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

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

On Mon, Jul 26, 2010 at 12:11:59PM +0800, Minchan Kim wrote:
> On Mon, Jul 26, 2010 at 12:27 PM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> > 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.
> 
> Why do you think it's a reasonable value?
> I mean why isn't it good 12.5% or 3.125%? Why do you select 6.25%?
> I am not against you. Just out of curiosity and requires more explanation.
> It might be thing _only I_ don't know. :(

It's more or less random selected. I'm also OK with 3.125%. It's an
threshold to turn on some _last resort_ mechanism, so don't need to be
optimal..

> >
> >> 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 :)
> 
> What I suggest is that couldn't we use recent_writeback/recent_scanned ratio?
> I think scan_control's new filed and counting wouldn't be a big
> overhead and complexity.
> I am not sure which ratio is best. but at least, it would make the
> logic scalable and sense to me. :)

..and don't need to be elaborated :)

Thanks,
Fengguang