Re: [PATCH v5 6/9] KVM: MMU: introduce pte_prefetch_topup_memory_cache()

[Avi Kivity <avi@redhat.com>]

On 07/12/2010 06:05 AM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>    
>> On 07/06/2010 01:49 PM, Xiao Guangrong wrote:
>>      
>>> Introduce this function to topup prefetch cache
>>>
>>>
>>>
>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>> index 3dcd55d..cda4587 100644
>>> --- a/arch/x86/kvm/mmu.c
>>> +++ b/arch/x86/kvm/mmu.c
>>> @@ -89,6 +89,8 @@ module_param(oos_shadow, bool, 0644);
>>>        }
>>>    #endif
>>>
>>> +#define PTE_PREFETCH_NUM        16
>>>
>>>        
>> Let's make it 8 to start with...  It's frightening enough.
>>
>> (8 = one cache line in both guest and host)
>>      
> Umm, before post this patchset, i have done the draft performance test for
> different prefetch distance, and it shows 16 is the best distance that we can
> get highest performance.
>    

What's the different between 8 and 16?

I'm worried that there are workloads that don't benefit from prefetch, 
and we may regress there.  So I'd like to limit it, at least at first.

btw, what about dirty logging? will prefetch cause pages to be marked dirty?

We may need to instantiate prefetched pages with spte.d=0 and examine it 
when tearing down the spte.

>>> +static int pte_prefetch_topup_memory_cache(struct kvm_vcpu *vcpu)
>>> +{
>>> +    return __mmu_topup_memory_cache(&vcpu->arch.mmu_rmap_desc_cache,
>>> +                    rmap_desc_cache, PTE_PREFETCH_NUM,
>>> +                    PTE_PREFETCH_NUM, GFP_ATOMIC);
>>> +}
>>> +
>>>
>>>        
>> Just make the ordinary topup sufficient for prefetch.  If we allocate
>> too much, we don't lose anything, the memory remains for the next time
>> around.
>>
>>      
> Umm, but at the worst case, we should allocate 40 items for rmap, it's heavy
> for GFP_ATOMIC allocation and holding mmu_lock.
>
>    

Why use GFP_ATOMIC at all?  Make mmu_topup_memory_caches() always assume 
we'll be prefetching.

Why 40?  I think all we need is PTE_PREFETCH_NUM rmap entries.

-- 
error compiling committee.c: too many arguments to function