Re: [PATCH v2] block: Improve shared tag set performance

[Bart Van Assche <bvanassche@acm.org>]

On 1/18/23 14:46, Bart Van Assche wrote:
> On 1/18/23 14:14, Keith Busch wrote:
>> On Tue, Jan 03, 2023 at 11:53:37AM -0800, Bart Van Assche wrote:
>>> This algorithm hurts performance for UFS devices because UFS devices
>>> have multiple logical units. One of these logical units (WLUN) is used
>>> to submit control commands, e.g. START STOP UNIT. If any request is
>>> submitted to the WLUN, the queue depth is reduced from 31 to 15 or
>>> lower for data LUNs.
>>
>> Can you give the WLUN it's own tagset instead?
> 
> Hi Keith,
> 
> Does that mean creating an additional tagset for the WLUN and statically 
> allocating all tags from the WLUN tagset that are used by other LUNs? 
> That approach would have the following disadvantages:
> - The maximum queue depth for other LUNs would be reduced from 31 to 30.
>    This would have a small but noticeable negative performance impact.
> - The code removed by this patch negatively impacts performance of all
>    SCSI hosts with two or more data LUNs and also of all NVMe controllers
>    that export two or more namespaces if there are significant
>    differences in the number of I/O operations per second for different
>    LUNs/namespaces. This is why I think that this should be solved in the
>    block layer instead of modifying each block driver individually.

(replying to my own e-mail)

Hi Keith,

An additional concern about the approach of using multiple tag sets per 
SCSI host: this approach is not compatible with any code that iterates 
over tag sets. Examples of functions that iterate over tag sets are 
blk_mq_tagset_busy_iter(), scsi_host_busy(), 
scsi_host_complete_all_commands() and scsi_host_busy_iter(). 
scsi_host_busy() is used by e.g. scsi_host_queue_ready() and 
scsi_eh_wakeup(). So I think this approach would break the SCSI core in 
a subtle way.

Bart.