linux-kernel - Re: [PATCH 13/18] writeback: limit max dirty pause time

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Date:	Tue, 06 Sep 2011 16:52:06 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/18] writeback: limit max dirty pause time

On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:

> +static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
> +				   unsigned long bdi_dirty)
> +{
> +	unsigned long hi = ilog2(bdi->write_bandwidth);
> +	unsigned long lo = ilog2(bdi->dirty_ratelimit);
> +	unsigned long t;
> +
> +	/* target for ~10ms pause on 1-dd case */
> +	t = HZ / 50;

1k/50 usually ends up being 20 something

> +	/*
> +	 * Scale up pause time for concurrent dirtiers in order to reduce CPU
> +	 * overheads.
> +	 *
> +	 * (N * 20ms) on 2^N concurrent tasks.
> +	 */
> +	if (hi > lo)
> +		t += (hi - lo) * (20 * HZ) / 1024;
> +
> +	/*
> +	 * Limit pause time for small memory systems. If sleeping for too long
> +	 * time, a small pool of dirty/writeback pages may go empty and disk go
> +	 * idle.
> +	 *
> +	 * 1ms for every 1MB; may further consider bdi bandwidth.
> +	 */
> +	if (bdi_dirty)
> +		t = min(t, bdi_dirty >> (30 - PAGE_CACHE_SHIFT - ilog2(HZ)));

Yeah, I would add the bdi->avg_write_bandwidth term in there, 1g/s as an
avg bandwidth is just too wrong..


> +
> +	/*
> +	 * The pause time will be settled within range (max_pause/4, max_pause).
> +	 * Apply a minimal value of 4 to get a non-zero max_pause/4.
> +	 */
> +	return clamp_val(t, 4, MAX_PAUSE);

So you limit to 50ms min? That still seems fairly large. Is that because
your min sleep granularity might be something like 10ms since you're
using jiffies?

> +}


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