On Thu, Aug 19, 2010 at 04:08:31PM +0800, Wu Fengguang wrote:
> On Thu, Aug 19, 2010 at 03:46:02PM +0800, Mel Gorman wrote:
> > On Thu, Aug 19, 2010 at 04:09:38PM +0900, Iram Shahzad wrote:
> > >> The loop should be waiting for the _other_ processes (doing direct
> > >> reclaims) to proceed. When there are _lots of_ ongoing page
> > >> allocations/reclaims, it makes sense to wait for them to calm down a bit?
> > >
> > > I have noticed that if I run other process, it helps the loop to exit.
> > > So is this (ie hanging until other process helps) intended behaviour?
> > >
> >
> > No, it's not but I'm not immediately seeing how it would occur either.
> > too_many_isolated() should only be true when there are multiple
> > processes running that are isolating pages be it due to reclaim or
> > compaction. These should be finishing their work after some time so
> > while a process may stall in too_many_isolated(), it should not stay
> > there forever.
> >
> > The loop around isolate_migratepages() puts back LRU pages it failed to
> > migrate so it's not the case that the compacting process is isolating a
> > large number of pages and then calling too_many_isolated() against itself.
>
> It seems the compaction process isolates 128MB pages at a time?
It should be one pageblock at a time for source migration and one pageblock
for target pages. Look at the values for low_pfn and end_pfn here;
static unsigned long isolate_migratepages(struct zone *zone,
struct compact_control *cc)
{
unsigned long low_pfn, end_pfn;
struct list_head *migratelist = &cc->migratepages;
/* Do not scan outside zone boundaries */
low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
/* Only scan within a pageblock boundary */
end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
....
and the loop around that looks like
while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
unsigned long nr_migrate, nr_remaining;
if (!isolate_migratepages(zone, cc))
continue;
nr_migrate = cc->nr_migratepages;
migrate_pages(&cc->migratepages, compaction_alloc,
(unsigned long)cc, 0);
update_nr_listpages(cc);
nr_remaining = cc->nr_migratepages;
count_vm_event(COMPACTBLOCKS);
count_vm_events(COMPACTPAGES, nr_migrate - nr_remaining);
if (nr_remaining)
count_vm_events(COMPACTPAGEFAILED, nr_remaining);
/* Release LRU pages not migrated */
if (!list_empty(&cc->migratepages)) {
putback_lru_pages(&cc->migratepages);
cc->nr_migratepages = 0;
}
}
Where is it isolating 128MB?
> That
> sounds risky, too_many_isolated() can easily be true, which will stall
> direct reclaim processes. I'm not seeing how exactly it makes
> compaction itself stall infinitely though.
>
> > > Also, the other process does help the loop to exit, but again it enters
> > > the loop and the compaction is never finished. That is, the process
> > > looks like hanging. Is this intended behaviour?
> >
> > Infinite loops are never intended behaviour.
>
> Yup.
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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