Re: [PATCH 1/2] virtio-fs: limit number of request queues

[Jingbo Xu <jefflexu@linux.alibaba.com>]

On 5/28/24 5:05 PM, Peter-Jan Gootzen wrote:
> Hi Jingbo,
> 
> On Tue, 2024-05-28 at 16:40 +0800, Jingbo Xu wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> Hi Peter-Jan,
>>
>> Thanks for the amazing work! Glad to see it is upstreamed.  We are
>> also
>> planning distribute virtiofs on DPU.
> Great to hear that virtio-fs is getting more attention with this
> powerful approach :)
> 
>>
>> BTW I have some questions when reading the patch, appreciate if you
>> could give a hint.
>>
>>
>> On 5/1/24 11:38 PM, Peter-Jan Gootzen wrote:
>>> Virtio-fs devices might allocate significant resources to virtio
>>> queues
>>> such as CPU cores that busy poll on the queue. The device indicates
>>> how
>>> many request queues it can support and the driver should initialize
>>> the
>>> number of queues that they want to utilize.
>>
>> I'm okay with truncating the num_request_queues to nr_cpu_ids if the
>> number of the available queues exceeds nr_cpu_ids, as generally 1:1
>> mapping between cpus and queues is enough and we can not make use more
>> queues in this case.
>>
>> I just don't understand the above commit message as how this relates
>> to
>> the resource footprint at the device side.  When the the number of the
>> queues actually used by the driver (e.g. nr_cpu_ids is 6) is less than
>> the amount (e.g. 8) that is advertised by the driver, will the device
>> side reclaim the resources allocated for the remaining unused queues?
>> The driver has no way notifying how many queues it actually utilizes.
> 
> The device resources allocated to queues is device implementation
> specific. So the cost of the driver creating more queues than it will
> ultimately use, is dependent on how the specific device handles these
> unsused queues.
> A smart device could at some point decide to spend less polling
> resources on a queue if it sees that it is unused (reclaiming the
> resources as you put it).
> 
> But in my opinion, the driver should not allocate more than it will use.
> The new scheme tries to map every core to a queue, so we know that we
> will not use >nr_cpu_ids queues.


Okay got it.  Many thanks for the reply.


-- 
Thanks,
Jingbo