On 2/3/20 10:28 AM, Peter Zijlstra wrote:

On Mon, Feb 03, 2020 at 09:59:12AM -0500, Waiman Long wrote:

quoted

On 2/3/20 8:45 AM, Peter Zijlstra wrote:

quoted

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?

Today, a 2-socket Rome server can have 128 cores and 256 threads. If we
scale up more, we could easily have more than 1000 threads in a system.
With that many logical cpus available, it is easy to envision some heavy
spinlock contention can happen fairly regularly. This patch can
alleviate the congestion and improve performance under that
circumstance. Of course, the specific locks that are contended will
depend on the workloads.

Not the point. If there isn't an issue today, we don't have anything to
fix.

Furthermore, we've always adressed specific issues by looking at the
locking granularity, first.

You are right in that. Unlike ticket spinlock where performance can drop
precipitately over a cliff when there is heavy contention, qspinlock
won't have this kind of performance drop. My suspicion is that slowdowns
caused by heavy spinlock contention in actual workloads are likely to be
more transient in nature and harder to pinpoint. These days, I seldom
get bug report that is related to heavy spinlock contention.

So again, what specific lock inspired all these patches?

Maybe Alex has some data to share.

Cheers,
Longman


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