On 2/4/20 12:27 PM, Peter Zijlstra wrote: > On Tue, Feb 04, 2020 at 11:54:02AM -0500, Alex Kogan wrote: >>> On Feb 3, 2020, at 10:47 AM, Waiman Long <longman@xxxxxxxxxx> wrote: >>> >>> On 2/3/20 10:28 AM, Peter Zijlstra wrote: >>>> On Mon, Feb 03, 2020 at 09:59:12AM -0500, Waiman Long wrote: >>>>> On 2/3/20 8:45 AM, Peter Zijlstra wrote: >>>>>> Presumably you have a workload where CNA is actually a win? That is, >>>>>> what inspired you to go down this road? Which actual kernel lock is so >>>>>> contended on NUMA machines that we need to do this? >> There are quite a few actually. files_struct.file_lock, file_lock_context.flc_lock >> and lockref.lock are some concrete examples that get very hot in will-it-scale >> benchmarks. > Right, that's all a variant of banging on the same resources across > nodes. I'm not sure there's anything fundamental we can fix there. > >> And then there are spinlocks in __futex_data.queues, >> which get hot when applications have contended (pthread) locks — >> LevelDB is an example. > A numa aware rework of futexes has been on the todo list for years :/ Now, we are going to get that for free with this patchset:-) > >> Our initial motivation was based on an observation that kernel qspinlock is not >> NUMA-aware. So what, you may ask. Much like people realized in the past that >> global spinning is bad for performance, and they switched from ticket lock to >> locks with local spinning (e.g., MCS), I think everyone would agree these days that >> bouncing a lock (and cache lines in general) across numa nodes is similarly bad. >> And as CNA demonstrates, we are easily leaving 2-3x speedups on the table by >> doing just that with the current qspinlock. > Actual benchmarks with performance numbers are required. It helps > motivate the patches as well as gives reviewers clues on how to > reproduce / inspect the claims made. > I think the cover-letter does have some benchmark results listed. Are you saying that some benchmark results should be put into individual patches themselves? Cheers, Longman
