Re: [PATCH V2 3/3] blk-mq: dequeue request one by one from sw queue iff hctx is busy

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On 6/29/18 2:12 AM, Ming Lei wrote:
> It won't be efficient to dequeue request one by one from sw queue,
> but we have to do that when queue is busy for better merge performance.
> 
> This patch takes EWMA to figure out if queue is busy, then only dequeue
> request one by one from sw queue when queue is busy.
> 
> Kashyap verified that this patch basically brings back rand IO perf
> on megasas_raid in case of none io scheduler. Meantime I tried this
> patch on HDD, and not see obvious performance loss on sequential IO
> test too.

Outside of the comments of others, please also export ->busy from
the blk-mq debugfs code.

> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index e3147eb74222..a5113e22d720 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -34,6 +34,7 @@ struct blk_mq_hw_ctx {
>  
>  	struct sbitmap		ctx_map;
>  
> +	unsigned int		busy;
>  	struct blk_mq_ctx	*dispatch_from;
>  
>  	struct blk_mq_ctx	**ctxs;

This adds another hole. Consider swapping it a bit, ala:

	struct blk_mq_ctx       *dispatch_from;
	unsigned int            busy;

	unsigned int            nr_ctx;
	struct blk_mq_ctx       **ctxs;

to eliminate a hole, instead of adding one more.

-- 
Jens Axboe




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