membarrier — issue memory barriers on a set of threads
#include <linux/membarrier.h>
int
membarrier( |
int cmd, |
int flags); |
The membarrier() system call
helps reducing the overhead of the memory barrier
instructions required to order memory accesses on multi-core
systems. However, this system call is heavier than a memory
barrier, so using it effectively is not as simple as replacing
memory barriers with this system call, but requires
understanding of the details below.
Use of memory barriers needs to be done taking into account that a memory barrier always needs to be either matched with its memory barrier counterparts, or that the architecture's memory model doesn't require the matching barriers.
There are cases where one side of the matching barriers
(which we will refer to as "fast side") is executed much more
often than the other (which we will refer to as "slow side").
This is a prime target for the use of membarrier(). The key idea is to replace,
for these matching barriers, the fast-side memory barriers by
simple compiler barriers, for example:
asm volatile ("" : : : "memory")
and replace the slow-side memory barriers by calls to
membarrier().
This will add overhead to the slow side, and remove
overhead from the fast side, thus resulting in an overall
performance increase as long as the slow side is infrequent
enough that the overhead of the membarrier() calls does not outweigh the
performance gain on the fast side.
The cmd argument
is one of the following:
MEMBARRIER_CMD_QUERYQuery the set of supported commands. The return
value of the call is a bit mask of supported commands.
MEMBARRIER_CMD_QUERY,
which has the value 0, is not itself included in this
bit mask. This command is always supported (on kernels
where membarrier() is
provided).
MEMBARRIER_CMD_SHAREDEnsure that all threads from all processes on the
system pass through a state where all memory accesses
to user-space addresses match program order between
entry to and return from the membarrier() system call. All threads
on the system are targeted by this command.
The flags argument
is currently unused and must be specified as 0.
All memory accesses performed in program order from each
targeted thread are guaranteed to be ordered with respect to
membarrier().
If we use the semantic barrier() to represent a compiler barrier
forcing memory accesses to be performed in program order
across the barrier, and smp_mb() to represent explicit memory
barriers forcing full memory ordering across the barrier, we
have the following ordering table for each pairing of
barrier(), membarrier() and smp_mb(). The pair ordering is detailed as
(O: ordered, X: not ordered):
barrier() smp_mb() membarrier() barrier() X X O smp_mb() X O O membarrier() O O O
On success, the MEMBARRIER_CMD_QUERY operation returns a
bit mask of supported commands and the MEMBARRIER_CMD_SHARED operation returns
zero. On error, −1 is returned, and errno is set appropriately.
For a given command, with flags set to 0, this system
call is guaranteed to always return the same value until
reboot. Further calls with the same arguments will lead to
the same result. Therefore, with flags set to 0, error handling
is required only for the first call to membarrier().
cmd is
invalid or flags is non-zero.
The membarrier()
system call is not implemented by this kernel.
A memory barrier instruction is part of the instruction set of architectures with weakly-ordered memory models. It orders memory accesses prior to the barrier and after the barrier with respect to matching barriers on other cores. For instance, a load fence can order loads prior to and following that fence with respect to stores ordered by store fences.
Program order is the order in which instructions are ordered in the program assembly code.
Examples where membarrier()
can be useful include implementations of Read-Copy-Update
libraries and garbage collectors.
Assuming a multithreaded application where "fast_path()"
is executed very frequently, and where "slow_path()" is
executed infrequently, the following code (x86) can be
transformed using membarrier():
#include <stdlib.h> static volatile int a, b; static void fast_path(void) { int read_a, read_b; read_b = b; asm volatile ("mfence" : : : "memory"); read_a = a; /* read_b == 1 implies read_a == 1. */ if (read_b == 1 && read_a == 0) abort(); } static void slow_path(void) { a = 1; asm volatile ("mfence" : : : "memory"); b = 1; } int main(int argc, char **argv) { /* * Real applications would call fast_path() and slow_path() * from different threads. Call those from main() to keep * this example short. */ slow_path(); fast_path(); exit(EXIT_SUCCESS); }
The code above transformed to use membarrier() becomes:
#define _GNU_SOURCE #include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <sys/syscall.h> #include <linux/membarrier.h> static volatile int a, b; static int membarrier(int cmd, int flags) { return syscall(__NR_membarrier, cmd, flags); } static int init_membarrier(void) { int ret; /* Check that membarrier() is supported. */ ret = membarrier(MEMBARRIER_CMD_QUERY, 0); if (ret < 0) { perror("membarrier"); return −1; } if (!(ret & MEMBARRIER_CMD_SHARED)) { fprintf(stderr, "membarrier does not support MEMBARRIER_CMD_SHARED\n"); return −1; } return 0; } static void fast_path(void) { int read_a, read_b; read_b = b; asm volatile ("" : : : "memory"); read_a = a; /* read_b == 1 implies read_a == 1. */ if (read_b == 1 && read_a == 0) abort(); } static void slow_path(void) { a = 1; membarrier(MEMBARRIER_CMD_SHARED, 0); b = 1; } int main(int argc, char **argv) { if (init_membarrier()) exit(EXIT_FAILURE); /* * Real applications would call fast_path() and slow_path() * from different threads. Call those from main() to keep * this example short. */ slow_path(); fast_path(); exit(EXIT_SUCCESS); }
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright 2015 Mathieu Desnoyers <mathieu.desnoyersefficios.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |