linux-kernel - Re: [PATCH] KVM: x86: lower default for halt_poll

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Date:   Tue, 16 May 2017 18:58:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com
Subject: Re: [PATCH] KVM: x86: lower default for halt_poll_ns



On 18/04/2017 12:41, Paolo Bonzini wrote:
> In some fio benchmarks, halt_poll_ns=400000 caused CPU utilization to
> increase heavily even in cases where the performance improvement was
> small.  In particular, bandwidth divided by CPU usage was as much as
> 60% lower.
> 
> To some extent this is the expected effect of the patch, and the
> additional CPU utilization is only visible when running the
> benchmarks.  However, halving the threshold also halves the extra
> CPU utilization (from +30-130% to +20-70%) and has no negative
> effect on performance.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Ping?

Paolo

> ---
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 74ef58c8ff53..68d4a33da392 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -45,7 +45,7 @@
>  
>  #define KVM_PIO_PAGE_OFFSET 1
>  #define KVM_COALESCED_MMIO_PAGE_OFFSET 2
> -#define KVM_HALT_POLL_NS_DEFAULT 400000
> +#define KVM_HALT_POLL_NS_DEFAULT 200000
>  
>  #define KVM_IRQCHIP_NUM_PINS  KVM_IOAPIC_NUM_PINS
>  
>