linux-kernel - Re: [PATCH] Skip I/O merges when disabled

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Message-ID: <20080429124427.GP12774@kernel.dk>
Date:	Tue, 29 Apr 2008 14:44:27 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Alan D. Brunelle" <Alan.Brunelle@...com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Skip I/O merges when disabled

On Tue, Apr 29 2008, Alan D. Brunelle wrote:

> 
> The block I/O + elevator + I/O scheduler code spend a lot of time trying
> to merge I/Os -- rightfully so under "normal" circumstances. However,
> if one were to know that the incoming I/O stream was /very/ random in
> nature, the cycles are wasted.
> 
> This patch adds a per-request_queue tunable that (when set) disables
> merge attempts (beyond the simple one-hit cache check), thus freeing up
> a non-trivial amount of CPU cycles.

Thanks Alan, applied. Made one little change (since I had to hand apply
the patch anyway):


> +static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
> +{
> +	int nm = test_bit(QUEUE_FLAG_NOMERGES, &q->queue_flags);
> +	return queue_var_show(nm, page);
> +}

static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
{
	return queue_var_show(blk_queue_nomerges(q), page);
}

-- 
Jens Axboe

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