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Date: Fri, 1 Nov 2002 20:55:32 +0000
From: Jamie Lokier <lk@tantalophile.demon.co.uk>
To: Andrew Morton <akpm@digeo.com>
Cc: Jens Axboe <axboe@suse.de>, Linux Kernel <linux-kernel@vger.kernel.org>,
       William Lee Irwin III <wli@holomorphy.com>
Subject: Re: rbtree scores (was Re: [patch] deadline-ioscheduler rb-tree sort)
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References: <20021031134315.GC6549@suse.de> <20021101113401.GE8428@suse.de> <3DC2D72B.B4D5707E@digeo.com>
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Andrew Morton wrote:
> Jens Axboe wrote:
> > 
> > As expected, the stock version O(N) insertion scan really hurts. Even
> > with 128 requests per list, rbtree version is far superior. Once bigger
> > lists are used, there's just no comparison whatsoever.
> > 
> 
> Jens,  the tree just makes sense.

Just a few comments about data structures - not important.

Technically I think that a priority queue, i.e. a heap (partially
ordered tree) is sufficient for the request queue.  I don't know the
request queue code well enough to be sure, though.

Both heaps and trees are O(N log N), the difference being that an
rbtree does a bit more constant-time work to balance the tree while
maintaining a stricter ordering.

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...

Implementing the latter would likely be a lot of work for little gain
though.

-- Jamie
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