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SubjectRe: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops

On 2017/11/13 18:53, Juergen Gross wrote:
> On 13/11/17 11:06, Quan Xu wrote:
>> From: Quan Xu <>
>> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called
>> in idle path which will poll for a while before we enter the real idle
>> state.
>> In virtualization, idle path includes several heavy operations
>> includes timer access(LAPIC timer or TSC deadline timer) which will
>> hurt performance especially for latency intensive workload like message
>> passing task. The cost is mainly from the vmexit which is a hardware
>> context switch between virtual machine and hypervisor. Our solution is
>> to poll for a while and do not enter real idle path if we can get the
>> schedule event during polling.
>> Poll may cause the CPU waste so we adopt a smart polling mechanism to
>> reduce the useless poll.
>> Signed-off-by: Yang Zhang <>
>> Signed-off-by: Quan Xu <>
>> Cc: Juergen Gross <>
>> Cc: Alok Kataria <>
>> Cc: Rusty Russell <>
>> Cc: Thomas Gleixner <>
>> Cc: Ingo Molnar <>
>> Cc: "H. Peter Anvin" <>
>> Cc:
>> Cc:
>> Cc:
>> Cc:
> Hmm, is the idle entry path really so critical to performance that a new
> pvops function is necessary?
Juergen, Here is the data we get when running benchmark netperf:
 1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0):
    29031.6 bit/s -- 76.1 %CPU

 2. w/ patch and disable kvm dynamic poll (halt_poll_ns=0):
    35787.7 bit/s -- 129.4 %CPU

 3. w/ kvm dynamic poll:
    35735.6 bit/s -- 200.0 %CPU

 4. w/patch and w/ kvm dynamic poll:
    42225.3 bit/s -- 198.7 %CPU

 5. idle=poll
    37081.7 bit/s -- 998.1 %CPU

 w/ this patch, we will improve performance by 23%.. even we could improve
 performance by 45.4%, if we use w/patch and w/ kvm dynamic poll. also the
 cost of CPU is much lower than 'idle=poll' case..

> Wouldn't a function pointer, maybe guarded
> by a static key, be enough? A further advantage would be that this would
> work on other architectures, too.

I assume this feature will be ported to other archs.. a new pvops makes code
clean and easy to maintain. also I tried to add it into existed pvops,
but it
doesn't match.

Alibaba Cloud
> Juergen

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