• Paolo Bonzini's avatar
    kvm: add halt_poll_ns module parameter · f7819512
    Paolo Bonzini authored
    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 (500000), 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.
    
    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_ns.  For the TSC
    deadline timer, thus, the effect is both a smaller average latency and
    a smaller variance.
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    f7819512
book3s.c 21.6 KB