linux-ext4 - Re: 3.2 and 3.1 filesystem scalability measurements

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Date:	Tue, 31 Jan 2012 20:55:18 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Chinner <david@...morbit.com>,
	Andreas Dilger <adilger@...ger.ca>,
	"aziro.linux.adm" <aziro.linux.adm@...il.com>,
	Eric Whitney <eric.whitney@...com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: 3.2 and 3.1 filesystem scalability measurements

On Tue, Jan 31, 2012 at 11:53:53AM +0100, Jan Kara wrote:
> On Tue 31-01-12 11:14:15, Dave Chinner wrote:
> > I also found this oddity on both XFS and ext4:
> > 
> >     flush-253:32-3400  [001] 1936151.384563: writeback_start:      bdi 253:32: sb_dev 0:0 nr_pages=-898403 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background
> >     flush-253:32-3400  [005] 1936151.455845: writeback_start:      bdi 253:32: sb_dev 0:0 nr_pages=-911663 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background
> >     flush-253:32-3400  [006] 1936151.596298: writeback_start:      bdi 253:32: sb_dev 0:0 nr_pages=-931332 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background
> >     flush-253:32-3400  [006] 1936151.719074: writeback_start:      bdi 253:32: sb_dev 0:0 nr_pages=-951001 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background
> > 
> > That's indicating the work->nr_pages is starting extremely negative,
> > which should not be the case. The highest I saw was around -2m.
> > Something is not working right there, as writeback is supposed to
> > terminate if work->nr_pages < 0....
>   Ugh, what kernel is this? The tracepoint is just a couple of lines after
>                 if (work->nr_pages <= 0)
>                         break;
>   so I really don't see how that could happen.

It should be a recent kernel judging from the "reason=background"
field.

I cannot find such "writeback_start.*nr_pages=-" pattern at all in my
huge pile of saved tracing logs. Since the background work is only
started by wb_check_background_flush() with .nr_pages = LONG_MAX,
I can only find such patterns:

      flush-0:27-5216  [005] ....   472.119012: writeback_start: bdi 0:27: sb_dev 0:0 nr_pages=9223372036854775807 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background
      flush-0:27-5216  [005] ....   472.119076: writeback_start: bdi 0:27: sb_dev 0:0 nr_pages=9223372036854775803 sync_mode=0 kupdate=0 range_cyclic=1 background=1 reason=background

       flush-9:0-5176  [025] ....   475.578426: writeback_start: bdi 9:0: sb_dev 0:0 nr_pages=361160 sync_mode=0 kupdate=1 range_cyclic=1 background=0 reason=periodic
       flush-9:0-5176  [025] ....   475.710138: writeback_start: bdi 9:0: sb_dev 0:0 nr_pages=328392 sync_mode=0 kupdate=1 range_cyclic=1 background=0 reason=periodic

Thanks,
Fengguang
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