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


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