freebsd/lib/libthr/libthr.3

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.Dd September 26, 2014
.Dt LIBTHR 3
.Os
.Sh NAME
.Nm libthr
.Nd "1:1 POSIX threads library"
.Sh LIBRARY
.Lb libthr
.Sh SYNOPSIS
.In pthread.h
.Sh DESCRIPTION
The
.Nm
library provides a 1:1 implementation of the
.Xr pthread 3
library interfaces for application threading.
It
has been optimized for use by applications expecting system scope thread
semantics, and can provide significant performance improvements
compared to
.Lb libkse .
.Pp
The library is tightly integrated with the run-time link editor
.Xr ld-elf.so.1 1
and
.Lb libc ;
all three components must be built from the same source tree.
Mixing
.Li libc
and
.Nm
libraries from different versions of
.Fx
is not supported.
The run-time linker
.Xr ld-elf.so.1 1
has some code to ensure backward-compatibility with older versions of
.Nm .
.Pp
The man page documents the quirks and tunables of the
.Nm .
When linking with
.Li -lpthread ,
the run-time dependency
.Li libthr.so.3
is recorded in the produced object.
.Sh MUTEX ACQUISITION
A locked mutex (see
.Xr pthread_mutex_lock 3 )
is represented by a volatile variable of type
.Dv lwpid_t ,
which records the global system identifier of the thread
owning the lock.
.Nm
performs a contested mutex acquisition in three stages, each of which
is more resource-consuming than the previous.
The first two stages are only applied for a mutex of
.Dv PTHREAD_MUTEX_ADAPTIVE_NP
type and
.Dv PTHREAD_PRIO_NONE
protocol (see
.Xr pthread_mutexattr 3 ) .
.Pp
First, on SMP systems, a spin loop
is performed, where the library attempts to acquire the lock by
.Xr atomic 9
operations.
The loop count is controlled by the
.Ev LIBPTHREAD_SPINLOOPS
environment variable, with a default value of 2000.
.Pp
If the spin loop
was unable to acquire the mutex, a yield loop
is executed, performing the same
.Xr atomic 9
acquisition attempts as the spin loop,
but each attempt is followed by a yield of the CPU time
of the thread using the
.Xr sched_yield 2
syscall.
By default, the yield loop
is not executed.
This is controlled by the
.Ev LIBPTHREAD_YIELDLOOPS
environment variable.
.Pp
If both the spin and yield loops
failed to acquire the lock, the thread is taken off the CPU and
put to sleep in the kernel with the
.Xr umtx 2
syscall.
The kernel wakes up a thread and hands the ownership of the lock to
the woken thread when the lock becomes available.
.Sh THREAD STACKS
Each thread is provided with a private user-mode stack area
used by the C runtime.
The size of the main (initial) thread stack is set by the kernel, and is
controlled by the
.Dv RLIMIT_STACK
process resource limit (see
.Xr getrlimit 2 ) .
.Pp
By default, the main thread's stack size is equal to the value of
.Dv RLIMIT_STACK
for the process.
If the
.Ev LIBPTHREAD_SPLITSTACK_MAIN
environment variable is present in the process environment
(its value does not matter),
the main thread's stack is reduced to 4MB on 64bit architectures, and to
2MB on 32bit architectures, when the threading library is initialized.
The rest of the address space area which has been reserved by the
kernel for the initial process stack is used for non-initial thread stacks
in this case.
The presence of the
.Ev LIBPTHREAD_BIGSTACK_MAIN
environment variable overrides
.Ev LIBPTHREAD_SPLITSTACK_MAIN ;
it is kept for backward-compatibility.
.Pp
The size of stacks for threads created by the process at run-time
with the
.Xr pthread_create 3
call is controlled by thread attributes: see
.Xr pthread_attr 3 ,
in particular, the
.Xr pthread_attr_setstacksize 3 ,
.Xr pthread_attr_setguardsize 3
and
.Xr pthread_attr_setstackaddr 3
functions.
