From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v5 6/9] KVM: MMU: introduce pte_prefetch_topup_memory_cache() Date: Tue, 13 Jul 2010 09:16:52 +0800 Message-ID: <4C3BBE84.30708@cn.fujitsu.com> References: <4C330918.6040709@cn.fujitsu.com> <4C330A37.8080709@cn.fujitsu.com> <4C39C1AB.6000606@redhat.com> <4C3A8694.1000401@cn.fujitsu.com> <4C3B09FD.3060307@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM list To: Avi Kivity Return-path: In-Reply-To: <4C3B09FD.3060307@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Avi Kivity wrote: > 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. > OK > 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. > Yeah, all speculative path are broken dirty bit tracking, and i guess it's need more review, so i plan to do it in the separate patch, i'll post it after this patchset merged, could you allow it? >>>> +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. > Oh, i see your mean now, i'll increase rmap entries in mmu_topup_memory_caches()