[Issue 17881] Provide mechanism to preallocate memory from the GC

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17881

--- Comment #6 from Steven Schveighoffer <schveiguy at yahoo.com> ---
(In reply to Rainer Schuetze from comment #5)
> A slightly simpler API could be to add allocating N same-sized chunks from
> the GC that returns them in a free-list-like chain.

I think an array is best, since the data already is an array, and a free-list
requires building a linked list before-hand.

> I agree with Andrei that we should not complicate the GC interface for every
> possible allocation pattern a user might want to optimize for, though.

Hm... I'm trying to find the most generic interface for this. It's not
necessarily limited to AA, as allocating a bunch of blocks in a loop isn't an
uncommon thing.

If there was a way to "re-flavor" the blocks from one giant block into
individual ones, then we could do this outside the GC.

> If you call GC.reserve(20_000_000*(Key.sizeof+Value.sizeof)) before
> inserting elements, there should be no collection while filling the AA.

Limiting the collection in the example I posted shaves off about 300-400msec,
but it still is 1.5 seconds vs. 0.1 for the local array version.

> If we add thread local free-lists to the GC, the overhead of allocating
> these from the GC instead of caching them in the AA would be rather small.

Agreed, I think keeping the lists inside the GC is the most useful, and does
not expose any implementation details to the user.

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