boxed data still imposes a small perf penalty

[nikomatsakis]

I did some quick benchmarks to see how effective #1493 was. The basis was comparing two versions of the binary tree shootout test, one based on boxes and one on unique pointers, running on my local machine with a parameter of 20. The results were:

• Before: 6.3s vs 22s
• After: 6.4s vs 7.5s

I did several runs but the results were fairly stable. So, there is a much smaller gap, but there is still a gap. Really, boxes ought to be faster, as they are purely stack local: we ought to be able to optimize allocation better.

[nikomatsakis]

It occurs to me I could be seeing ref-counting costs.

[marijnh]

What do you mean 'purely stack-local'? I don't really see a scenario in which a refcounted box is faster than a unique one.

[nikomatsakis]

I'm sorry, I meant "task-local". Re: being faster, you are probably right, but I don't think boxes need to be slower, and certainly not this much slower. I recognize that unique pointers don't require any form of GC, ref-counting or otherwise, thus giving them an innate advantage; box allocation, OTOH, is always thread-local, so that could work in box's favor. OTOOH, optimized malloc implementations already reserve big pools per thread, so that may not make much of a difference in practice.

I can think of two factors that work against boxes right now, and I'd like to know their relative impact:

• Space overhead: boxes have a header in front which is non-trivial in size.
• Ref-counting overhead: we inc and dec refs and do a bit of other bookkeeping for cycle collection.

So as to get some hard data, I was thinking of running an experiment where I disable all ref-counting, cycle collection, and freeing of unique pointers, and compare their performance. This ought to eliminate the RC overhead. A further experiment might be to avoid allocating the header as well for boxes. Are there other factors to consider I am not thinking of?

[graydon]

I don't think this is quite concrete enough to act on, or at least not in any way that's not covered elsewhere. We're working on regions, arenas, cycle collection / GC and ARC elsewhere. Closing this as vague. Reopen if you disagree.