From: Dave Chinner <david@fromorbit.com>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH RFC 0/5] IO-less balance_dirty_pages() v2 (simple approach)
Date: Tue, 29 Mar 2011 13:14:58 +1100 [thread overview]
Message-ID: <20110329021458.GF3008@dastard> (raw)
In-Reply-To: <20110328024445.GA11816@localhost>
On Mon, Mar 28, 2011 at 10:44:45AM +0800, Wu Fengguang wrote:
> On Sat, Mar 26, 2011 at 07:05:44AM +0800, Jan Kara wrote:
> > And actually the NFS traces you pointed to originally seem to be different
> > problem, in fact not directly related to what balance_dirty_pages() does...
> > And with local filesystem the results seem to be reasonable (although there
> > are some longer sleeps in your JBOD measurements I don't understand yet).
>
> Yeah the NFS case can be improved on the FS side (for now you may just
> reuse my NFS patches and focus on other generic improvements).
>
> The JBOD issue is also beyond my understanding.
>
> Note that XFS will also see one big IO completion per 0.5-1 seconds,
> when we are to increase the write chunk size from the current 4MB to
> near the bdi's write bandwidth. As illustrated by this graph:
>
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/4G/xfs-1dd-1M-8p-3927M-20%25-2.6.38-rc6-dt6+-2011-02-27-22-58/global_dirtied_written-500.png
Which is _bad_.
Increasing the writeback chunk size simply causes dirty queue
starvation issues when there are lots of dirty files and lots more
memory than there is writeback bandwidth. Think of a machine with
1TB of RAM (that's a 200GB dirty limit) and 1GB/s of disk
throughput. Thats 3 minutes worth of writeback and increasing the
chunk size to ~1s worth of throughput means that the 200th dirty
file won't get serviced for 3 minutes....
We used to have behaviour similar to this this (prior to 2.6.16, IIRC),
and it caused all sorts of problems where people were losing 10-15
minute old data when the system crashed because writeback didn't
process the dirty inode list fast enough in the presence of lots of
large files....
A small writeback chunk size has no adverse impact on XFS as long as
the elevator does it's job of merging IOs (which in 99.9% of cases
it does) so I'm wondering what the reason for making this change
is.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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next prev parent reply other threads:[~2011-03-29 2:15 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-08 22:31 [PATCH RFC 0/5] IO-less balance_dirty_pages() v2 (simple approach) Jan Kara
2011-03-08 22:31 ` [PATCH 1/5] writeback: account per-bdi accumulated written pages Jan Kara
2011-03-08 22:31 ` [PATCH 2/5] mm: Properly reflect task dirty limits in dirty_exceeded logic Jan Kara
2011-03-09 21:02 ` Vivek Goyal
2011-03-14 20:44 ` Jan Kara
2011-03-15 15:21 ` Vivek Goyal
2011-03-08 22:31 ` [PATCH 3/5] mm: Implement IO-less balance_dirty_pages() Jan Kara
2011-03-10 0:07 ` Vivek Goyal
2011-03-14 20:48 ` Jan Kara
2011-03-15 15:23 ` Vivek Goyal
2011-03-16 21:26 ` Curt Wohlgemuth
2011-03-16 22:53 ` Curt Wohlgemuth
2011-03-16 16:53 ` Vivek Goyal
2011-03-16 19:10 ` Jan Kara
2011-03-16 19:31 ` Vivek Goyal
2011-03-16 19:58 ` Jan Kara
2011-03-16 20:22 ` Vivek Goyal
2011-03-08 22:31 ` [PATCH 4/5] mm: Remove low limit from sync_writeback_pages() Jan Kara
2011-03-08 22:31 ` [PATCH 5/5] mm: Autotune interval between distribution of page completions Jan Kara
2011-03-17 15:46 ` [PATCH RFC 0/5] IO-less balance_dirty_pages() v2 (simple approach) Curt Wohlgemuth
2011-03-17 15:51 ` Christoph Hellwig
2011-03-17 16:24 ` Curt Wohlgemuth
2011-03-17 16:43 ` Christoph Hellwig
2011-03-17 17:32 ` Jan Kara
2011-03-17 18:55 ` Curt Wohlgemuth
2011-03-17 22:56 ` Vivek Goyal
2011-03-18 14:30 ` Wu Fengguang
2011-03-22 21:43 ` Jan Kara
2011-03-23 4:41 ` Dave Chinner
2011-03-25 12:59 ` Wu Fengguang
2011-03-25 13:44 ` Wu Fengguang
2011-03-25 23:05 ` Jan Kara
2011-03-28 2:44 ` Wu Fengguang
2011-03-28 15:08 ` Jan Kara
2011-03-29 1:44 ` Wu Fengguang
2011-03-29 2:14 ` Dave Chinner [this message]
2011-03-29 2:41 ` Wu Fengguang
2011-03-29 5:59 ` Dave Chinner
2011-03-29 7:31 ` Wu Fengguang
2011-03-29 7:52 ` Wu Fengguang
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