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

[David Rientjes]

On Thu, 2 Jul 2015, Vlastimil Babka wrote:

> Memory compaction can be currently performed in several contexts:
> 
> - kswapd balancing a zone after a high-order allocation failure
> - direct compaction to satisfy a high-order allocation, including THP page
>   fault attemps
> - khugepaged trying to collapse a hugepage
> - manually from /proc
> 
> The purpose of compaction is two-fold. The obvious purpose is to satisfy a
> (pending or future) high-order allocation, and is easy to evaluate. The other
> purpose is to keep overal memory fragmentation low and help the
> anti-fragmentation mechanism. The success wrt the latter purpose is more
> difficult to evaluate.
> 
> The current situation wrt the purposes has a few drawbacks:
> 
> - compaction is invoked only when a high-order page or hugepage is not
>   available (or manually). This might be too late for the purposes of keeping
>   memory fragmentation low.
> - direct compaction increases latency of allocations. Again, it would be
>   better if compaction was performed asynchronously to keep fragmentation low,
>   before the allocation itself comes.
> - (a special case of the previous) the cost of compaction during THP page
>   faults can easily offset the benefits of THP.
> 
> To improve the situation, we need an equivalent of kswapd, but for compaction.
> E.g. a background thread which responds to fragmentation and the need for
> high-order allocations (including hugepages) somewhat proactively.
> 
> One possibility is to extend the responsibilities of kswapd, which could
> however complicate its design too much. It should be better to let kswapd
> handle reclaim, as order-0 allocations are often more critical than high-order
> ones.
> 
> Another possibility is to extend khugepaged, but this kthread is a single
> instance and tied to THP configs.
> 
> This patch goes with the option of a new set of per-node kthreads called
> kcompactd, and lays the foundations. The lifecycle mimics kswapd kthreads.
> 
> The work loop of kcompactd currently mimics an pageblock-order direct
> compaction attempt each 15 seconds. This might not be enough to keep
> fragmentation low, and needs evaluation.
> 
> When there's not enough free memory for compaction, kswapd is woken up for
> reclaim only (not compaction/reclaim).
> 
> Further patches will add the ability to wake up kcompactd on demand in special
> situations such as when hugepages are not available, or when a fragmentation
> event occured.
> 

Thanks for looking at this again.

The code is certainly clean and the responsibilities vs kswapd and 
khugepaged are clearly defined, but I'm not sure how receptive others 
would be of another per-node kthread.

Khugepaged benefits from the periodic memory compaction being done 
immediately before it attempts to compact memory, and that may be lost 
with a de-coupled approach like this.

Initially, I suggested implementing this inside khugepaged for that 
purpose, and the full compaction could be done on the next 
scan_sleep_millisecs wakeup before allocating a hugepage and when 
kcompactd_sleep_millisecs would have expired.  So the true period between 
memory compaction events could actually be
kcompactd_sleep_millisecs - scan_sleep_millisecs.

You bring up an interesting point, though, about non-hugepage uses of 
memory compaction and its effect on keeping fragmentation low.  I'm not 
sure of any reports of that actually being an issue in the wild?

I know that the networking layer has done work recently to reduce page 
allocator latency for high-order allocations that can easily fallback to 
order-0 memory: see commit fb05e7a89f50 ("net: don't wait for order-3 page 
allocation").

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.

So I'm inclined to think that the current trouble spot for memory 
compaction is thp allocations.  I may live to find differently :)

How would you feel about implementing this as part of the khugepaged loop 
before allocating a hugepage and scanning memory?
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