Re: [PATCH v2.5 3/6] scsi_debug: add multiple queue support

[Bart Van Assche <bart.vanassche@sandisk.com>]

On 05/02/2016 01:35 AM, Hannes Reinecke wrote:
> On 05/01/2016 04:44 AM, Douglas Gilbert wrote:
>> Add submit_queue parameter (minimum and default: 1; maximum:
>> nr_cpu_ids) that controls how many queues are built, each with
>> their own lock and in_use bit vector. Add statistics parameter
>> which is default on.
>>
>> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
>> ---
>>   drivers/scsi/scsi_debug.c | 680 +++++++++++++++++++++++++++++-----------------
>>   1 file changed, 426 insertions(+), 254 deletions(-)
>>
> Two general questions for this:
>
> - Why do you get rid of the embedded command payload?
>    Where's the benefit of allocating the commands yourself?
> - Wouldn't it be better to move to a per-cpu structure per queue?
>    Each queue will be tacked to a CPU anyway, so you could be using
>    per-cpu structures. Otherwise you'll run into synchronization
>    issues, and any performance gain you might get from scsi-mq is
>    lost as you to synchronize on the lower level.

With submit_queues == nr_cpu_ids the block layer will ensure that the 
each submit queue is always run on the same CPU core. This means that 
there is a high chance that sdebug_queue.qc_lock will be present in the 
L1 cache and hence that the locking overhead will be minimal.

Bart.