Re: [RFC 1/4] mm, compaction: introduce kcompactd

[Joonsoo Kim]

On Thu, Jul 23, 2015 at 01:58:20PM -0700, David Rientjes wrote:
> On Thu, 23 Jul 2015, Joonsoo Kim wrote:
> 
> > > The slub allocator does try to allocate its high-order memory with 
> > > __GFP_WAIT before falling back to lower orders if possible.  I would think 
> > > that this would be the greatest sign of on-demand memory compaction being 
> > > a problem, especially since CONFIG_SLUB is the default, but I haven't seen 
> > > such reports.
> > 
> > In fact, some of our product had trouble with slub's high order
> > allocation 5 months ago. At that time, compaction didn't make high order
> > page and compaction attempts are frequently deferred. It also causes many
> > reclaim to make high order page so I suggested masking out __GFP_WAIT
> > and adding __GFP_NO_KSWAPD when trying slub's high order allocation to
> > reduce reclaim/compaction overhead. Although using high order page in slub
> > has some gains that reducing internal fragmentation and reducing management
> > overhead, benefit is marginal compared to the cost at making high order
> > page. This solution improves system response time for our case. I planned
> > to submit the patch but it is delayed due to my laziness. :)
> > 
> 
> Hi Joonsoo,

Hello David.

> 
> On a fragmented machine I can certainly understand that the overhead 
> involved in allocating the high-order page outweighs the benefit later and 
> it's better to fallback more quickly to page orders if the cache allows 
> it.
> 
> I believe that this would be improved by the suggestion of doing 
> background synchronous compaction.  So regardless of whether __GFP_WAIT is 
> set, if the allocation fails then we can kick off background compaction 
> that will hopefully defragment memory for future callers.  That should 
> make high-order atomic allocations more successful as well.

Yep! I also think __GFP_NO_KSWAPD isn't appropriate for general case.
Reason I suggested __GFP_NO_KSWAPD to our system is that reclaim/compaction
continually fails to make high order page so we don't want to invoke
reclaim/compaction even though it works in background. But, on almost of
other system, reclaim/compaction could succeed so adding __GFP_NO_KSWAPD
doens't make sense for general case.

Thanks.
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