Re: [PATCH RFC 4/4] xen, blkback: add support for multiple block rings

[Bob Liu <bob.liu@oracle.com>]

On 09/12/2014 07:45 AM, Arianna Avanzini wrote:
> On Fri, Aug 22, 2014 at 02:15:58PM +0100, David Vrabel wrote:
>> On 22/08/14 12:20, Arianna Avanzini wrote:
>>> This commit adds to xen-blkback the support to retrieve the block
>>> layer API being used and the number of available hardware queues,
>>> in case the block layer is using the multi-queue API. This commit
>>> also lets the driver advertise the number of available hardware
>>> queues to the frontend via XenStore, therefore allowing for actual
>>> multiple I/O rings to be used.
>>
>> Does it make sense for number of queues should be dependent on the
>> number of queues available in the underlying block device?  
> 
> Thank you for raising that point. It probably is not the best solution.
> 
> Bob Liu suggested to have the number of I/O rings depend on the number
> of vCPUs in the driver domain. Konrad Wilk suggested to compute the
> number of I/O rings according to the following formula to preserve the
> possibility to explicitly define the number of hardware queues to be
> exposed to the frontend:
> what_backend_exposes = some_module_parameter ? :
>                    min(nr_online_cpus(), nr_hardware_queues()).
> io_rings = min(nr_online_cpus(), what_backend_exposes);
> 
> (Please do correct me if I misunderstood your point)

Since xen-netfront/xen-netback driver have already implemented
multi-queue, I'd like we can use the same way as the net driver
negotiate of number of queues.

Thanks,
-Bob