RE: [Xen-devel] VT-d scalability issue

["Han, Weidong" <weidong.han@xxxxxxxxx>]

Espen Skoglund wrote:
> Not regarding the other questions/objections in this thread for a
> moment --- what kind of performance improvments are we talking of here
> if the vcpus are pinned?  Is it close to 1 VM or is there still some
> performance degradation due to IOTLB pressure?

Definitely performance will degrade due to IOTLB pressure when there are
many VMs which exhausts IOTLB.

Randy (Weidong)

> 
> [And talking of IOTLB pressure, why can't Intel document the IOTLB
> sizes in the chipset docs?  Or even better, why can't these values be
> queried from the chipset?]
> 
>       eSk
> 
> 
> [Edwin Zhai]
>> Keir,
>> I have found a VT-d scalability issue and want to some feed backs.
> 
>> When I assign a pass-through NIC to a linux VM and increase the num
>> of VMs, the iperf throughput for each VM drops greatly. Say, start 8
>> VM running on a machine with 8 physical cpus, start 8 iperf client
>> to connect each of them, the final result is only 60% of 1 VM.
> 
>> Further investigation shows vcpu migration cause "cold" cache for
>> pass-through domain.  following code in vmx_do_resume try to
>> invalidate orig processor's cache when 14 migration if this domain
>> has pass-through device and no support for wbinvd vmexit.
> 
>> 16 if ( has_arch_pdevs(v->domain) && !cpu_has_wbinvd_exiting ) {
>>     int cpu = v->arch.hvm_vmx.active_cpu;
>>     if ( cpu != -1 )
>>         on_selected_cpus(cpumask_of_cpu(cpu), wbinvd_ipi, NULL, 1,
> 
>> }
> 
>> So we want to pin vcpu to free processor for domains with
>> pass-through device in creation process, just like what we did for
>> NUMA system.
> 
>> What do you think of it? Or have other ideas?
> 
>> Thanks,
> 
> 
>> --
>> best rgds,
>> edwin
> 
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> 
> 
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