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

Waiman Long
2020-01-24T14:43:08Z

On 1/24/20 2:52 AM, Peter Zijlstra wrote:
> On Thu, Jan 23, 2020 at 04:33:54PM -0500, Alex Kogan wrote:
>> Let me put this question to you. What do you think the number should be?
> I think it would be very good to keep the inter-node latency below 1ms.
It is hard to guarantee that given that lock hold times can vary quite a
lot depending on the workload. What we can control is just how many
later lock waiters can jump ahead before a given waiter.
> But to realize that we need data on the lock hold times. Specifically
> for the heavily contended locks that make CNA worth it in the first
> place.
>
> I don't see that data, so I don't see how we can argue about this let
> alone call something reasonable.
>
In essence, CNA lock is for improving throughput on NUMA machines at the
expense of increasing worst case latency. If low latency is important,
it should be disabled. If CONFIG_PREEMPT_RT is on,
CONFIG_NUMA_AWARE_SPINLOCKS should be off.

Cheers,
Longman