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
