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Diffstat (limited to 'Documentation/virt/kvm/halt-polling.rst')
| -rw-r--r-- | Documentation/virt/kvm/halt-polling.rst | 25 |
1 files changed, 19 insertions, 6 deletions
diff --git a/Documentation/virt/kvm/halt-polling.rst b/Documentation/virt/kvm/halt-polling.rst index 4922e4a15f18..c82a04b709b4 100644 --- a/Documentation/virt/kvm/halt-polling.rst +++ b/Documentation/virt/kvm/halt-polling.rst @@ -14,7 +14,7 @@ before giving up the cpu to the scheduler in order to let something else run. Polling provides a latency advantage in cases where the guest can be run again very quickly by at least saving us a trip through the scheduler, normally on the order of a few micro-seconds, although performance benefits are workload -dependant. In the event that no wakeup source arrives during the polling +dependent. In the event that no wakeup source arrives during the polling interval or some other task on the runqueue is runnable the scheduler is invoked. Thus halt polling is especially useful on workloads with very short wakeup periods where the time spent halt polling is minimised and the time @@ -112,13 +112,26 @@ powerpc kvm-hv case. | | function. | | +-----------------------+---------------------------+-------------------------+ -These module parameters can be set from the debugfs files in: +These module parameters can be set from the sysfs files in: /sys/module/kvm/parameters/ -Note: that these module parameters are system wide values and are not able to +Note: these module parameters are system-wide values and are not able to be tuned on a per vm basis. +Any changes to these parameters will be picked up by new and existing vCPUs the +next time they halt, with the notable exception of VMs using KVM_CAP_HALT_POLL +(see next section). + +KVM_CAP_HALT_POLL +================= + +KVM_CAP_HALT_POLL is a VM capability that allows userspace to override halt_poll_ns +on a per-VM basis. VMs using KVM_CAP_HALT_POLL ignore halt_poll_ns completely (but +still obey halt_poll_ns_grow, halt_poll_ns_grow_start, and halt_poll_ns_shrink). + +See Documentation/virt/kvm/api.rst for more information on this capability. + Further Notes ============= @@ -129,12 +142,12 @@ Further Notes global max polling interval (halt_poll_ns) then the host will always poll for the entire block time and thus cpu utilisation will go to 100%. -- Halt polling essentially presents a trade off between power usage and latency and +- Halt polling essentially presents a trade-off between power usage and latency and the module parameters should be used to tune the affinity for this. Idle cpu time is essentially converted to host kernel time with the aim of decreasing latency when entering the guest. - Halt polling will only be conducted by the host when no other tasks are runnable on that cpu, otherwise the polling will cease immediately and schedule will be invoked to - allow that other task to run. Thus this doesn't allow a guest to denial of service the - cpu. + allow that other task to run. Thus this doesn't allow a guest to cause denial of service + of the cpu. |