Re: [PATCH] Speed up the cdrw packet writing driver

[Jens Axboe <axboe@suse.de>]

On Mon, Aug 23 2004, Peter Osterlund wrote:
> Jens Axboe <axboe@suse.de> writes:
> 
> > On Sat, Aug 14 2004, Peter Osterlund wrote:
> > > Hi!
> > > 
> > > This patch replaces the pd->bio_queue linked list with an rbtree.  The
> > > list can get very long (>200000 entries on a 1GB machine), so keeping
> > > it sorted with a naive algorithm is far too expensive.
> > 
> > It looks like you are assuming that bio->bi_sector is unique which isn't
> > necessarily true. In that respect, list -> rbtree conversion isn't
> > trivial (or, at least it requires extra code to handle this).
> 
> I don't think that is assumed anywhere.
> 
> The pkt_rbtree_find() function returns the first node with a sector
> number >= s, even if there are multiple bios with bi_sector == s. Note
> that the code branches to the left if s == tmp->bio->bi_sector.
> 
> The pkt_rbtree_insert() function is careful to insert a bio after any
> already existing bios with the same sector number. Note that it
> branches to the right if s == tmp->bio->bi_sector.
> 
> The tree rotations done internally in rbtree.c also can't mess things
> up, because tree rotations don't change the inorder traversal order of
> a tree.

You are right, the code looks fine indeed. The bigger problem is
probably that a faster data structure is needed at all, having hundreds
of thousands bio's pending for a packet writing device is not nice at
all.

-- 
Jens Axboe