On all architectures
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.Nm
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is implemented using per-CPU data fields that are specially aligned
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in memory, to avoid inter-CPU bus traffic due to shared use
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of the variables between CPUs.
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These are allocated using
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.Va UMA_ZONE_PCPU
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.Xr uma 9
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zone.
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The update operation only touches the field that is private to current CPU.
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Fetch operation loops through all per-CPU fields and obtains a snapshot
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sum of all fields.
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.Pp
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On amd64 a
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.Nm counter
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update is implemented as a single instruction without lock semantics,
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operating on the private data for the current CPU,
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which is safe against preemption and interrupts.
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.Pp
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On i386 architecture, when machine supports the cmpxchg8 instruction,
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this instruction is used.
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The multi-instruction sequence provides the same guarantees as the
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amd64 single-instruction implementation.
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.Pp
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On some architectures updating a counter require a
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.Xr critical 9
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section.
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.Sh SEE ALSO
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.Xr atomic 9 ,
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.Xr critical 9 ,
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.Xr locking 9 ,
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.Xr malloc 9 ,
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.Xr sysctl 9 ,
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.Xr uma 9
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.Sh HISTORY
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The
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.Nm
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facility first appeared in
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.Fx 10.0 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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facility was written by
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.An Gleb Smirnoff
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and
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.An Konstantin Belousov .
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