Re: [PATCH] block: do not reverse request order when flushing plug list

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

On Mon, Mar 13, 2023 at 10:30:02AM +0100, Jan Kara wrote:
> Commit 26fed4ac4eab ("block: flush plug based on hardware and software
> queue order") changed flushing of plug list to submit requests one
> device at a time. However while doing that it also started using
> list_add_tail() instead of list_add() used previously thus effectively
> submitting requests in reverse order. Also when forming a rq_list with
> remaining requests (in case two or more devices are used), we
> effectively reverse the ordering of the plug list for each device we
> process. Submitting requests in reverse order has negative impact on
> performance for rotational disks (when BFQ is not in use). We observe
> 10-25% regression in random 4k write throughput, as well as ~20%
> regression in MariaDB OLTP benchmark on rotational storage on btrfs
> filesystem.
> 
> Fix the problem by preserving ordering of the plug list when inserting
> requests into the queuelist as well as by appending to requeue_list
> instead of prepending to it.

Also in case of !plug->multiple_queues && !plug->has_elevator, requests
are still sent to device in reverse order, do we need to cover that case?


Thanks,
Ming