Re: [boost] [Container] interest in a flat_unordered_map

Subject: Re: [boost] [Container] interest in a flat_unordered_map
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-09-18 20:31:12

On Fri, Sep 18, 2015 at 11:09 PM, Treb Connell <trebconnell_at_[hidden]> wrote:
> I'm hoping to port a flat hash map that I've written to boost.
>
> *Performance (primary reason for use):*
> It uses memory and cache very effectively and ends up being *very fast*.
> It's hard to benchmark real world situations, but in inserting, finding,
> erasing, and iterating over a dictionary it was up to six orders of
> magnitude faster than VS2013's unordered_map.  This isn't a totally fair
> test because the cache efficiency really shines.

I believe, much of the benefit depends on the nature of keys and
values (e.g. ints vs. strings)? Is there a more detailed description
of the tests and results, at different container sizes?

> The most optimal implementation of iterators for this container *does not
> have constant time traversal*.  The implementation is a vector with
> elements scattered across it.  To increment an iterator I continue to
> increment the internal vector iterator until I hit a new element.  In
> practice elements should be scattered evenly, and will be close to one
> another, but in the worst case this is N-1.  Additional memory or logic
> could be added to make this O(1), but I don't think that trade is worth
> it.  In my applications of unordered_map I rarely iterate, and would not
> want to penalize insert, find, or erase to make iteration faster.

Can the iterator tradeoff be made a compile time option?

One frequently used operation that generally amounts to iterating
through elements is clear(). Do I understand correctly that its
complexity can be O(N) where N>size()? I believe the same applies to
range erase().

Also, is size() constant time?

> I could make a new concept for this, pay the cost for making it truly O(1),
> or *since the common case is not O(N) would it be okay to consider these
> bidirectional iterators*?

I think the iterators should have bidirectional traversal category
regardless of the choice on the complexity. It's the closest standard
category that fits anyway.