128 lines
6 KiB
Plaintext
Executable file
128 lines
6 KiB
Plaintext
Executable file
The KVM halt polling system
|
|
===========================
|
|
|
|
The KVM halt polling system provides a feature within KVM whereby the latency
|
|
of a guest can, under some circumstances, be reduced by polling in the host
|
|
for some time period after the guest has elected to no longer run by cedeing.
|
|
That is, when a guest vcpu has ceded, or in the case of powerpc when all of the
|
|
vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
|
|
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
|
|
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
|
|
savings of not invoking the scheduler are distinguishable.
|
|
|
|
The generic halt polling code is implemented in:
|
|
|
|
virt/kvm/kvm_main.c: kvm_vcpu_block()
|
|
|
|
The powerpc kvm-hv specific case is implemented in:
|
|
|
|
arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked()
|
|
|
|
Halt Polling Interval
|
|
=====================
|
|
|
|
The maximum time for which to poll before invoking the scheduler, referred to
|
|
as the halt polling interval, is increased and decreased based on the perceived
|
|
effectiveness of the polling in an attempt to limit pointless polling.
|
|
This value is stored in either the vcpu struct:
|
|
|
|
kvm_vcpu->halt_poll_ns
|
|
|
|
or in the case of powerpc kvm-hv, in the vcore struct:
|
|
|
|
kvmppc_vcore->halt_poll_ns
|
|
|
|
Thus this is a per vcpu (or vcore) value.
|
|
|
|
During polling if a wakeup source is received within the halt polling interval,
|
|
the interval is left unchanged. In the event that a wakeup source isn't
|
|
received during the polling interval (and thus schedule is invoked) there are
|
|
two options, either the polling interval and total block time[0] were less than
|
|
the global max polling interval (see module params below), or the total block
|
|
time was greater than the global max polling interval.
|
|
|
|
In the event that both the polling interval and total block time were less than
|
|
the global max polling interval then the polling interval can be increased in
|
|
the hope that next time during the longer polling interval the wake up source
|
|
will be received while the host is polling and the latency benefits will be
|
|
received. The polling interval is grown in the function grow_halt_poll_ns() and
|
|
is multiplied by the module parameter halt_poll_ns_grow.
|
|
|
|
In the event that the total block time was greater than the global max polling
|
|
interval then the host will never poll for long enough (limited by the global
|
|
max) to wakeup during the polling interval so it may as well be shrunk in order
|
|
to avoid pointless polling. The polling interval is shrunk in the function
|
|
shrink_halt_poll_ns() and is divided by the module parameter
|
|
halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0.
|
|
|
|
It is worth noting that this adjustment process attempts to hone in on some
|
|
steady state polling interval but will only really do a good job for wakeups
|
|
which come at an approximately constant rate, otherwise there will be constant
|
|
adjustment of the polling interval.
|
|
|
|
[0] total block time: the time between when the halt polling function is
|
|
invoked and a wakeup source received (irrespective of
|
|
whether the scheduler is invoked within that function).
|
|
|
|
Module Parameters
|
|
=================
|
|
|
|
The kvm module has 3 tuneable module parameters to adjust the global max
|
|
polling interval as well as the rate at which the polling interval is grown and
|
|
shrunk. These variables are defined in include/linux/kvm_host.h and as module
|
|
parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
|
|
powerpc kvm-hv case.
|
|
|
|
Module Parameter | Description | Default Value
|
|
--------------------------------------------------------------------------------
|
|
halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
|
|
| which defines the ceiling value |
|
|
| of the polling interval for | (per arch value)
|
|
| each vcpu. |
|
|
--------------------------------------------------------------------------------
|
|
halt_poll_ns_grow | The value by which the halt | 2
|
|
| polling interval is multiplied |
|
|
| in the grow_halt_poll_ns() |
|
|
| function. |
|
|
--------------------------------------------------------------------------------
|
|
halt_poll_ns_shrink | The value by which the halt | 0
|
|
| polling interval is divided in |
|
|
| the shrink_halt_poll_ns() |
|
|
| function. |
|
|
--------------------------------------------------------------------------------
|
|
|
|
These module parameters can be set from the debugfs files in:
|
|
|
|
/sys/module/kvm/parameters/
|
|
|
|
Note: that these module parameters are system wide values and are not able to
|
|
be tuned on a per vm basis.
|
|
|
|
Further Notes
|
|
=============
|
|
|
|
- Care should be taken when setting the halt_poll_ns module 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. This is because even if a guest has
|
|
wakeups during which very little work is done and which are quite far apart, if
|
|
the period is shorter than the 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 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.
|