Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 The KVM halt polling system 2 =========================== 3 4 The KVM halt polling system provides a feature within KVM whereby the latency 5 of a guest can, under some circumstances, be reduced by polling in the host 6 for some time period after the guest has elected to no longer run by cedeing. 7 That is, when a guest vcpu has ceded, or in the case of powerpc when all of the 8 vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions 9 before giving up the cpu to the scheduler in order to let something else run. 10 11 Polling provides a latency advantage in cases where the guest can be run again 12 very quickly by at least saving us a trip through the scheduler, normally on 13 the order of a few micro-seconds, although performance benefits are workload 14 dependant. In the event that no wakeup source arrives during the polling 15 interval or some other task on the runqueue is runnable the scheduler is 16 invoked. Thus halt polling is especially useful on workloads with very short 17 wakeup periods where the time spent halt polling is minimised and the time 18 savings of not invoking the scheduler are distinguishable. 19 20 The generic halt polling code is implemented in: 21 22 virt/kvm/kvm_main.c: kvm_vcpu_block() 23 24 The powerpc kvm-hv specific case is implemented in: 25 26 arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked() 27 28 Halt Polling Interval 29 ===================== 30 31 The maximum time for which to poll before invoking the scheduler, referred to 32 as the halt polling interval, is increased and decreased based on the perceived 33 effectiveness of the polling in an attempt to limit pointless polling. 34 This value is stored in either the vcpu struct: 35 36 kvm_vcpu->halt_poll_ns 37 38 or in the case of powerpc kvm-hv, in the vcore struct: 39 40 kvmppc_vcore->halt_poll_ns 41 42 Thus this is a per vcpu (or vcore) value. 43 44 During polling if a wakeup source is received within the halt polling interval, 45 the interval is left unchanged. In the event that a wakeup source isn't 46 received during the polling interval (and thus schedule is invoked) there are 47 two options, either the polling interval and total block time[0] were less than 48 the global max polling interval (see module params below), or the total block 49 time was greater than the global max polling interval. 50 51 In the event that both the polling interval and total block time were less than 52 the global max polling interval then the polling interval can be increased in 53 the hope that next time during the longer polling interval the wake up source 54 will be received while the host is polling and the latency benefits will be 55 received. The polling interval is grown in the function grow_halt_poll_ns() and 56 is multiplied by the module parameter halt_poll_ns_grow. 57 58 In the event that the total block time was greater than the global max polling 59 interval then the host will never poll for long enough (limited by the global 60 max) to wakeup during the polling interval so it may as well be shrunk in order 61 to avoid pointless polling. The polling interval is shrunk in the function 62 shrink_halt_poll_ns() and is divided by the module parameter 63 halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0. 64 65 It is worth noting that this adjustment process attempts to hone in on some 66 steady state polling interval but will only really do a good job for wakeups 67 which come at an approximately constant rate, otherwise there will be constant 68 adjustment of the polling interval. 69 70 [0] total block time: the time between when the halt polling function is 71 invoked and a wakeup source received (irrespective of 72 whether the scheduler is invoked within that function). 73 74 Module Parameters 75 ================= 76 77 The kvm module has 3 tuneable module parameters to adjust the global max 78 polling interval as well as the rate at which the polling interval is grown and 79 shrunk. These variables are defined in include/linux/kvm_host.h and as module 80 parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the 81 powerpc kvm-hv case. 82 83 Module Parameter | Description | Default Value 84 -------------------------------------------------------------------------------- 85 halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT 86 | which defines the ceiling value | 87 | of the polling interval for | (per arch value) 88 | each vcpu. | 89 -------------------------------------------------------------------------------- 90 halt_poll_ns_grow | The value by which the halt | 2 91 | polling interval is multiplied | 92 | in the grow_halt_poll_ns() | 93 | function. | 94 -------------------------------------------------------------------------------- 95 halt_poll_ns_shrink | The value by which the halt | 0 96 | polling interval is divided in | 97 | the shrink_halt_poll_ns() | 98 | function. | 99 -------------------------------------------------------------------------------- 100 101 These module parameters can be set from the debugfs files in: 102 103 /sys/module/kvm/parameters/ 104 105 Note: that these module parameters are system wide values and are not able to 106 be tuned on a per vm basis. 107 108 Further Notes 109 ============= 110 111 - Care should be taken when setting the halt_poll_ns module parameter as a 112 large value has the potential to drive the cpu usage to 100% on a machine which 113 would be almost entirely idle otherwise. This is because even if a guest has 114 wakeups during which very little work is done and which are quite far apart, if 115 the period is shorter than the global max polling interval (halt_poll_ns) then 116 the host will always poll for the entire block time and thus cpu utilisation 117 will go to 100%. 118 119 - Halt polling essentially presents a trade off between power usage and latency 120 and the module parameters should be used to tune the affinity for this. Idle 121 cpu time is essentially converted to host kernel time with the aim of decreasing 122 latency when entering the guest. 123 124 - Halt polling will only be conducted by the host when no other tasks are 125 runnable on that cpu, otherwise the polling will cease immediately and 126 schedule will be invoked to allow that other task to run. Thus this doesn't 127 allow a guest to denial of service the cpu.