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

[David Rientjes <rientjes@google.com>]

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,

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.

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