85 lines
2.3 KiB
ReStructuredText
85 lines
2.3 KiB
ReStructuredText
==================
|
|
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 sysfs 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.
|