Re: [PATCH 0/3] [RFC][PATCH] clustered writeback

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Mon, Aug 27, 2007 at 05:03:36AM -0700, Arjan van de Ven wrote:
> On Mon, 27 Aug 2007 19:21:52 +0800
> > 
> > Because it does the work in small batches of 10 inodes, when the
> > system has <=10 dirty inodes, its behavior will reduce to:
> > - do a full sweep *at once* on every 25s
> > Which means the disk will flicker once every 25s, not bad :)
> 
> 25 seconds is quite not good already though.... it takes a disk a
> second or two of no activity to go into low power mode, every 25
> seconds means you now have at least a 10% constant power cost....
> 
> I don't know the right answer (well other than "make sure inodes aren't
> dirty", which involves fixing apps to not do as much file operations,
> as well as relatime) but just "every 25s is no big deal" isn't really
> the case ;-(

Yeah, 25s may be too frequent... What I meant is that the old behavior
could be "write 1-3 inodes on every 5s" if the inodes are dirtied at
random times. Now it becomes "write 10 inodes on every 25s". So it is
actually better ;-)

It's interesting that we want writeback to be smooth on heavy loads
and to be 'bursty' on light loads. Increasing dirty_expire_centisecs
and decreasing dirty_writeback_centisecs could help it somehow.