Re: [PATCH v3 3/4] block: drop shared-tag fairness throttling

[Hannes Reinecke <hare@xxxxxxx>]

On 6/9/26 14:18, Sumit Saxena wrote:
> From: Bart Van Assche <bvanassche@xxxxxxx>
>
> Original patch [1] by Bart Van Assche; this version is rebased onto the
> current tree.  In testing it improves IOPS by roughly 16-18% by removing
> the fair-sharing throttle on shared tag queues.
>
> This patch removes the following code and structure members:
> - The function hctx_may_queue().
> - blk_mq_hw_ctx.nr_active and request_queue.nr_active_requests_shared_tags
>    and also all the code that modifies these two member variables.
>
> [1]: 
> https://lore.kernel.org/linux-block/20240529213921.3166462-1-bvanassche@xxxxxxx/
>
> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
> Signed-off-by: Sumit Saxena <sumit.saxena@xxxxxxxxxxxx>
> ---
>   block/blk-core.c       |   2 -
>   block/blk-mq-debugfs.c |  22 ++++++++-
>   block/blk-mq-tag.c     |   4 --
>   block/blk-mq.c         |  17 +------
>   block/blk-mq.h         | 100 -----------------------------------------
>   include/linux/blk-mq.h |   6 ---
>   include/linux/blkdev.h |   2 -
>   7 files changed, 22 insertions(+), 131 deletions(-)
What tests did you perform?
I'm pretty sure you see an improvement when having just a few drives,
but what about having a lot of them (ie tens of drives)?
The whole point of this was to increase fairness between drives, so
of course removing it will make an individual drive going faster ...

Maybe it's an idea to move the fairness algorithm into an I/O scheduler;
that way we can keep the original behaviour yet get the performance
increase if people want it.

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@xxxxxxx                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich