Re: rbtree scores (was Re: [patch] deadline-ioscheduler rb-tree sort)

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Jens Axboe wrote:
> > If it was worth it (I suspect not), you can make a data structure
> > which has O(1) amortised insertion time for a number of common cases,
> > such as runs of ascending block numbers.  Seems a likely pattern for a
> > filesystem...
> 
> Fibonacci heaps, for instance. I looked into that as well. However, it's
> actually more important to have (if possible) O(1) extraction than
> insert. Extraction is typically run from interrupt context, when a
> driver wants to requeue more requests because it has completed one (or
> some). That was a worry with the rbtree solution. The linked list may
> have had sucky O(N) insert, but extraction was a nice O(1). So far I
> haven't been able to notice any regression in this area, regardless.

There's a skip list variant which offers O(1) extraction from the head
of the list, and probabilistic O(log n) insertion, fwiw.

-- Jamie