On Fri, Feb 29, 2008 at 01:34:34PM -0800, Christoph Lameter wrote: > On Fri, 29 Feb 2008, Andrea Arcangeli wrote: > > > On Fri, Feb 29, 2008 at 01:03:16PM -0800, Christoph Lameter wrote: > > > That means we need both the anon_vma locks and the i_mmap_lock to become > > > semaphores. I think semaphores are better than mutexes. Rik and Lee saw > > > some performance improvements because list can be traversed in parallel > > > when the anon_vma lock is switched to be a rw lock. > > > > The improvement was with a rw spinlock IIRC, so I don't see how it's > > related to this. > > AFAICT The rw semaphore fastpath is similar in performance to a rw > spinlock.
read side is taken in the slow path. write side is taken in the fast path. pagefault is fast path, VM during swapping is slow path. > > Perhaps the rwlock spinlock can be changed to a rw semaphore without > > measurable overscheduling in the fast path. However theoretically > > Overscheduling? You mean overhead? The only possible overhead that a rw semaphore could ever generate vs a rw lock is overscheduling. > > speaking the rw_lock spinlock is more efficient than a rw semaphore in > > case of a little contention during the page fault fast path because > > the critical section is just a list_add so it'd be overkill to > > schedule while waiting. That's why currently it's a spinlock (or rw > > spinlock). > > On the other hand a semaphore puts the process to sleep and may actually > improve performance because there is less time spend in a busy loop. > Other processes may do something useful and we stay off the contended > cacheline reducing traffic on the interconnect. Yes, that's the positive side, the negative side is that you'll put the task in uninterruptible sleep and call schedule() and require a wakeup, because a list_add taking <1usec is running in the other cpu. No other downside. But that's the only reason it's a spinlock right now, infact there can't be any other reason. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
