Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and ioq0

[jianchao.wang]

Hi sagi

Thanks for your kindly response.

On 03/01/2018 05:28 PM, Sagi Grimberg wrote:
> 
>> Note that we originally allocates irqs this way, and Keith changed
>> it a while ago for good reasons. So I'd really like to see good
>> reasons for moving away from this, and some heuristics to figure
>> out which way to use. E.g. if the device supports more irqs than
>> I/O queues your scheme might always be fine.
> 
> I still don't understand what this buys us in practice. Seems redundant
> to allocate another vector without any (even marginal) difference.
> 

When the adminq is free, ioq0 irq completion path has to invoke nvme_irq twice, one for itself,
one for adminq completion irq action.
We are trying to save every cpu cycle across the nvme host path, why we waste nvme_irq cycles here.
If we have enough vectors, we could allocate another irq vector for adminq to avoid this.

Sincerely
Jianchao