Re: compaction: trying to understand the code

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Aug 19, 2010 at 04:15:54PM +0800, Mel Gorman wrote:
> 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;

Ah sorry! I confused it with section size..

Thanks,
Fengguang

> 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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>