Hi folks,

(I think Lihao is travelling at the moment, so he may be delayed in his
replies)

On Wed, Jan 22, 2020 at 12:24:58PM -0500, Waiman Long wrote:

On 1/22/20 6:45 AM, Lihao Liang wrote:

quoted

On Wed, Jan 22, 2020 at 10:28 AM Alex Kogan [off-list ref] wrote:

quoted

Summary
-------

Lock throughput can be increased by handing a lock to a waiter on the
same NUMA node as the lock holder, provided care is taken to avoid
starvation of waiters on other NUMA nodes. This patch introduces CNA
(compact NUMA-aware lock) as the slow path for qspinlock. It is
enabled through a configuration option (NUMA_AWARE_SPINLOCKS).

Thanks for your patches. The experimental results look promising!

I understand that the new CNA qspinlock uses randomization to achieve
long-term fairness, and provides the numa_spinlock_threshold parameter
for users to tune. As Linux runs extremely diverse workloads, it is not
clear how randomization affects its fairness, and how users with
different requirements are supposed to tune this parameter.

To this end, Will and I consider it beneficial to be able to answer the
following question:

With different values of numa_spinlock_threshold and
SHUFFLE_REDUCTION_PROB_ARG, how long do threads running on different
sockets have to wait to acquire the lock? This is particularly relevant
in high contention situations when new threads keep arriving on the same
socket as the lock holder.

In this email, I try to provide some formal analysis to address this
question. Let's assume the probability for the lock to stay on the
same socket is *at least* p, which corresponds to the probability for
the function probably(unsigned int num_bits) in the patch to return *false*,
where SHUFFLE_REDUCTION_PROB_ARG is passed as the value of num_bits to the
function.

That is not strictly true from my understanding of the code. The
probably() function does not come into play if a secondary queue is
present. Also calling cna_scan_main_queue() doesn't guarantee that a
waiter in the same node can be found. So the simple mathematical
analysis isn't that applicable in this case. One will have to do an
actual simulation to find out what the actual behavior will be.

It's certainly true that the analysis is based on the worst-case scenario,
but I think it's still worth considering. For example, the secondary queue
does not exist initially so it seems a bit odd that we only instantiate it
with < 1% probability.

That said, my real concern with any of this is that it makes formal
modelling and analysis of the qspinlock considerably more challenging. I
would /really/ like to see an update to the TLA+ model we have of the
current implementation [1] and preferably also the userspace version I
hacked together [2] so that we can continue to test and validate changes
to the code outside of the usual kernel stress-testing.

Will

[1] https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/
[2] https://mirrors.edge.kernel.org/pub/linux/kernel/people/will/spinbench/

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