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@xxxxxxxxxx> 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
