Re: [PATCH RFC] kvm: x86: add halt_poll module parameter

[Rik van Riel <riel@redhat.com>]

On 02/05/2015 11:05 AM, Paolo Bonzini wrote:
> This patch introduces a new module parameter for the KVM module; when it
> is present, KVM attempts a bit of polling on every HLT before scheduling
> itself out via kvm_vcpu_block.
> 
> This parameter helps a lot for latency-bound workloads---in particular
> I tested it with O_DSYNC writes with a battery-backed disk in the host.
> In this case, writes are fast (because the data doesn't have to go all
> the way to the platters) but they cannot be merged by either the host or
> the guest.  KVM's performance here is usually around 30% of bare metal,
> or 50% if you use cache=directsync or cache=writethrough (these
> parameters avoid that the guest sends pointless flush requests, and
> at the same time they are not slow because of the battery-backed cache).
> The bad performance happens because on every halt the host CPU decides
> to halt itself too.  When the interrupt comes, the vCPU thread is then
> migrated to a new physical CPU, and in general the latency is horrible
> because the vCPU thread has to be scheduled back in.
> 
> With this patch performance reaches 60-65% of bare metal and, more
> important, 99% of what you get if you use idle=poll in the guest.  This
> means that the tunable gets rid of this particular bottleneck, and more
> work can be done to improve performance in the kernel or QEMU.
> 
> Of course there is some price to pay; every time an otherwise idle vCPUs
> is interrupted by an interrupt, it will poll unnecessarily and thus
> impose a little load on the host.  The above results were obtained with
> a mostly random value of the parameter (2000000), and the load was around
> 1.5-2.5% CPU usage on one of the host's core for each idle guest vCPU.
> 
> The patch also adds a new stat, /sys/kernel/debug/kvm/halt_successful_poll,
> that can be used to tune the parameter.  It counts how many HLT
> instructions received an interrupt during the polling period; each
> successful poll avoids that Linux schedules the VCPU thread out and back
> in, and may also avoid a likely trip to C1 and back for the physical CPU.

In the long run, this value should probably be auto-tuned.
However, it seems like a good idea to introduce this kind
of thing one step at a time.

> While the VM is idle, a Linux 4 VCPU VM halts around 10 times per second.
> Of these halts, almost all are failed polls.  During the benchmark,
> instead, basically all halts end within the polling period, except a more
> or less constant stream of 50 per second coming from vCPUs that are not
> running the benchmark.  The wasted time is thus very low.  Things may
> be slightly different for Windows VMs, which have a ~10 ms timer tick.
> 
> The effect is also visible on Marcelo's recently-introduced latency
> test for the TSC deadline timer.  Though of course a non-RT kernel has
> awful latency bounds, the latency of the timer is around 8000-10000 clock
> cycles compared to 20000-120000 without setting halt_poll.  For the TSC
> deadline timer, thus, the effect is both a smaller average latency and
> a smaller variance.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Acked-by: Rik van Riel <riel@redhat.com>