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SubjectRe: [PATCH 0/3] Add NUMA-awareness to qspinlock
On Wed, Jan 30, 2019 at 10:01:32PM -0500, Alex Kogan wrote:
> Lock throughput can be increased by handing a lock to a waiter on the
> same NUMA socket as the lock holder, provided care is taken to avoid
> starvation of waiters on other NUMA sockets. This patch introduces CNA
> (compact NUMA-aware lock) as the slow path for qspinlock.

Since you use NUMA, use the term node, not socket. The two are not
strictly related.

> CNA is a NUMA-aware version of the MCS spin-lock. Spinning threads are
> organized in two queues, a main queue for threads running on the same
> socket as the current lock holder, and a secondary queue for threads
> running on other sockets. Threads record the ID of the socket on which
> they are running in their queue nodes. At the unlock time, the lock
> holder scans the main queue looking for a thread running on the same
> socket. If found (call it thread T), all threads in the main queue
> between the current lock holder and T are moved to the end of the
> secondary queue, and the lock is passed to T. If such T is not found, the
> lock is passed to the first node in the secondary queue. Finally, if the
> secondary queue is empty, the lock is passed to the next thread in the
> main queue.
>
> Full details are available at https://arxiv.org/abs/1810.05600.

Full details really should also be in the Changelog. You can skip much
of the academic bla-bla, but the Changelog should be self contained.

> We have done some performance evaluation with the locktorture module
> as well as with several benchmarks from the will-it-scale repo.
> The following locktorture results are from an Oracle X5-4 server
> (four Intel Xeon E7-8895 v3 @ 2.60GHz sockets with 18 hyperthreaded
> cores each). Each number represents an average (over 5 runs) of the
> total number of ops (x10^7) reported at the end of each run. The stock
> kernel is v4.20.0-rc4+ compiled in the default configuration.
>
> #thr stock patched speedup (patched/stock)
> 1 2.710 2.715 1.002
> 2 3.108 3.001 0.966
> 4 4.194 3.919 0.934

So low contention is actually worse. Funnily low contention is the
majority of our locks and is _really_ important.

> 8 5.309 6.894 1.299
> 16 6.722 9.094 1.353
> 32 7.314 9.885 1.352
> 36 7.562 9.855 1.303
> 72 6.696 10.358 1.547
> 108 6.364 10.181 1.600
> 142 6.179 10.178 1.647
>
> When the kernel is compiled with lockstat enabled, CNA

I'll ignore that, lockstat/lockdep enabled runs are not what one would
call performance relevant.

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