On 12/3/23 23:52, Christoph Hellwig wrote:
On Thu, Nov 30, 2023 at 11:31:27AM -0800, Bart Van Assche wrote:
The fair tag sharing algorithm reduces performance for UFS devices
significantly. This is because UFS devices have multiple logical units, a
limited queue depth (32 for UFS 3.1 devices), because it happens often that
multiple logical units are accessed and also because it takes time to
give tags back after activity on a request queue has stopped. This patch series
restores UFS device performance to that of the legacy block layer by disabling
fair tag sharing for UFS devices.
I feel like a broken record:
fair tag sharing exists for a reason. Opting out of it for a specific
driver does not make any sense. Either you can make a good argument
why you don't want it at all, or for specific configurations you
can clearly explain, or you make it work faster. A "treat my driver
special" flag is never acceptable.
Hi Christoph,
Feedback that helps improving a patch series is always welcome.
Here is how I see fair tag sharing:
* Users probably want to configure minimum and maximum bandwidth or IOPS
values instead of an equal number of tags per request queue. I assume
that the fair tag sharing algorithm assigns an equal number of tags to
each request queue since the run-time cost of the latter algorithm is
much lower than the run-time cost of the previous algorithms.
* Any algorithm that is more sophisticated than the current fair tag
sharing algorithm would have a higher runtime cost. We don't want to
increase the cost of command processing in the block layer core.
Hence my proposal to disable the fair tag sharing algorithm if it's not
needed. I don't see a good alternative to this approach.
Thanks,
Bart.
