On Mon, 2007-05-14 at 14:07 +0200, Nick Piggin wrote: > On Fri, May 11, 2007 at 07:18:33PM +0200, Peter Zijlstra wrote: > > On Fri, 2007-05-11 at 18:52 +0200, Eric Dumazet wrote: > > > > > > But I personally find this new rw_mutex not scalable at all if you have > > > some > > > writers around. > > > > > > percpu_counter_sum is just a L1 cache eater, and O(NR_CPUS) > > > > Yeah, that is true; there are two occurences, the one in > > rw_mutex_read_unlock() is not strictly needed for correctness. > > > > Write locks are indeed quite expensive. But given the ratio of > > reader:writer locks on mmap_sem (I'm not all that familiar with other > > rwsem users) this trade-off seems workable. > > I guess the problem with that logic is assuming the mmap_sem read side > always needs to be scalable. Given the ratio of threaded:unthreaded > apps, maybe the trade-off swings away from favour?
Could be; I've been bashing my head against the wall trying to find a scalable write side solution. But so far only got a massive dent in my brain from the effort. Perhaps I can do a similar optimistic locking for my rcu-btree as I did for the radix tree. That way most of the trouble would be endowed upon the vmas instead of the mm itself. And then it would be up to user-space to ensure it has in the order of nr_cpu_ids arenas to work in. Also, as Hugh pointed out in an earlier thread; mmap_sem's write side also protects the page tables, so we'd need to fix that up too; assumedly the write side equivalent of the vma lock would then protect all underlying page tables.... /me drifting away, rambling incoherently,.. - 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/
