guest-halt-polling.txt

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:

	1) 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.

	2) 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).

Module Parameters
=================

The haltpoll governor has 5 tunable module parameters:

1) guest_halt_poll_ns:
Maximum amount of time, in nanoseconds, that polling is
performed before halting.

Default: 200000

2) guest_halt_poll_shrink:
Division factor used to shrink per-cpu guest_halt_poll_ns when
wakeup event occurs after the global guest_halt_poll_ns.

Default: 2

3) guest_halt_poll_grow:
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.

Default: 2

4) guest_halt_poll_grow_start:
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).

Default: 50000

5) guest_halt_poll_allow_shrink:

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).

Default: Y

The module parameters can be set from the debugfs files in:

	/sys/module/haltpoll/parameters/

Further Notes
=============

- 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.