Re: [GIT PULL] adaptive spinning mutexes

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > Has anyone found a non-synthetic benchmark where this makes a 
> > significant difference?  Aside from btrfs, I mean.
> 
> Yea, if you have some particular filesystem (or other subsystem) that 
> uses a global mutex, you'll obviously see way more contention. Btrfs may 
> not be _unique_ in this regard, but it's definitely doing something that 
> isn't good.
> 
> Btw, it's doing something that ext3 also used to do iirc, until we fixed 
> it to use spinlocks instead (the block group lock in particular).
> 
> Yeah - just double-checked. Commit c12b9866ea52 in the historical Linux 
> archive, from 2003. Which made block allocation protected by a per-group 
> spinlock, rather than lock_super().

btw., i think spin-mutexes have a design advantage here: in a lot of code 
areas it's quite difficult to use spinlocks - cannot allocate memory, 
cannot call any code that can sporadically block (but does not _normally_ 
block), etc.

With mutexes those atomicity constraints go away - and the performance 
profile should now be quite close to that of spinlocks as well.

	Ingo