[dm-devel] btree vs. linear seach in device mapper

[Mikulas Patocka]

On Thu, 4 Apr 2013, Joe Thornber wrote:

> On Thu, Apr 04, 2013 at 10:27:02AM -0400, Mikulas Patocka wrote:
> > BTW. when I see that btree code in dm-table.c, I ask - why doesn't it use 
> > binary search?
> > 
> > We only append targets at the end when constructing the device, we never 
> > insert or remove them, so we don't need a tree. For these operations 
> > binary search would be as good as btree and it is simpler.
> 
> I originally expected dm tables to have many, many more entries than
> they do these days (I remember benchmarking it with 1 million
> entries).  I used a btree to try and be nicer to the cpu cache; the
> idea being that each btree node could fit into a cache line.  Plain
> binary search would have caused many more cache faults.
> 
> Given how dm is used these days I wouldn't mind a switch to a binary
> search.
> 
> - Joe

I see.

If the btree helps to save a few cachelines and doesn't hurt, we can leave 
it there. If it ever causes some code maintainability difficulties, we can 
switch to a binary search...

Mikulas