C++ Logo

SG14

Advanced search

Subject: Re: New colony proposal - feedback requested
From: Matthew Bentley (mattreecebentley_at_[hidden])
Date: 2020-05-05 19:15:08


For anyone reading this in gmail, please click on the ... in my previous
response, as gmail hides the forwarded portion of the message.

On Wed, 6 May 2020 at 11:59, Matthew Bentley <mattreecebentley_at_[hidden]>
wrote:

> > Thanks again, and once more, take anything I don't comment on as
> accepted criticism/explanation.
>
>>
>> Your response didn't go to the list. I've added the list again.
>>
>
> Ah, whoops - thanks. But you added the incorrect list address- correcting
> here.
>
> You either need to say which constraints a user-supplied type for Skipfield
>> is required to satisfy (note it's a template parameter, so the user can
>> choose), or the template parameter must go. Currently, the wording is
>> silent on any constraints for "Skipfield".
>>
>
> Torn here between obviating future improvements and being specific. It
> needs to stay in order for current implementations to be useful to, in
> particular, embedded folk, so it would be an unsigned integer type. If
> future implementations don't use it or use something different, it can be
> gracefully deprecated I guess.
>
>
>>
>> I haven't yet understood the "reverse iterator", --end(), empty container,
>> and undefined behavior train of thought. I believe you can only do
>> --end()
>> on a non-empty container, as a matter of precondition for that operation.
>>
>>
> I don't think the C++ spec specifies that iterators have to be bounded?
> But certainly including bounding creates performance detriment and should
> be optional.
> In a bounded iterator --end() would be non-problematic, in a non-bounded
> would result in undefined behaviour.
>
>
>
>>
>> This area probably needs a bit more thought / design work.
>> It does not seem to be a frequent operation to change min/max
>> mid-way during your container's lifetime, so offering a small
>> interface surface here is probably a good idea.
>>
>
> I'm going to look into whether or not specifying min/max limits for block
> capacities without changing the capacities of existing blocks can have any
> adverse effects ie. if block 1 is capacity 4, and set_block_capacity_limits
> sets min capacity to 10, whether leaving block 1 at capacity 4 could have
> any adverse effects.
> If not, will re-implement so that it's a simple "set variables" operation
> without any side effects.
> If so, will re-implement so that set_block_capacity_limits doesn't include
> the reallocation code when the type is non-copyable/movable (via constexpr
> and type_traits).
>
>
>
>>
>> Some ABIs can return trivially-copyable structs of sufficiently small size
>> in registers. The structured binding above has a good chance that no
>> stack memory will ever be used, because min/max will be kept in a register
>> in the caller, too.
>>
>> If you pass-by-reference, ABIs generally convert that into a pointer,
>> i.e. the caller needs to pass a memory region and later needs to load
>> the data from the filled memory region. That is more work.
>>
>>
> I tried implementing it your way, but the code-smell got bad pretty
> quickly.
> You end up with code like:
> plf::colony<int, my_allocator_type>::minmax block_sizes;
> block_sizes = i_colony.get_block_capacity_limits();
>
> You have to make the struct internal to colony otherwise you can't
> guarantee that the allocator will supply the same size_type to block_sizes
> as it does to colony. You could just go with size_t I guess. But the main
> issues were with using it with constructors, where you end up with this:
> plf::colony<int, my_allocator_type>::minmax block_sizes = {50, 500};
> plf::colony<int, my_allocator_type> i(block_sizes, my_allocator_instance);
>
> or worse:
>
> plf::colony<int, my_allocator_type> i(plf::colony<int,
> my_allocator_type>::minmax block_sizes(50, 500), my_allocator_instance);
>
> I tried passing via a bracketed pair of ints, but of course that makes it
> go into the initializer_list constructor instead.
>
> I think in this instance, the way I've done it is best, or results in less
> ugly code for the end user. I think the frequency of usage of
> get_block_capacity_limits does not justify creation of a struct for this
> purpose. The standard does use pass-by-reference for return parameters, in
> places - eg. istream& getline - so it's not a big deal I feel.
>
>
>
>> >> Any sorting near O(n^2) is not worth talking about, so you should
>> simply
