Re: [Xen-devel] Xen virtual IOMMU high level design doc

[Lan Tianyu <tianyu.lan@xxxxxxxxx>]

On 2016年08月31日 20:02, Jan Beulich wrote:
>>>> On 31.08.16 at 10:39, <tianyu.lan@xxxxxxxxx> wrote:
>> > On 2016年08月25日 19:11, Jan Beulich wrote:
>>>>>> >>>>> On 17.08.16 at 14:05, <tianyu.lan@xxxxxxxxx> wrote:
>>>> >>> 1 Motivation for Xen vIOMMU
>>>> >>> ============================================================================
>>>> >>> ===
>>>> >>> 1.1 Enable more than 255 vcpu support
>>>> >>> HPC virtualization requires more than 255 vcpus support in a single VM
>>>> >>> to meet parallel computing requirement. More than 255 vcpus support
>>>> >>> requires interrupt remapping capability present on vIOMMU to deliver
>>>> >>> interrupt to #vcpu >255 Otherwise Linux guest fails to boot up with 
>>>> >>> >255
>>>> >>> vcpus if interrupt remapping is absent.
>>> >> 
>>> >> I continue to question this as a valid motivation at this point in
>>> >> time, for the reasons Andrew has been explaining.
>> > 
>> > If we want to support Linux guest with >255 vcpus, interrupt remapping
>> > is necessary.
> I don't understand why you keep repeating this, without adding
> _why_ you think there is a demand for such guests and _what_
> your plans are to eliminate Andrew's concerns.
> 

The motivation for such huge VM is for HPC(High-performance computing)
Cloud service which requires high performance parallel computing.
We just create single VM on one machine and expose more than 255 pcpus
to VM in order to make sure high performance parallel computing in VM.
One vcpu is pinged on pcpu.

For performance, we achieved high performance data(>95% native
data of stream, dgemm and sgemm benchmarks in VM) after some tuning and
optimizations. We presented these on Xen summit of this year.

For stability, Andrew found some issues of huge VM with watchdog
enabled and cause hypervisor reboot. We will reproduce and fix them.

-- 
Best regards
Tianyu Lan

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