Lightning Talk: Execution Policies: Not a Forward Progress Guarantee - Ruslan Arutyunyan

Added: 2026-05-26

[00:00:00]Honestly, it's the hallway track. Like the biggest the spaces where I've learned the most at CPPCON have been in the evenings hanging out with people over a drink or at dinner and just chatting.

[00:00:26]So yesterday we had a talk which basically talked about like some forward progress guarantee and somehow you know combine it together with execution policies and like I think like at least partially this talk was not true. The point is like I'm not going to blame this person.

[00:00:47]I'm going to blame blame myself as the C++ committee member how bad we taught people about execution policies.

[00:00:57]There are some pretty simple things but I kind of feel people just don't understand them still and confuse them very often uh with for progress guarantees.

[00:01:11]I'm going to use my example from the lightning talk yesterday.

[00:01:16]Uh like it doesn't compile of course because there is no like variable text here like on the screen. It compiles like in my slides. But the point is um I mean yesterday slides like the point the question that I have to all of you we have like let's focus on execution policy which is the first parameters this set execution part raise your hand who think this is prescription to run this algorithm in parallel.

[00:01:48]Okay, you are wrong.

[00:01:51]That means I am wrong as the C++ committee member who thinks that this is a recommendation or like yeah basically asking for run it in parallel.

[00:02:06]Okay, you are closer to the answer. But actually what execution policy mean is basically giving permission what is allowed to do with your like lambda so like callables and believe me every algorithm in C++ standard library hope I'm not lying but I'm pretty sure everyone has a some callable to call even sto sort without a comparator it does have implicit callable which is like less operator by default And that's what you where you're saying what is allowed.

[00:02:44]Um you give a permission what is allowed to do with your lambda. That's it. That's all execution policies are about.

[00:02:56]And this is basically the property of your lambda. You might ask why we haven't attached them to lambdas. I think there are various reasons. One of them is on the screen. We have two lambdas per algorithm. If you attach the different policies to different lambdas, it's would involve like some algebra to have a common type out of policies and this is like sort of over complication.

[00:03:20]I think the committee didn't decide to do that and just stick to to algorithm.

[00:03:26]So parent seek is a maximum permission that users give to implementation. It may still run in sequentially but it's a maximum permission. My lambda is doesn't involve data races and my lambda uh doesn't have unsafe uh unse like unsequenced or vectorizable operations.

[00:03:49]That's it like do with this information whatever you want.

[00:03:53]I mean implementation do with this information whatever you want. Um one of the examples like you don't have uh any mutxes uh and you have like uh each index of data read um individually like without like any data races. PAR is a permission for paralization only. So you might have some synchronization within your lambda for example like atomics or whatever whatever synchronization you want. Unseek is uh permission to do the operation unsequentially but like on the one thread. So yeah, vectorization would be a proper uh thinking like it's one way of implementing it. I don't have a very quick good example uh what would be like the lambda that doesn't require that doesn't allow paralization but allows unsequence operation but that's it what it's for and sequential means zero permissions you don't know like you can't assume anything safe about your lambda um you must execute it sequentially on the calling thread that's it that's all the permissions that I gave you basically zero Forward progress guarantee is separate story to execution policies. It's basically how your agents make progress.

[00:05:08]There is a talk of Olivia Jahoo on this conference 2023. If I'm saying something contradictory to what he said, he is probably right.

[00:05:19]He's likely right. But other than that, I hope this is a good summary for you guys and you know a lot about execution policies right now. I hope you enjoyed it. [applause]