Re: [PATCH 3/5] mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback

Mel Gorman
2011-07-14T06:33:38Z

On Thu, Jul 14, 2011 at 09:41:50AM +1000, Dave Chinner wrote:
> On Wed, Jul 13, 2011 at 03:31:25PM +0100, Mel Gorman wrote:
> > Workloads that are allocating frequently and writing files place a
> > large number of dirty pages on the LRU. With use-once logic, it is
> > possible for them to reach the end of the LRU quickly requiring the
> > reclaimer to scan more to find clean pages. Ordinarily, processes that
> > are dirtying memory will get throttled by dirty balancing but this
> > is a global heuristic and does not take into account that LRUs are
> > maintained on a per-zone basis. This can lead to a situation whereby
> > reclaim is scanning heavily, skipping over a large number of pages
> > under writeback and recycling them around the LRU consuming CPU.
> >
> > This patch checks how many of the number of pages isolated from the
> > LRU were dirty. If a percentage of them are dirty, the process will be
> > throttled if a blocking device is congested or the zone being scanned
> > is marked congested. The percentage that must be dirty depends on
> > the priority. At default priority, all of them must be dirty. At
> > DEF_PRIORITY-1, 50% of them must be dirty, DEF_PRIORITY-2, 25%
> > etc. i.e. as pressure increases the greater the likelihood the process
> > will get throttled to allow the flusher threads to make some progress.
>
> It still doesn't take into account how many pages under writeback
> were skipped. If there are lots of pages that are under writeback, I
> think we still want to throttle to give IO a chance to complete and
> clean those pages before scanning again....
>

An earlier revision did take them into account but in these tests at
least, 0 pages at the end of the LRU were PageWriteback. I expect this
to change when multiple processes and CPUs were in use but am ignoring
it for the moment.

--
Mel Gorman
SUSE Labs