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.
