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    SubjectRe: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs
    On Fri, Aug 27, 2010 at 09:42:54AM +0800, Wu Fengguang wrote:
    > On Fri, Aug 27, 2010 at 02:17:35AM +0800, Johannes Weiner wrote:
    > > On Thu, Aug 26, 2010 at 06:42:45PM +0100, Mel Gorman wrote:
    > > > On Fri, Aug 27, 2010 at 02:38:43AM +0900, Minchan Kim wrote:
    > > > > On Thu, Aug 26, 2010 at 04:14:16PM +0100, Mel Gorman wrote:
    > > > > > If congestion_wait() is called with no BDIs congested, the caller will
    > > > > > sleep for the full timeout and this is an unnecessary sleep. This patch
    > > > > > checks if there are BDIs congested. If so, it goes to sleep as normal.
    > > > > > If not, it calls cond_resched() to ensure the caller is not hogging the
    > > > > > CPU longer than its quota but otherwise will not sleep.
    > > > > >
    > > > > > This is aimed at reducing some of the major desktop stalls reported during
    > > > > > IO. For example, while kswapd is operating, it calls congestion_wait()
    > > > > > but it could just have been reclaiming clean page cache pages with no
    > > > > > congestion. Without this patch, it would sleep for a full timeout but after
    > > > > > this patch, it'll just call schedule() if it has been on the CPU too long.
    > > > > > Similar logic applies to direct reclaimers that are not making enough
    > > > > > progress.
    > > > > >
    > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    > > > > > ---
    > > > > > mm/backing-dev.c | 20 ++++++++++++++------
    > > > > > 1 files changed, 14 insertions(+), 6 deletions(-)
    > > > > >
    > > > > > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    > > > > > index a49167f..6abe860 100644
    > > > > > --- a/mm/backing-dev.c
    > > > > > +++ b/mm/backing-dev.c
    > > > >
    > > > > Function's decripton should be changed since we don't wait next write any more.
    > > > >
    > > >
    > > > My bad. I need to check that "next write" thing. It doesn't appear to be
    > > > happening but maybe that side of things just broke somewhere in the
    > > > distant past. I lack context of how this is meant to work so maybe
    > > > someone will educate me.
    > >
    > > On every retired io request the congestion state on the bdi is checked
    > > and the congestion waitqueue woken up.
    > >
    > > So without congestion, we still only wait until the next write
    > > retires, but without any IO, we sleep the full timeout.
    > >
    > > Check __freed_requests() in block/blk-core.c.
    >
    > congestion_wait() is tightly related with pageout() and writeback,
    > however it may have some intention for the no-IO case as well.
    >
    > - if write congested, maybe we are doing too much pageout(), so wait.
    > it might also reduce some get_request_wait() stalls (the normal way
    > is to explicitly check for congestion before doing write out).
    >
    > - if any write completes, it may free some PG_reclaim pages, so proceed.
    > (when not congested)
    >

    For these cases, would it make sense for wait_iff_congested() to compare
    nr_writeback to nr_inactive and decide to wait on congestion if more
    than half the inactive list is in writeback?

    > - if no IO at all, the 100ms sleep might still prevent a page reclaimer
    > from stealing lots of slices from a busy computing program that
    > involves no page allocation at all.
    >

    I don't think this is a very strong arguement because cond_reched() is
    being called.

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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