Re: Make yield_task_fair more efficient

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Feb 21 2008, Peter Zijlstra wrote:
> 
> On Thu, 2008-02-21 at 15:37 +0530, Balbir Singh wrote:
> 
> > You use the empty pointer (missing right child), so why do we need a list. May
> > be I am missing something.
> 
> A fully threaded tree also has back-pointer to traverse backwards
> through the ordered elements.
> 
> That said, overloading the right child pointer might not be the best
> thing for the linux kernel, as it will impact all the rb-tree lookups
> which are open-coded and often performance critical (this is the reason
> the colour isn't bit encoded in either of the child pointers either).
> 
> But if you only want a uni directional thread, I guess we can stick it
> in the unsigned long we use for the node colour.
> 
> Still, perhaps it's worth it to grow rb_node to 4 words and do the fully
> threaded thing as there are also a lot of rb_prev() users in the kernel.
> Who knows..
> 
> Anyway, I agree that improving rb_next() is worth looking into for the
> scheduler.

For the IO scheduler as well, it's used quite extensively! So speeding
up rb_next() would definitely help, as it's typically invoked for every
bio queued (attempting to back merge with the next request). CFQ and AS
additionally does an rb_next() and rb_prev() when trying to decide which
request to do next.

-- 
Jens Axboe