Reserving capacity in associative arrays

[Steven Schveighoffer via Digitalmars-d-learn]

On 2/14/16 10:22 PM, Jon D wrote:
> Is there a way to reserve capacity in associative arrays? In some
> programs I've been writing I've been getting reasonable performance up
> to about 10 million entries, but beyond that performance is impacted
> considerably (say, 30 million or 50 million entries). GC stats (via the
> "--DRT-gcopt=profile:1" option) indicate dramatic increases in gc time,
> which I'm assuming comes from resizing the underlying hash table. I'm
> guessing that by preallocating a large size the performance degradation
> would not be quite so dramatic. The underlying implementation of
> associative arrays appears to take an initial number of buckets, and
> there's a private resize() method, but it's not clear if there's a
> public way to use these.

There is not a public way to access these methods unfortunately.

It would be a good addition to druntime I believe.

Recently, I added a clear method to the AA, which does not reduce 
capacity. So if you frequently build large AAs, and then throw them 
away, you could instead reuse the memory.

I would caution to be sure of this cause, however, before thinking it 
would solve the problem. The AA not only uses an array for buckets, but 
allocates a memory location for each element as well. I'm often wrong 
when I assume what the problem is when it comes to GC issues...

-Steve