On Wed, Mar 09, 2005 at 12:21:01PM +0100, Andi Kleen wrote: > Arjan van de Ven <[EMAIL PROTECTED]> writes: > > > On Wed, 2005-03-09 at 16:34 +0530, Suparna Bhattacharya wrote: > >> Any sense of how costly it is to use spin_lock_irq's vs spin_lock > >> (across different architectures) ? Isn't rwsem used very widely ? > > On P4s cli/sti is quite costly, let's says 100+ cycles. That is mostly > because it synchronizes the CPU partly. The Intel tables say 26/36 cycles > latency, but in practice it seems to take longer because of the > synchronization. > > I would assume this is the worst case, everywhere else it should > be cheaper (except perhaps in some virtualized environments) > On P-M and AMD K7/K8 it is only a few cycles difference. > > > > oh also rwsems aren't used all that much simply because they are quite > > more expensive than regular semaphores, so that you need a HUGE bias in > > reader/writer ratio to make it even worth it... > > I agree. I think in fact once Christopher L's lockless page fault fast path > goes in it would be a good idea to reevaluate if mmap_sem should > be really a rwsem and possibly change it back to a normal semaphore > that perhaps gets dropped only on a page cache miss.
OK - makes sense. Thanks ! Regards Suparna -- Suparna Bhattacharya ([EMAIL PROTECTED]) Linux Technology Center IBM Software Lab, India - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
