On Fri, Feb 08, 2013 at 03:49:02PM +0100, Jens Axboe wrote: > On Fri, Feb 08 2013, Tejun Heo wrote: > > (cc'ing Andrew) > > > > On Wed, Dec 26, 2012 at 06:00:02PM -0800, Kent Overstreet wrote: > > > This implements a refcount with similar semantics to > > > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t > > > but dynamically switches to per cpu refcounting when the rate of > > > gets/puts becomes too high. > > > > > > It also implements two stage shutdown, as we need it to tear down the > > > percpu counts. Before dropping the initial refcount, you must call > > > percpu_ref_kill(); this puts the refcount in "shutting down mode" and > > > switches back to a single atomic refcount with the appropriate barriers > > > (synchronize_rcu()). > > > > > > It's also legal to call percpu_ref_kill() multiple times - it only > > > returns true once, so callers don't have to reimplement shutdown > > > synchronization. > > > > > > For the sake of simplicity/efficiency, the heuristic is pretty simple - > > > it just switches to percpu refcounting if there are more than x gets > > > in one second (completely arbitrarily, 4096). > > > > > > It'd be more correct to count the number of cache misses or something > > > else more profile driven, but doing so would require accessing the > > > shared ref twice per get - by just counting the number of gets(), we can > > > stick that counter in the high bits of the refcount and increment both > > > with a single atomic64_add(). But I expect this'll be good enough in > > > practice. > > > > > > Signed-off-by: Kent Overstreet <koverstreet@xxxxxxxxxx> > > > > What's the status of this series? The percpu-refcnt part is still > > going through review and the merge window is opening up pretty soon. > > Kent, Andrew? > > I'd feel a lot better deferring the whole aio/dio performance series for > one merge window. There's very little point in rushing it, and I don't > think it's been reviewed/tested enough yet. It could probably use more review, but it has been sitting in linux-next and the issues that showed up there are all fixed. You going to help review it? :) I'm not really set on it going in this merge cycle, but testing wise I do think it's in pretty good shape and I'm not sure where we're going to get more testing from before it goes in. And Andrew - apologies for not getting you the benchmarks you asked for, getting hardware for it has turned out to be more troublesome than I expected. Still don't know what's going on with that. (It turned out though that when I readded that second kiocb refcount that io_submit_one() owns, that was a real performance regression. So I've got a patch that refactors aio_rw_vect_retry() so it can safely be dropped, and I'm going to _try_ to refactor cancellation so that I can get rid of kiocb refcounting entirely. But I'm not really in a big hurry to pile more stuff into this patch series, I'd prefer to get what I've got in...) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
