Re: [PATCH 01/35] writeback: enabling gate limit for light dirtied bdi

[Wu Fengguang <fengguang.wu@intel.com>]

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On Fri, Jan 14, 2011 at 03:26:10AM +0800, Peter Zijlstra wrote:
> On Thu, 2011-01-13 at 11:44 +0800, Wu Fengguang wrote:
> > When testing 10-disk JBOD setup, I
> > find that bdi_dirty_limit fluctuations too much. So I'm considering
> > use global_dirty_limit as control target. 
> 
> Is this because the bandwidth is equal or larger than the dirty period?

The patchset will call ->writepages(N) with
N=rounddown_pow_of_two(bdi->write_bandwidth). XFS will then typically
do endio batches in the same amount. I see in practice XFS's
xfs_end_io() work get queued and executed ~2 times per second,
normally clearing 32MB worth of PG_writeback. I guess this is one
major source of fluctuation.

The attached XFS graphs can confirm this. The "written" and
"writeback" curves are skipping at 32MB size.

As for the dirty period,

        calc_period_shift()
        = 2 + ilog2(dirty_total - 1)
        = 2 + ilog2(380000)             # a 8GB test box, 20% dirty_ratio
        = 19

So period = (1 << 18) = 256k pages = 1GB. It's much larger than 32MB.
(Please correct me if wrong).

The problem is not limited to XFS. ext2/ext3/ext4 are also fluctuating
in a range up to bdi->write_bandwidth.

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/16G-10HDD-JBOD/ext2-fio-jbod-sync-128k-24p-15977M-2.6.37-rc8-dt5+-2010-12-31-19-36/balance_dirty_pages-pages.png
http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/16G-10HDD-JBOD/ext4_wb-fio-jbod-sync-128k-24p-15977M-2.6.37-rc8-dt5+-2010-12-31-12-24/balance_dirty_pages-pages.png

I noticed (ext2/ext3 graphs attached) that they are clearing
PG_writeback in much smaller batches at least in 1-disk case.
However the writeback pages can go low for 1-2 times in every 10
seconds.

Thanks,
Fengguang

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