Re: [PATCH 0/3] Add NUMA-awareness to qspinlock
From: Alex Kogan
Date: Fri Feb 01 2019 - 16:21:38 EST
> On Jan 31, 2019, at 4:56 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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.
Got it, thanks.
>> 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://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=7sFZrsdpLJxLRHIFWN_sE6zgKy20Ti8lOoepiEyipAo&s=5VRAQVjw0B1SCjvBLzzwxkHQ6TZ3FIl_tGDfvn3FXvo&e=.
> 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.
This can be most certainly engineered out, e.g., by caching the node ID on which a task is running.
We will look into that.
>> 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.
Please, note that only one set of results has lockstat enabled.
The rest of the results (will-it-scale included) do not have it.