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xen-devel
Re: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID
It begs the question whether it's worth complicating code for an
optimisation with no measurable benefit, doesn't it?
-- Keir
On 12/01/2011 12:41, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:
> Hi Tim,
> Flush by ASID provides more flexible control of tlb flushing. The most
> advantage is to allow hypervisor to flush tagged tlb selectively. Using this
> feature, HV is able to flush tlb entries associated with a guest VM directly
> instead of allocating a new asid . The whole tlb flush will also be reduced
> by reducing asid allocation.
>
> So far, we did not measure drastic performance improvement in testing with
> kernbench and X11perf. Actually, we found out that, reducing tlb flushes
> accompanying with vmrun does not improve performance very much.
> we sent out a patch to optimize hvm_flush_guest_tlbs last week, which reduces
> over 90% tlb flushes for vmrun, and we even cannot see signification speedup
> with it. Maybe, the latency of vmrun is too big so that the overhead of tlb
> flush is negligible?
>
> Thanks,
> Wei
>
>
> On Wednesday 12 January 2011 11:17:00 Tim Deegan wrote:
>> At 17:55 +0000 on 11 Jan (1294768552), Wei Wang2 wrote:
>>> Future AMD SVM supports a new feature called flush by ASID. The idea is
>>> to allow CPU to flush TLBs associated with the ASID assigned to guest VM.
>>> So hypervisor doesn't have to reassign a new ASID in order to flush
>>> guest's VCPU. Please review it.
>>
>> What advantage does the new system have? Intuitively it seems like it
>> might be a tiny bit fairer and a tiny bit faster (by explicitly flushing
>> instead of relying on LRO) but I'm not convinced that it will be visible
>> in macro-benchmarks. Have you measured it?
>>
>> Cheers,
>>
>> Tim.
>>
>>> Thanks,
>>> Wei
>>>
>>> Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
>>> Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
>>> --
>>> Advanced Micro Devices GmbH
>>> Sitz: Dornach, Gemeinde Aschheim,
>>> Landkreis München Registergericht München,
>>> HRB Nr. 43632
>>> WEEE-Reg-Nr: DE 12919551
>>> Geschäftsführer:
>>> Alberto Bozzo, Andrew Bowd
>>>
>>>
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>> http://lists.xensource.com/xen-devel
>
>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel
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