Re: [PATCH 3/3] writeback: Do not congestion sleep when there areno congested BDIs

[Wu Fengguang]

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@xxxxxxxxx>
> > > > ---
> > > >  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)

- 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.

Thanks,
Fengguang
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