Comment Feed for Channel 9 - Threads and Shared Variables in C++11 http://ch9files.blob.core.windows.net/ch9/b54e/8ceeaa3d-551e-4184-a964-9fd4012cb54e/GoingNative2012ThreadsSharedVar_220_ch9.jpg Channel 9 - Threads and Shared Variables in C++11 The C++11 standard introduces threads into the language, and carefully defines the meaning of variables shared between threads. The design is based on the idea that meaningful multithreaded programs should, by default, behave as though threads were executed in simple interleaved fashion. This allows efficient execution only because "data races" (simultaneous non-read-only accesses to shared variables) are disallowed.  Although this basic approach is shared with some other languages, and with threads interfaces like pthreads, we tried hard to avoid their mistakes.  For example, C++11 provides a comprehensive atomic operations library, which makes it much more feasible to avoid data races in real low-level code. I'll give a very quick overview of thread support in C++11, and then focus on the behavior of shared variables, often called the "memory model". I'll try to explain why, after more than 40 years of programming with shared variables, shared variables and atomic operations were still controversial, and why the C++11 approach is the right one. en Tue, 21 May 2013 17:39:39 GMT Tue, 21 May 2013 17:39:39 GMT Rev9 Re: Threads and Shared Variables in C++11 Are slides going to be published at some point?

posted by AndresPetralli

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634638101951715540 Thu, 02 Feb 2012 20:09:55 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634638101951715540 AndresPetralli Re: Threads and Shared Variables in C++11 I'm interested in the slides too, I missed the keynote and the first part of Hans's session

posted by tocsa

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634638108468172653 Thu, 02 Feb 2012 20:20:46 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634638108468172653 tocsa Re: Threads and Shared Variables in C++11 @El Cubico That's utterly useless...  Recommended to not download, it's just El Cubico's bad manner upon Boehm's talk concerning his foreign accent. I actually want to hear the content not your ridiculous edit.

posted by HeavensRevenge

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634639050607260529 Fri, 03 Feb 2012 22:31:00 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634639050607260529 HeavensRevenge Re: Threads and Shared Variables in C++11

, HeavensRevenge wrote

@El Cubico That's utterly useless...  Recommended to not download, it's just El Cubico's bad manner upon Boehm's talk concerning his foreign accent. I actually want to hear the content not your ridiculous edit.

I agree. Removed.

C

posted by Charles

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634640056935775354 Sun, 05 Feb 2012 02:28:13 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634640056935775354 Charles Re: Threads and Shared Variables in C++11
Frequently breaks repeatably in practice.

Should read:

Frequently breaks repeatedly in practice.

posted by Javier Estrada

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634642288721266635 Tue, 07 Feb 2012 16:27:52 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634642288721266635 Javier Estrada Re: Threads and Shared Variables in C++11 @Javier Estrada: No, I don't think it's a typo. He actually means 'repeatably', as in 'in a repeatable way'.

He actually explains during the talk that compilers will typically treat the 'done' variable as a loop invariant and only read it once, which means the loop will either execute forever or not at all - a predictable, repeatable, behaviour (not the more-common unpredictable mess that you may get from programs containing data races).

posted by bog

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634645702435464056 Sat, 11 Feb 2012 15:17:23 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634645702435464056 bog Re: Threads and Shared Variables in C++11
I understand what it says. But isn't this a too high performance hit? On the one hand one gives the compiler lots of freedom and one has to ensure proper memory barriers for a single variable. But for variables that are next to each other everything has to be synchronized properly by the compiler.

For current architectures protecting neighbouring variables might be a different issue than protecting the same variable. But how about possible future systems where neighbouring variables have to be treated in the same way as a single one when it comes to concurrent access by different cpus? When the number of cpus in a system goes into the thousands the necessary cache coherence protocol traffic might forbid the current model.

posted by Christoph

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634646005509566035 Sat, 11 Feb 2012 23:42:30 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634646005509566035 Christoph Re: Threads and Shared Variables in C++11 We definitely need more info on what we can and can't do.
There's a missing bracket on slide 52. 8 - )

posted by ANet

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634647471152725949 Mon, 13 Feb 2012 16:25:15 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634647471152725949 ANet Re: Threads and Shared Variables in C++11
I guess an object "read" equates to a const method call, and an object "write" equates to a non-const method call.

Does anyone know if this is actually part of the standard? Can I assume that multiple threads calling const methods on an object is safe from any C++11 compiler/library?

posted by djw

]]> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634671412067658956 Mon, 12 Mar 2012 09:26:46 GMT http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/Threads-and-Shared-Variables-in-C-11#c634671412067658956 djw