linux-kernel - Re: [PATCH 3/3] blk-mq: Fix formula to calculate fair share of tags

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Message-ID: <54077D61.3030707@kernel.dk>
Date:	Wed, 03 Sep 2014 14:43:13 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Alexander Gordeev <agordeev@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] blk-mq: Fix formula to calculate fair share of tags

On 09/03/2014 02:33 PM, Alexander Gordeev wrote:
> Fair share of tags is number of tags diveded on number
> of users. Not sure why it is different.
> 
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> ---
>  block/blk-mq-tag.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d1eb579..1b9c949 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -127,7 +127,7 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
>  	/*
>  	 * Allow at least some tags
>  	 */
> -	depth = max((bt->depth + users - 1) / users, 4U);
> +	depth = max(bt->depth / users, 4U);
>  	return atomic_read(&hctx->nr_active) < depth;

It's normal rounding, you'll round down otherwise. Say you have a tag
depth of 31 (SATA), and 4 active users. Your change would make that 7
tags per user, leaving 3 idle. If you round up, you end up with 8 tags
instead. That will potentially just leave some sleeping for a new tag,
but at least you'll exhaust the space.

-- 
Jens Axboe

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