Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

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

On Sun, Jul 25, 2010 at 08:03:45PM +0800, Minchan Kim wrote:
> On Sun, Jul 25, 2010 at 07:43:20PM +0900, KOSAKI Motohiro wrote:
> > Hi
> > 
> > sorry for the delay.
> > 
> > > Will you be picking it up or should I? The changelog should be more or less
> > > the same as yours and consider it
> > > 
> > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > > 
> > > It'd be nice if the original tester is still knocking around and willing
> > > to confirm the patch resolves his/her problem. I am running this patch on
> > > my desktop at the moment and it does feel a little smoother but it might be
> > > my imagination. I had trouble with odd stalls that I never pinned down and
> > > was attributing to the machine being commonly heavily loaded but I haven't
> > > noticed them today.
> > > 
> > > It also needs an Acked-by or Reviewed-by from Kosaki Motohiro as it alters
> > > logic he introduced in commit [78dc583: vmscan: low order lumpy reclaim also
> > > should use PAGEOUT_IO_SYNC]
> > 
> > My reviewing doesn't found any bug. however I think original thread have too many guess
> > and we need to know reproduce way and confirm it.
> > 
> > At least, we need three confirms.
> >  o original issue is still there?
> >  o DEF_PRIORITY/3 is best value?
> 
> I agree. Wu, how do you determine DEF_PRIORITY/3 of LRU?
> I guess system has 512M and 22M writeback pages. 
> So you may determine it for skipping max 32M writeback pages.
> Is right?

For 512M mem, DEF_PRIORITY/3 means 32M dirty _or_ writeback pages.
Because shrink_inactive_list() first calls
shrink_page_list(PAGEOUT_IO_ASYNC) then optionally 
shrink_page_list(PAGEOUT_IO_SYNC), so dirty pages will first be
converted to writeback pages and then optionally be waited on.

The dirty/writeback pages may go up to 512M*20% = 100M. So 32M looks
a reasonable value.

> And I have a question of your below comment. 
> 
> "As the default dirty throttle ratio is 20%, sync write&wait
> will hardly be triggered by pure dirty pages"
> 
> I am not sure exactly what you mean but at least DEF_PRIOIRTY/3 seems to be
> related to dirty_ratio. It always can be changed by admin.
> Then do we have to determine magic value(DEF_PRIORITY/3)  proportional to dirty_ratio?

Yes DEF_PRIORITY/3 is already proportional to the _default_
dirty_ratio. We could do explicit comparison with dirty_ratio
just in case dirty_ratio get changed by user. It's mainly a question
of whether deserving to add such overheads and complexity. I'd prefer
to keep the current simple form :)

Thanks,
Fengguang