** No longer affects: linux (Ubuntu) ** No longer affects: linux (Ubuntu Xenial)
-- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/1724614 Title: [KVM] Lower the default for halt_poll_ns to 200000 ns Status in linux source package in Zesty: Won't Fix Status in linux source package in Groovy: New Bug description: [Environment] Distributor ID: Ubuntu Description: Ubuntu 16.04.3 LTS Release: 16.04 Codename: xenial Linux porygon 4.4.0-112-generic #135-Ubuntu SMP Fri Jan 19 11:48:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux [Description] We've identified a constant high (~90%) system time load at the host level when a VCPU in a KVM guest remains or switches/resumes in/from halt/idle state in a constant frequency, usually for a slightly smaller time than the default polling period. The halt polling mechanism has the intention to reduce latency in the cases on which the guest is quickly resumed saving a call to the scheduler. We've performed some testing by adjusting the /sys/module/kvm/parameters/halt_poll_ns value which defines the max time that should be spend polling before calling the scheduler to allow it to run other tasks (which defaults to 400000 ns in Ubuntu). With the default value the tests shows that the load remains nearly on 90% on a VCPU that has a single task in the run queue. We've also tested altering the halt_poll_ns value to 200000 ns and the results seems to drop the system time usage from 90% to ~25%. root@porygon:/home/ubuntu# echo 200000 > /sys/module/kvm/parameters/halt_poll_ns root@porygon:/home/ubuntu# mpstat 1 -P 6 5 Linux 4.4.0-112-generic (porygon) 01/24/2018 _x86_64_ (64 CPU) 02:06:08 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle 02:06:09 PM 6 0.00 0.00 4.85 0.00 0.00 0.00 0.00 16.50 0.00 78.64 [...] Average: 6 0.00 0.00 4.26 0.00 0.00 0.00 0.00 17.83 0.00 77.91 root@porygon:/home/ubuntu# echo 400000 > /sys/module/kvm/parameters/halt_poll_ns root@porygon:/home/ubuntu# mpstat 1 -P 6 5 Linux 4.4.0-112-generic (porygon) 01/24/2018 _x86_64_ (64 CPU) 02:06:20 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle 02:06:21 PM 6 0.00 0.00 87.13 0.00 0.00 0.00 0.00 11.88 0.00 0.99 [...] Average: 6 0.00 0.00 89.59 0.00 0.00 0.00 0.00 8.45 0.00 1.96 [Reproducer] 1) Configure a KVM guest with a single pinned VCPU. 2) Run the following program (http://pastebin.ubuntu.com/25731919/) at the KVM guest. $ gcc test.c -lpthread -o test && ./test 250 0 3) Run mpstat at the host on the pinned CPU and compare the stats $ sudo mpstat 1 -P 6 5 [Fix] Change the halt polling max time to half of the current value. In some fio benchmarks, halt_poll_ns=400000 caused CPU utilization to increase heavily even in cases where the performance improvement was small. In particular, bandwidth divided by CPU usage was as much as 60% lower. To some extent this is the expected effect of the patch, and the additional CPU utilization is only visible when running the benchmarks. However, halving the threshold also halves the extra CPU utilization (from +30-130% to +20-70%) and has no negative effect on performance. Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> * https://github.com/torvalds/linux/commit/b401ee0b85a53e89739ff68a5b1a0667d664afc9 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/zesty/+source/linux/+bug/1724614/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
