Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

Waiman Long
2020-02-04T17:39:30Z

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@redhat.com> 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