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

Peter Zijlstra
2020-01-21T13:50:51Z

On Tue, Jan 21, 2020 at 02:29:49PM +0100, 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.

Daniel, IIRC you just did a paper on constructing worst case latencies
from measuring pieces. Do you have data on average lock hold times?