Guest halt polling¶
The cpuidle_haltpoll driver, with the haltpoll governor, allows the guest vcpus to poll for a specified amount of time before halting.
This provides the following benefits to host side polling:
- The POLL flag is set while polling is performed, which allows a remote vCPU to avoid sending an IPI (and the associated cost of handling the IPI) when performing a wakeup.
- The VM-exit cost can be avoided.
The downside of guest side polling is that polling is performed even with other runnable tasks in the host.
The basic logic as follows: A global value, guest_halt_poll_ns, is configured by the user, indicating the maximum amount of time polling is allowed. This value is fixed.
Each vcpu has an adjustable guest_halt_poll_ns (“per-cpu guest_halt_poll_ns”), which is adjusted by the algorithm in response to events (explained below).
The haltpoll governor has 5 tunable module parameters:
Maximum amount of time, in nanoseconds, that polling is performed before halting.
Division factor used to shrink per-cpu guest_halt_poll_ns when wakeup event occurs after the global guest_halt_poll_ns.
Multiplication factor used to grow per-cpu guest_halt_poll_ns when event occurs after per-cpu guest_halt_poll_ns but before global guest_halt_poll_ns.
The per-cpu guest_halt_poll_ns eventually reaches zero in case of an idle system. This value sets the initial per-cpu guest_halt_poll_ns when growing. This can be increased from 10000, to avoid misses during the initial growth stage:
10k, 20k, 40k, … (example assumes guest_halt_poll_grow=2).
Bool parameter which allows shrinking. Set to N to avoid it (per-cpu guest_halt_poll_ns will remain high once achieves global guest_halt_poll_ns value).
The module parameters can be set from the debugfs files in:
- Care should be taken when setting the guest_halt_poll_ns parameter as a large value has the potential to drive the cpu usage to 100% on a machine which would be almost entirely idle otherwise.