[PATCH] nvmet: Limit num of queues to num of cpus on traget

[Max Gurtovoy]

On 12/6/2017 2:54 PM, Sagi Grimberg wrote:
> 
>> Sagi/Christoph,
>> In case the NVMEoF host asks num_io_queues << num_target_cpus there is
>> a waste of memory allocated in the target side for the sq's/cq's that
>> we never use. We might want to check the number of needed queues in
>> "set_features" cmd and allocate the minimum(wanted, target_capable).
>> This will block the option of creating more queues in a later stage
>> of the host lifecycle, but saves resources.
>>
>> thoughts ?
> 
> This is not the way to go at all. If you really care about saving a few
> Kbytes of nvmet sq/cq and see that its a real problem, you need to
> allocate them on demand instead of posing this limit to the host.
> 

Linux host side sets "count = min(*count, nr_io_queues)" in 
nvme_set_queue_count so there is no chance it will ask more queues in 
the future. We don't really care of this scenario (wanted_queues < 
target_capable_queues) but we do care about the case that this patch 
comes to fix.