Messages in this thread | | | Date | Thu, 15 Jan 2009 19:08:44 +0100 | From | Ingo Molnar <> | Subject | Re: [GIT PULL] adaptive spinning mutexes |
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* 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
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