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

Waiman Long
2020-01-21T15:45:59Z

On 1/21/20 8:29 AM, Peter Zijlstra wrote:
> On Mon, Dec 30, 2019 at 02:40:41PM -0500, Alex Kogan wrote:
>
>> +/*
>> + * Controls the threshold for the number of intra-node lock hand-offs before
>> + * the NUMA-aware variant of spinlock is forced to be passed to a thread on
>> + * another NUMA node. By default, the chosen value provides reasonable
>> + * long-term fairness without sacrificing performance compared to a lock
>> + * that does not have any fairness guarantees. The default setting can
>> + * be changed with the "numa_spinlock_threshold" boot option.
>> + */
>> +int intra_node_handoff_threshold __ro_after_init = 1 << 16;
> There is a distinct lack of quantitative data to back up that
> 'reasonable' claim there.
>
> Where is the table of inter-node latencies observed for the various
> values tested, and on what criteria is this number deemed reasonable?
>
> To me, 64k lock hold times seems like a giant number, entirely outside
> of reasonable.

I actually had similar question before, but having the capability of
changing the default with boot time parameter alleviate some of my
concern. I will certainly like to see actual data on how different
values will affect the performance of the code.

Cheers,
Longman