Re: [PATCH v4 0/3] Support disabling fair tag sharing

[Ming Lei <ming.lei@redhat.com>]

Hi Bart,

On Mon, Oct 23, 2023 at 01:36:32PM -0700, Bart Van Assche wrote:
> Hi Jens,
> 
> Performance of UFS devices is reduced significantly by the fair tag sharing
> algorithm. This is because UFS devices have multiple logical units and a
> limited queue depth (32 for UFS 3.1 devices) and also because it takes time to
> give tags back after activity on a request queue has stopped. This patch series
> addresses this issue by introducing a flag that allows block drivers to
> disable fair sharing.
> 
> Please consider this patch series for the next merge window.

In previous post[1], you mentioned that the issue is caused by non-IO
queue of WLUN, but in this version, looks there isn't such story any more.

IMO, it isn't reasonable to account non-IO LUN for tag fairness, so
solution could be to not take non-IO queue into account for fair tag
sharing. But disabling fair tag sharing for this whole tagset could be
too over-kill.

And if you mean normal IO LUNs, can you share more details about the
performance drop? such as the test case, how many IO LUNs, and how to
observe performance drop, cause it isn't simple any more since multiple
LUN's perf has to be considered.


[1] https://lore.kernel.org/linux-block/20231018180056.2151711-1-bvanassche@acm.org/

Thanks,
Ming