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Subject: Re: Question re: colony and new SIMD-related function
From: Matthew Bentley (mattreecebentley_at_[hidden])
Date: 2019-08-04 23:23:14


Not sure what you're replying to Niall, but it's not something I've
personally written. Most importantly, colony isn't like anything in the
STL. Even a half-minute browse of the project page would let you know that.

On Fri, 2 Aug 2019 at 21:58, Niall Douglas via SG14 <sg14_at_[hidden]>
wrote:

> On 01/08/2019 22:50, Matt Bentley via SG14 wrote:
> > Thanks Staffan & Niall,
> > my understanding is that gather-scatter has greatly improved in AVX512
>
> Sure, but I've not personally tested how well it works. It's much like
> hardware memory transactions were billed as amazing, but in fact are
> amazingly slow on Intel, and are not actually of much use at all in the
> real world.
>
> > In terms of my query, I guess the main point is:
> > If SIMD processing of elements is worthwhile in a given use-case, do you
> > think it's worth exposing colony internals so that the programmer can
> > create a gather-mask by parallel-processing the internal skipfields,
> > or do you think it's going to be just as well-performing for the
> > programmer to construct the mask in serial via iterating over the colony
> > as per usual?
>
> I'll be blunt here.
>
> I remain very unconvinced that any of the alt-containers which have
> landed before WG21 in recent years are worth the committee time. I even
> don't think Abseil's or Folly's containers are worth the committee time,
> for the cost-benefits they supply.
>
> I would observe that a lot of what committee folk do nowadays in their
> day jobs is write custom containers for bespoke use cases - indeed, just
> last week I wrote yet another open addressed hash table.
>
> Why do we all keep reimplementing containers? Because if one is tightly
> integrating data layout with the container, that yields a unique
> container design each and every time. And by definition, if the STL
> containers aren't good enough, that's because performance in this area
> is important, and now it's worth rolling a bespoke localised solution.
>
> Hence, for me personally, any container which looks like a STL container
> isn't worth the committee time. I'd personally be much keener on
> containers which look *very* different to STL containers. Like ones
> which are coroutinised, and have no allocators, because all dynamic
> memory allocation is implemented using commit-on-first-write page
> faulting, and during that page fault service the coroutine suspends and
> the CPU does other work until the TLB shootdown has completed.
>
> Now *those* are containers worth standardising because they deliver
> *orders of magnitude* gains over the conventional ones. Even your basic
> vector<T> becomes oodles better during capacity expansion, because
> instead of doing no work during capacity expansion, you are always doing
> as much work as possible.
>
> Anyway, that's my ha'pennies worth. And please don't be dissuaded from
> your efforts and work, it's just my opinion about cost-benefit
> tradeoffs. I am obviously quite biased by writing the kind of containers
> I want all day, and being consistently sad that WG21 doesn't and
> probably can't understand.
>
> Niall
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