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Special Forums UNIX and Linux Applications High Performance Computing Memory Barriers for (Ubuntu) Linux (i686) Post 302430438 by Corona688 on Thursday 17th of June 2010 03:00:49 PM
Old 06-17-2010
Small world, how so? Smilie
Quote:
Originally Posted by gorga
I hadn't heard of futexes until you mentioned them, but I did some reading and it seems they still use atomic instructions to update shared variables.
Well, yes. It has to synchronize somehow. One way or another you must interrupt other cores with this change in status, or they may never know.
Quote:
In that case I could just use one of GCC's built-in atomic operations like "__sync_fetch_ and_ add" or "__sync_bool_compare_and_swap" as described here...

Atomic Builtins - Using the GNU Compiler Collection (GCC)
Wow, those are nice.

Quote:
The thing with these is they use the asm op-code "lock", which issues a hardware lock on the data-bus effectively locking every other process out of memory.
I think you're overreacting... Any memory I/O monopolizes the bus*, LOCK just guarantees one instruction gets two ops in a row.

Also. The original 8088 has precisely one instruction worth of cache, so locking the bus stalls it instantly... The huge caches, multiple independent memory buses, and cache communication systems in recent NUMA systems usually let cores keep going or find something else to do. I'm not sure LOCK XCGH even forces a real memory fetch anymore(might be simple to test, try to get back to you on that.)

Lastly, if you're doing no mutexing, what are you doing instead -- polling? That's not going to be more efficient, untold amounts of CPU will be expended on what amounts to a while(1) loop.

I really think pthreads is still what you're looking for. They've made it as fast as they know how, significantly changing the kernel to accommodate it.

* Exceptions exist for very special-purpose memory chips like video RAM.
 

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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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