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High Performance Computing
Memory Barriers for (Ubuntu) Linux (i686)
Post 302430438 by Corona688 on Thursday 17th of June 2010 03:00:49 PM
06-17-2010
Small world, how so? 
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.
Wow, those are nice.
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.
Quote:
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)
Atomic Builtins - Using the GNU Compiler Collection (GCC)
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.
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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