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Special Forums UNIX and Linux Applications High Performance Computing Memory Barriers for (Ubuntu) Linux (i686) Post 302429949 by gorga on Wednesday 16th of June 2010 08:20:50 AM
Old 06-16-2010
Memory Barriers for (Ubuntu) Linux (i686)

Hi all,

(Hope this is the right forum for this question)

I have some multi-threaded C code (compiled with GCC 4.4.3) which accesses shared variables. Although I've marked those variables with volatile to guard against compiler re-ordering, I'm concerned that processor out-of-order execution may cause my code to fail, and I'm looking for a "low-cost" method of guaranteeing ordering is maintained in my code.

For example, I have something like...

Code:
memset(&task, 0, sizeof(task_t));/* null memory */
task.id.prefix = prefix_id;
task.id.instance = instance_id;
/* write-memorybarrier required here */
task.state = task_ready;

Where I need to ensure that the "task state" is only set to "task_ready" after the previous instructions have been committed. As the "task" is shared between threads, another thread seeing the state as "ready" may try to access its member variables, so it's vital that the tasks "prefix" and "instance" have been updated.

I know this is a common problem and mutexes and semaphores provide in-built memory barriers to address this problem but I'm trying to build a scalable application and I want to avoid their use if possible. I also know GCC provides built-in atomic operations but I see they involve locking the data-bus, and I've heard about system primitives like "smp_wmb()" but I'm not sure how to incorporate these into my "user-space" program as they are platform dependent.

Therefore can anyone provide pointers or advise on how best (in terms of scalability and speed) to guarantee ordering is maintained?

Thanks.
 

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libmlib_mt(3LIB)														  libmlib_mt(3LIB)

NAME
libmlib_mt - multi-threaded mediaLib SYNOPSIS
cc [ flag... ] file... -lmlib_mt -lmlib [ library... ] #include <mlib.h> Interfaces in this library provide functions for multimedia processing. Multi-threaded (MT) mediaLib is a software layer developed on top of mediaLib using OpenMP. When it is used with a large data set on a multi-processor system, MT mediaLib will partition data into subsets and process the subsets in parallel, thus greatly improving performance of applications that use mediaLib. INTERFACES
The shared object libmlib_mt.so.2 provides the same public interfaces as those defined in libmlib(3lib). See intro(3) for additional infor- mation on shared object interfaces. There are two ways to use MT mediaLib. 1. Pre-load a multi-threaded mediaLib library during runtime by setting the LD_PRELOAD environment variable as follows before starting your application, in Bourne/Korn shell: LD_PRELOAD=libmlib_mt.so export LD_PRELOAD or in C shell: setenv LD_PRELOAD libmlib_mt.so In this way, you can take advantage of MT mediaLib without rebuilding your application. 2. Link your application with a multi-threaded mediaLib library directly as shown under . In this way, an MT mediaLib library is always used whenever your application is started. The parallelization of MT mediaLib is controlled, in part, by the PARALLEL environment variable. You can change its setting to adjust the degree of parallelization before starting your application, in Bourne/Korn shell: PARALLEL=n export PARALLEL or in C shell: setenv PARALLEL n where n is a positive integer for number of threads. Note that other factors also affect the degree of parallelization in MT mediaLib. /usr/lib/libmlib_mt.so.2 shared object /usr/lib/64/libmlib_mt.so.2 64-bit shared object See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWmlibt | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ Intro(3), libmlib(3lib), attributes(5) 30 Sep 2005 libmlib_mt(3LIB)
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