Re: Announce: Semaphore-Removal tree

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sat, 2008-04-26 at 05:30 -0400, Christoph Hellwig wrote:
> On Fri, Apr 25, 2008 at 02:22:31PM -0700, Daniel Walker wrote:
> > If you can make a case for converting some semaphores to spinlocks be my
> > guest .. If you have good reasoning I wouldn't stand in the way.. (Real
> > time converts all the spinlocks to mutexes anyway ..)
> 
> Right at hand I have the XFS inode hash lock was converted from a rw_semaphore
> to a rwlock_t becuase the context switch overhead was killing
> performance in various benchmarks. This is a very typical scenary for
> locks that are taken often and held for a rather short time.  Add to
> that fact that a spinlock is compltely optimized away for an UP kernel
> while a mutex is not and the amount of memory that any mutex takes
> compared to a spinlock you have a clear winner.

I'm guessing RCU would be a bit more work?

The problem with rwlock_t is that for it to be a spinning lock the hold
times should be short, for it to be a rwlock over a spinlock there
should be a significant amount of concurrency, these two things together
make for a cache-line bouncing fest.