Re: [PATCH 15/17] fs/ext4: use cached rbtrees

[Davidlohr Bueso]

On Wed, 19 Jul 2017, Jan Kara wrote:

> Hum, so I'm somewhat undecided whether this is worth the churn. For free
> blocks rb_tree we use rb_first() only in ext4_mb_generate_from_freelist()
> which gets called only when generating new buddy bitmap from on-disk bitmap
> and we traverse the whole tree after that - thus the extra cost of
> rb_first() is a) well hidden in the total cost of iteration, b) rather rare
> anyway.

Yes, during some of the testing I did for this patch, I could not find trees
with more than just a handful of nodes, ie: while removing file(s). 

> Similarly for the H-tree directory code, we call rb_first() in
> ext4_dx_readdir() only to start an iteration over the whole B-tree and in
> such case I don't think optimizing rb_first() makes a big difference
> (maintaining cached value is going to have about the same cost as we save
> by using it).

Your reaction is rather expected, actually. Lets just leave this patch out.

Thanks,
Davidlohr