Re: [PATCH] blk-mq: only alloc hctx point as many as nr_hw_queues

Jens Axboe
2017-07-13T16:38:40Z

On 07/13/2017 10:30 AM, weiping zhang wrote:
> On Thu, Jul 13, 2017 at 10:14:31AM -0600, Jens Axboe wrote:
>> On 07/13/2017 10:10 AM, weiping zhang wrote:
>>> one hw queue only has one hctx, reduce it's number same as nr_hw_queues.
>>
>> What happens if a device registers with eg 4 hw queues, then later
>> calls blk_mq_update_nr_hw_queues()?
>>
>
> your means, 10 cpu, and device has 10 hw queues, but only register 4,
> and then update it to 10, no place store new hctx. Am i understanding
> right ? if yes, could you tell me why driver do that ? thanks.

It doesn't matter why the driver would do that, the fact is that this is
a feature we support and it's a part of the API. NBD uses it, for
instance. Your patch breaks it, for saving a bit of memory. So I will
not include it.

--
Jens Axboe