If no attributes for the thread stack size are specified, the default
non-initial thread stack size is 2MB for 64bit architectures, and 1MB
for 32bit architectures.
.Sh RUN-TIME SETTINGS
The following environment variables are recognized by
.Nm
and adjust the operation of the library at run-time:
.Bl -tag -width LIBPTHREAD_SPLITSTACK_MAIN
.It Ev LIBPTHREAD_BIGSTACK_MAIN
Disables the reduction of the initial thread stack enabled by
.Ev LIBPTHREAD_SPLITSTACK_MAIN .
.It Ev LIBPTHREAD_SPLITSTACK_MAIN
Causes a reduction of the initial thread stack, as described in the
section
.Sx THREAD STACKS .
This was the default behaviour of
.Nm
before
.Fx 11.0 .
.It Ev LIBPTHREAD_SPINLOOPS
The integer value of the variable overrides the default count of
iterations in the
.Li spin loop
of the mutex acquisition.
The default count is 2000, set by the
.Dv MUTEX_ADAPTIVE_SPINS
constant in the
.Nm
sources.
.It Ev LIBPTHREAD_YIELDLOOPS
A non-zero integer value enables the yield loop
in the process of the mutex acquisition.
The value is the count of loop operations.
.It Ev LIBPTHREAD_QUEUE_FIFO
The integer value of the variable specifies how often blocked
threads are inserted at the head of the sleep queue, instead of its tail.
Bigger values reduce the frequency of the FIFO discipline.
The value must be between 0 and 255.
.El
.Sh INTERACTION WITH RUN-TIME LINKER
The
.Nm
library must appear before
.Li libc
in the global order of depended objects.
.Pp
Loading
.Nm
with the
.Xr dlopen 3
call in the process after the program binary is activated
is not supported, and causes miscellaneous and hard-to-diagnose misbehaviour.
This is due to
.Nm
interposing several important
.Li libc
symbols to provide thread-safe services.
In particular,
.Dv errno
and the locking stubs from
.Li libc
are affected.
This requirement is currently not enforced.
.Pp
If the program loads any modules at run-time, and those modules may require
threading services, the main program binary must be linked with
.Li libpthread ,
even if it does not require any services from the library.
.Pp
.Nm
cannot be unloaded; the
.Xr dlclose 3
function does not perform any action when called with a handle for
.Nm .
One of the reasons is that the interposing of
.Li libc
functions cannot be undone.
.Sh SIGNALS
The implementation also interposes the user-installed
.Xr signal 3
handlers.
This interposing is done to postpone signal delivery to threads which
entered (libthr-internal) critical sections, where the calling
of the user-provided signal handler is unsafe.
An example of such a situation is owning the internal library lock.
When a signal is delivered while the signal handler cannot be safely
called, the call is postponed and performed until after the exit from
the critical section.
This should be taken into account when interpreting
.Xr ktrace 1
logs.
.Sh SEE ALSO
.Xr ktrace 1 ,
.Xr ld-elf.so.1 1 ,
.Xr getrlimit 2 ,
.Xr umtx 2 ,
.Xr dlclose 3 ,
.Xr dlopen 3 ,
.Xr errno 3 ,
.Xr getenv 3 ,
.Xr libc 3 ,
.Xr pthread_attr 3 ,
.Xr pthread_attr_setstacksize 3 ,
.Xr pthread_create 3 ,
.Xr signal 3 ,
.Xr atomic 9
.Sh AUTHORS
.An -nosplit
The
.Nm
library
was originally created by
.An Jeff Roberson Aq Mt jeff@FreeBSD.org ,
and enhanced by
.An Jonathan Mini Aq Mt mini@FreeBSD.org
and
.An Mike Makonnen Aq Mt mtm@FreeBSD.org .
It has been substantially rewritten and optimized by
.An David Xu Aq Mt davidxu@FreeBSD.org .