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Message-ID: <20110620213818.GI4810@quack.suse.cz>
Date: Mon, 20 Jun 2011 23:38:18 +0200
From: Jan Kara <jack@...e.cz>
To: Wu Fengguang <fengguang.wu@...el.com>
Cc: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
Christoph Hellwig <hch@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] writeback: timestamp based bdi dirty_exceeded state
On Sun 19-06-11 23:01:15, Wu Fengguang wrote:
> When there are only one (or several) dirtiers, dirty_exceeded is always
> (or mostly) off. Converting to timestamp avoids this problem. It helps
> to use smaller write_chunk for smoother throttling.
Hmm, what do you mean by "dirty_exceeded is always (or mostly) off"? I
agree that dirty_exceeded handling is broken in current kernel when there
are more dirtiers because of tasks having different dirty limits. Is this
the problem you are speaking about?
If yes, I think I have a cleaner fix for that - setting dirty_exceeded
when bdi_dirty enters the range where per-task dirty limits can be and
resetting it only when we leave that range. I can refresh the fix and
send it to you...
Honza
> Before patch, the wait time in balance_dirty_pages() are ~200ms:
>
> [ 1093.397700] write_bandwidth: comm=swapper pages=1536 time=204ms
> [ 1093.594319] write_bandwidth: comm=swapper pages=1536 time=196ms
> [ 1093.796642] write_bandwidth: comm=swapper pages=1536 time=200ms
>
> After patch, ~25ms:
>
> [ 90.261339] write_bandwidth: comm=swapper pages=192 time=20ms
> [ 90.293168] write_bandwidth: comm=swapper pages=192 time=24ms
> [ 90.323853] write_bandwidth: comm=swapper pages=192 time=24ms
> [ 90.354510] write_bandwidth: comm=swapper pages=192 time=28ms
> [ 90.389890] write_bandwidth: comm=swapper pages=192 time=28ms
> [ 90.421787] write_bandwidth: comm=swapper pages=192 time=24ms
>
> include/linux/backing-dev.h | 2 +-
> mm/backing-dev.c | 2 --
> mm/page-writeback.c | 24 ++++++++++++------------
> 3 files changed, 13 insertions(+), 15 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2011-06-19 22:59:49.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-06-19 22:59:53.000000000 +0800
> @@ -483,6 +483,15 @@ unsigned long bdi_dirty_limit(struct bac
> return bdi_dirty;
> }
>
> +/*
> + * last time exceeded (limit - limit/DIRTY_BRAKE)
> + */
> +static bool dirty_exceeded_recently(struct backing_dev_info *bdi,
> + unsigned long time_window)
> +{
> + return jiffies - bdi->dirty_exceed_time <= time_window;
> +}
> +
> static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
> unsigned long elapsed,
> unsigned long written)
> @@ -621,7 +630,6 @@ static void balance_dirty_pages(struct a
> unsigned long bdi_thresh;
> unsigned long pages_written = 0;
> unsigned long pause = 1;
> - bool dirty_exceeded = false;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long start_time = jiffies;
>
> @@ -669,14 +677,9 @@ static void balance_dirty_pages(struct a
> * bdi or process from holding back light ones; The latter is
> * the last resort safeguard.
> */
> - dirty_exceeded = (bdi_dirty > bdi_thresh) ||
> - (nr_dirty > dirty_thresh);
> -
> - if (!dirty_exceeded)
> + if (bdi_dirty <= bdi_thresh && nr_dirty <= dirty_thresh)
> break;
> -
> - if (!bdi->dirty_exceeded)
> - bdi->dirty_exceeded = 1;
> + bdi->dirty_exceed_time = jiffies;
>
> bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty, bdi_dirty,
> start_time);
> @@ -719,9 +722,6 @@ static void balance_dirty_pages(struct a
> pause = HZ / 10;
> }
>
> - if (!dirty_exceeded && bdi->dirty_exceeded)
> - bdi->dirty_exceeded = 0;
> -
> if (writeback_in_progress(bdi))
> return;
>
> @@ -775,7 +775,7 @@ void balance_dirty_pages_ratelimited_nr(
> return;
>
> ratelimit = ratelimit_pages;
> - if (mapping->backing_dev_info->dirty_exceeded)
> + if (dirty_exceeded_recently(bdi, MAX_PAUSE))
> ratelimit = 8;
>
> /*
> --- linux-next.orig/include/linux/backing-dev.h 2011-06-19 22:54:58.000000000 +0800
> +++ linux-next/include/linux/backing-dev.h 2011-06-19 22:59:53.000000000 +0800
> @@ -79,7 +79,7 @@ struct backing_dev_info {
> unsigned long avg_write_bandwidth;
>
> struct prop_local_percpu completions;
> - int dirty_exceeded;
> + unsigned long dirty_exceed_time;
>
> unsigned int min_ratio;
> unsigned int max_ratio, max_prop_frac;
> --- linux-next.orig/mm/backing-dev.c 2011-06-19 22:59:49.000000000 +0800
> +++ linux-next/mm/backing-dev.c 2011-06-19 22:59:53.000000000 +0800
> @@ -670,8 +670,6 @@ int bdi_init(struct backing_dev_info *bd
> goto err;
> }
>
> - bdi->dirty_exceeded = 0;
> -
> bdi->bw_time_stamp = jiffies;
> bdi->written_stamp = 0;
>
>
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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