Re: [GIT PULL] adaptive spinning mutexes

[Nick Piggin]

On Thu, Jan 15, 2009 at 10:16:53AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 15 Jan 2009, Ingo Molnar wrote:
> > 
> > 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.
> 
> Umm. Except if you wrote the code nicely and used spinlocks, you wouldn't 
> hold the lock over all those unnecessary and complex operations.
> 
> IOW, if you do pre-allocation instead of holding a lock over the 
> allocation, you win. So yes, spin-mutexes makes it easier to write the 
> code, but it also makes it easier to just plain be lazy.

Yeah, I agree often it is harder to get the locking right but you end up
with a better result. With mutexes, on the off chance you do have t oblock
while holding the lock, performance and latency of other threads will tank.
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