virt/kvm/halt-polling.rst

1 .. SPDX-License-Identifier: GPL-2.0
7 The KVM halt polling system provides a feature within KVM whereby the latency
11 vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
14 Polling provides a latency advantage in cases where the guest can be run again
15 very quickly by at least saving us a trip through the scheduler, normally on
16 the order of a few micro-seconds, although performance benefits are workload
17 dependant. In the event that no wakeup source arrives during the polling
20 wakeup periods where the time spent halt polling is minimised and the time
27 The powerpc kvm-hv specific case is implemented in:
39 	kvm_vcpu->halt_poll_ns
41 or in the case of powerpc kvm-hv, in the vcore struct:
43 	kvmppc_vcore->halt_poll_ns
47 During polling if a wakeup source is received within the halt polling interval,
48 the interval is left unchanged. In the event that a wakeup source isn't
57 will be received while the host is polling and the latency benefits will be
64 max) to wakeup during the polling interval so it may as well be shrunk in order
76 		      invoked and a wakeup source received (irrespective of
86 powerpc kvm-hv case.
88 +-----------------------+---------------------------+-------------------------+
90 +-----------------------+---------------------------+-------------------------+
96 +-----------------------+---------------------------+-------------------------+
102 +-----------------------+---------------------------+-------------------------+
107 +-----------------------+---------------------------+-------------------------+
113 +-----------------------+---------------------------+-------------------------+
125 - Care should be taken when setting the halt_poll_ns module parameter as a large value
132 - Halt polling essentially presents a trade off between power usage and latency and
134   essentially converted to host kernel time with the aim of decreasing latency when
137 - Halt polling will only be conducted by the host when no other tasks are runnable on