Re: [PATCH] nvme: don't memset() the normal read/write command

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 10/12/2021 11:13 AM, Jens Axboe wrote:
> External email: Use caution opening links or attachments
> 
> 
> This memset in the fast path costs a lot of cycles on my setup. Here's a
> top-of-profile of doing ~6.7M IOPS:
> 
> +    5.90%  io_uring  [nvme]            [k] nvme_queue_rq
> +    5.32%  io_uring  [nvme_core]       [k] nvme_setup_cmd
> +    5.17%  io_uring  [kernel.vmlinux]  [k] io_submit_sqes
> +    4.97%  io_uring  [kernel.vmlinux]  [k] blkdev_direct_IO
> 
> and a perf diff with this patch:
> 
>       0.92%     +4.40%  [nvme_core]       [k] nvme_setup_cmd
> 
> reducing it from 5.3% to only 0.9%. This takes it from the 2nd most
> cycle consumer to something that's mostly irrelevant.
> 
> Retain the full clear for the other commands to avoid doing any audits
> there, and just clear the fields in the rw command manually that we
> don't already fill.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
>

Looks good to me.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>