On Wed, 2013-04-10 at 11:48 -0400, Olivier Langlois wrote: > Please explain how expensive it is. All I am seeing is a couple of > additions.
Let me start with this, since your earlier argument also refers to this. So yes it does look simple and straight fwd, only one addition. However its an atomic operation across all threads of the same process. Imagine a single process with 512 threads, all running on a separate cpu. Do you see the problem? The cacheline contention of that one atomic is enough to bring a machine that size to its knees. People tried, it works. This is a fundamentally unscalable problem that is part of the POSIX interface. Also, since its a concurrent problem, the entire question: "what is the current runtime of the process" is uncertain and fuzzy. I prefer to look at it as a Heisenberg uncertainty principle of SMP computing; you cannot know the exact state of your SMP system and have it run (fast). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
