Re: VirtIO vs Emulation Netperf benchmark results

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On 11/10/12 15:42, Christoffer Dall wrote:
> On Thu, Oct 11, 2012 at 8:54 AM, Alexander Graf <agraf@xxxxxxx> wrote:
>>
>> On 11.10.2012, at 12:37, Peter Maydell wrote:
>>
>>> On 11 October 2012 11:27, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>> On 11/10/12 10:55, Alexander Graf wrote
>>>>> On 11.10.2012, at 11:46, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>>>> - Our memory bandwidth is reduced by the number of TLB entries we waste
>>>>>> by not using section mappings instead of 4kB pages. Running hackbench on
>>>>>> a guest shows quite a slowdown that should mostly go away if/when
>>>>>> userspace switches to huge pages as backing store. I expect virtio to
>>>>>> suffer from the same problem.
>>>>>
>>>>> That one should be in a completely different ballpark. I'd be very
>>>> surprised if you get more than 10% slowdowns in TLB miss intensive
>>>> workloads. Definitely not as low of a hanging fruit as we see here.
>>>
>>>> The A15 uses separate TLBs for stage-1 and stage-2 translation, which
>>>> means that each random memory access costs us two TLBs, effectively
>>>> reducing the efficiency of the translation hardware by 50%.
>>>>
>>>> Using 2MB sections as the backing store (stage-2) should definitely be a
>>>> quite an improvement.
>>>
>>> Having done a quick google and looked at the QEMU code, I think that
>>> hugepage support should be a purely kernel-side thing: if you add
>>> transparent hugepage support then QEMU will just DTRT without special
>>> config.
>>>
>>> (Alex: does that sound right?)
>>
>> Don't bother with transparent hugepage support for now. Just use -mem-path and a directly mounted hugetlbfs with a page pool. But yes, no changes needed in QEMU at all. All the infrastructure for that is there already.
>>
>> If you need code to look at on how this works, check out
>>
>>   http://lxr.free-electrons.com/source/arch/powerpc/kvm/e500_tlb.c?a=powerpc#L498
>>
>>
> I'm working on this already.

QEMU or kernel side?

	M.
-- 
Jazz is not dead. It just smells funny...


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