So I'm toying around with a promising structure that I call "free tree" for std.allocator. There's some detail with code and docs here: http://forum.dlang.org/thread/mi1qph$cgr$1@digitalmars.com.

A free tree allocator is akin to a free list. Instead of just a singly linked list, it also maintains a binary search tree sorted by size. So each free chunk contains the size, the "next" node, and "left"/"right" children.

Insertion in the free tree is done to the root, i.e. the newly inserted block becomes the root. Just like the free list. So we got nice locality etc. and no need to worry about rebalancing. Code came in really small and clean, textbook-like.

All in all free tree is like an adaptive battery of freelists - it just adapts to whichever sizes you allocate the most.

And herein lies the danger. As allocation patterns come and go, chunks of less-frequently-used lengths get pushed toward the leaves of the tree and there's nothing to limit growth. So we have fragmentation because the free tree is holding onto all those old chunks etc.

What's needed is a good eviction strategy that's cheap (e.g. doesn't require me to hold age info or run complex algos) and effective. One hamfisted solution is to just blow away the tree once in a while (e.g. when it gets to a specific size or after some time/number of allocations) but I feel there's something more principled out there.

I'd love to hear your thoughts.


Thanks,

Andrei

On 5/05/2015 5:56 a.m., Andrei Alexandrescu wrote:
> So I'm toying around with a promising structure that I call "free tree"
> for std.allocator. There's some detail with code and docs here:
> http://forum.dlang.org/thread/mi1qph$cgr$1@digitalmars.com.
>
> A free tree allocator is akin to a free list. Instead of just a singly
> linked list, it also maintains a binary search tree sorted by size. So
> each free chunk contains the size, the "next" node, and "left"/"right"
> children.
>
> Insertion in the free tree is done to the root, i.e. the newly inserted
> block becomes the root. Just like the free list. So we got nice locality
> etc. and no need to worry about rebalancing. Code came in really small
> and clean, textbook-like.
>
> All in all free tree is like an adaptive battery of freelists - it just
> adapts to whichever sizes you allocate the most.
>
> And herein lies the danger. As allocation patterns come and go, chunks
> of less-frequently-used lengths get pushed toward the leaves of the tree
> and there's nothing to limit growth. So we have fragmentation because
> the free tree is holding onto all those old chunks etc.
>
> What's needed is a good eviction strategy that's cheap (e.g. doesn't
> require me to hold age info or run complex algos) and effective. One
> hamfisted solution is to just blow away the tree once in a while (e.g.
> when it gets to a specific size or after some time/number of
> allocations) but I feel there's something more principled out there.
>
> I'd love to hear your thoughts.
>
>
> Thanks,
>
> Andrei

This may sound crazy BUT:

struct MyPointer {
	void* ptr;
	alias ptr this;
}

void* allocate() {
	// do the actual allocation
	MyPointer* ret = new MyPointer(thePtr);// something better?
	// store ret
	return ret;
}

void cleaner() {
	MyPointer*[] toMerge;
	foreach(pointer; pointers) {
		// detect small but mergable items
	}
	foreach(toM; toMerge) {
		// re assign the ptr
	}
}

In essence make small allocations big by reallocating them to be bigger.

On Monday, 4 May 2015 at 17:56:23 UTC, Andrei Alexandrescu wrote:
> So I'm toying around with a promising structure that I call "free tree" for std.allocator. There's some detail with code and docs here: http://forum.dlang.org/thread/mi1qph$cgr$1@digitalmars.com.
>
> A free tree allocator is akin to a free list. Instead of just a singly linked list, it also maintains a binary search tree sorted by size. So each free chunk contains the size, the "next" node, and "left"/"right" children.
>
> Insertion in the free tree is done to the root, i.e. the newly inserted block becomes the root. Just like the free list. So we got nice locality etc. and no need to worry about rebalancing. Code came in really small and clean, textbook-like.
>
> All in all free tree is like an adaptive battery of freelists - it just adapts to whichever sizes you allocate the most.
>
> And herein lies the danger. As allocation patterns come and go, chunks of less-frequently-used lengths get pushed toward the leaves of the tree and there's nothing to limit growth. So we have fragmentation because the free tree is holding onto all those old chunks etc.
>
> What's needed is a good eviction strategy that's cheap (e.g. doesn't require me to hold age info or run complex algos) and effective. One hamfisted solution is to just blow away the tree once in a while (e.g. when it gets to a specific size or after some time/number of allocations) but I feel there's something more principled out there.
>
> I'd love to hear your thoughts.
>
>
> Thanks,
>
> Andrei

Taking the tree analogy further, would it be possible to have autumn (fall) for the leaves. Either the tree gets rid of old leaves (as in nature) or the leaves have some self-destruction mechanism like cells in the body.