Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock

Waiman Long
2020-01-25T01:59:49Z

On 1/24/20 7:57 PM, Paul E. McKenney wrote:
> On Fri, Jan 24, 2020 at 06:39:02PM -0500, Alex Kogan wrote:
>> Hi, Paul.
>>
>> Thanks for running those experiments!
>>
>>> On Jan 24, 2020, at 5:24 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
>>>
>>> On Tue, Jan 14, 2020 at 10:59:15PM -0500, Alex Kogan wrote:
>>>> Minor changes from v8 based on feedback from Longman:
>>>> -----------------------------------------------------
>>>>
>>>> - Add __init to cna_configure_spin_lock_slowpath().
>>>>
>>>> - Fix the comment for cna_scan_main_queue().
>>>>
>>>> - Change the type of intra_node_handoff_threshold to unsigned int.
>>>>
>>>>
>>>> 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).
>>>>
>>>> CNA is a NUMA-aware version of the MCS lock. Spinning threads are
>>>> organized in two queues, a main queue for threads running on the same
>>>> node as the current lock holder, and a secondary queue for threads
>>>> running on other nodes. Threads store the ID of the node on which
>>>> they are running in their queue nodes. After acquiring the MCS lock and
>>>> before acquiring the spinlock, the lock holder scans the main queue
>>>> looking for a thread running on the same node (pre-scan). 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. If such T
>>>> is not found, we make another scan of the main queue after acquiring
>>>> the spinlock when unlocking the MCS lock (post-scan), starting at the
>>>> node where pre-scan stopped. If both scans fail to find such T, the
>>>> MCS lock is passed to the first thread in the secondary queue. If the
>>>> secondary queue is empty, the MCS lock is passed to the next thread in the
>>>> main queue. To avoid starvation of threads in the secondary queue, those
>>>> threads are moved back to the head of the main queue after a certain
>>>> number of intra-node lock hand-offs.
>>>>
>>>> More 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=1KUGGZYTHnQ25fgRFppdNvpJfI0rOO_Usdu18RDu_14&s=F12nhHutwnPNt_TQ2ELER0DhtsHlEI9EiW1nDPhm5-Y&e= <https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=1KUGGZYTHnQ25fgRFppdNvpJfI0rOO_Usdu18RDu_14&s=F12nhHutwnPNt_TQ2ELER0DhtsHlEI9EiW1nDPhm5-Y&e=> .
>>>>
>>>> The series applies on top of v5.5.0-rc6, commit b3a987b026.
>>>> Performance numbers are available in previous revisions
>>>> of the series.
>>>>
>>>> Further comments are welcome and appreciated.
>>> I ran this on a large system with a version of locktorture that was
>>> modified to print out the maximum and minimum per-CPU lock-acquisition
>>> counts, and with CPU hotplug disabled. I also modified the LOCK01 and
>>> LOCK04 scenarios to use 220 hardware threads.
>>>
>>> Here is what the test ended up with at the end of a one-hour run:
>>>
>>> LOCK01 (exclusive):
>>> Writes: Total: 1241107333 Max/Min: 9206962/60902 ??? Fail: 0
>>>
>>> LOCK04 (rwlock):
>>> Writes: Total: 232991963 Max/Min: 2631574/74582 ??? Fail: 0
>>> Reads : Total: 216935386 Max/Min: 2735939/28665 ??? Fail: 0
>>>
>>> The "???" strings are printed because the ratio of maximum to minimum exceeds
>>> a factor of two.
>> Is this what you expect / have seen with the existing qspinlock?
>>
>>> I also ran 30-minute runs on my laptop, which has 12 hardware threads:
>>>
>>> LOCK01 (exclusive):
>>> Writes: Total: 3992072782 Max/Min: 259368782/97231961 ??? Fail: 0
>>>
>>> LOCK04 (rwlock):
>>> Writes: Total: 131063892 Max/Min: 13136206/5876157 ??? Fail: 0
>>> Reads : Total: 144876801 Max/Min: 19999535/4873442 ??? Fail: 0
>> I assume the system above is multi-socket, but your laptop is probably not?
>>
>> If that’s the case, CNA should not be enabled on your laptop (grep
>> kernel logs for "Enabling CNA spinlock” to be sure).
>>
>>> These also exceed the factor-of-two cutoff, but not as dramatically.
>>> The readers for the reader-writer lock fared worst, with a 4-to-1 ratio.
>>>
>>> These tests did run within guest OSes.
>> So I really wonder if CNA was enabled here, or whether this is what you get
>> with paravirt qspinlock.
>>
>>> Is that configuration out of
>>> scope for this locking algorithm? In addition (as might well also have
>>> been the case for the locktorture runs in your paper), these tests run
>>> a pair of stress-test tasks for each hardware thread.
>>>
>>> Is this expected behavior?
>> The results do appear skewed a bit too much, but it would be helpful to know
>> what qspinlock we are looking at, and how they compare to the existing qspinlock,
>> in case it is indeed CNA.
> You called it! I will play with QEMU's -numa argument to see if I can get
> CNA to run for me. Please accept my apologies for the false alarm.
>
> Thanx, Paul
>
CNA is not currently supported in a VM guest simply because the numa
information is not reliable. You will have to run it on baremetal to
test it. Sorry for that.

Regards,
Longman