>> >>prescribe O(n log n) with a "remarks" paragraph that this might allocate
>> >> memory (which might cause std::bad_alloc to be thrown).
>> >
>> > The time complexity doesn't relate to the speed of sorting (if it did,
>> std::list sorting would be as fast as std::vector sorting). The schema I'm
>> discussing simply allows for faster sort algorithms which rely on random
>> access, as opposed to relying on a slower algorithm which doesn't ala list.
>>
>> I'm guessing that std::list::sort also does some tricks with an
>> intermediate pointer array, so I'm not seeing the point.
>>
>
> It doesn't - it does a basic merge-sort in all implementations I've seen.
> I pretty much came up with the pointer-sort dereferencing hack (
> https://plflib.org/list.htm#details) - or at least I haven't seen it
> elsewhere - that's why plf::list sorting is 77% faster than std::list (see:
> https://plflib.org/benchmarks_haswell_gcc.htm#sort).
> From my old reading of gcc/boost posts, I think the libstdC++ people are
> pretty against allocating memory during a sort.
>
>
>
>> > Because that's an edge case outside of get_index_from_iterator and
>> results in performance loss in general use when you specialize for it.
>>
>> One missed branch, I hope?
>>
>
> More than one.
>
>
>>
>> Anyway, there seems to be rationale for going beyond
>> std::distance(begin(), it)
>> that should be recorded in the "Design Decisions" section.
>>
>
> Fair enough.
>
>
>
>> > I don't think that's not the right place for the explanation, the right
>> location is in Design decisions.
>> > You wouldn't expect an explanation of how a map/vector/deque works, or
>> why some approaches allow for better performance, to be included in the
>> tech spec, so I don't think you can expect the same of this.
>>
>> std::deque doesn't allow me to specify block size parameters
>> from the outside, so I don't care on the specification level.
>>
>> The interface for std::colony that you offered does allow me to
>> influence min/max, so you need to normatively specify what such
>> influence does, otherwise the parameter is meaningless and
>> therefore needs to go.
>>
>
> I don't think there are abstract explanations for all the parameters which
> the other containers are able to influence either, at least not to the
> depth you're talking about. Prove me wrong?
>
>
>
>> >
>> > The C++ abstract machine has no way of expressing "cache locality",
>> btw.
>> >
>> >
>> > Not really my concern. I'm interested in high-performance containers,
>> not the C++ abstract machine.
>>
>> Then you're in the wrong forum here.
>>
>
> Not really. I think the C++ abstract machine is based on some fairly old
> understandings which don't hold true anymore ie. that time complexity is
> strongly relevant in some capacity (latency, overall performance) which it
> isn't outside of large N or large sizeof(N). That being the case, it's not
> something I should overly concern myself with, as the computers it was
> designed for no longer exist.
>
>
>
> My understanding is that std::colony does not provide random-access
>> iterators.
>> Only those have relational operators, though, in the general scheme of
>> iterator operations [random.access.iterators].
>>
>> There is no specification that the iterators std::colony provides have
>> relational operators.
>>
>
> There has been no pushback so far from colony having bidirectional
> iterators with additional relational operators. The iterators have always
> been specified as bidirectional. If you have a rational objection I'd be
> interested in hearing it.
>
>
>
>> > Okay, you're trying to get me to move some prose to the tech spec and
>> some to the design decisions. Is there a guide as to what prose belongs
>> where?
>>
>> Anything explaining "why" should go to the "Design Decisions".
>>
>> Anything allowing to judge whether my independent (assume bad-faith)
>> implementation
>> of std::colony is conforming or not (given arbitrarily hostile template
>> parameters
>> and arbitrarily hostile sequences of operations) needs to into the
>> specification.
>>
>> Example:
>> As I said above, "cache-locality" (for instance) is nothing you can talk
>> about
>> in the C++ standard (because the abstract machine doesn't provide
>> vocabulary
>> for it), but you might want to talk about only allowing at most one memory
>> allocation (observable via the Allocator template parameter) per "min"
>> inserts or so.
>>
>
> Okay. Will contemplate that.
> Thanks once again,
> Matt
>



SG14 list run by sg14-owner@lists.isocpp.org

Older Archives on Google Groups