Re: [RFC PATCH 5/5] mm/compaction: run the compaction whenever fragmentation ratio exceeds the threshold

[Vlastimil Babka <vbabka@suse.cz>]

On 01/13/2017 08:14 AM, js1304@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> Until now, we invoke the compaction whenever allocation request is stall
> due to non-existence of the high order freepage. It is effective since we
> don't need a high order freepage in usual and cost of maintaining
> high order freepages is quite high. However, it increases latency of high
> order allocation request and decreases success rate if allocation request
> cannot use the reclaim/compaction. Since there are some workloads that
> require high order freepage to boost the performance, it is a matter of
> trade-off that we prepares high order freepage in advance. Now, there is
> no way to prepare high order freepages, we cannot consider this trade-off.
> Therefore, this patch introduces a way to invoke the compaction when
> necessary to manage trade-off.
> 
> Implementation is so simple. There is a theshold to invoke the full
> compaction. If fragmentation ratio reaches this threshold in given order,
> we ask the full compaction to kcompactd with a hope that it restores
> fragmentation ratio.
> 
> If fragmentation ratio is unchanged or worse after full compaction,
> further compaction attempt would not be useful. So, this patch
> stops the full compaction in this case until the situation changes
> to avoid useless compaction effort.
> 
> Now, there is no scientific code to detect the situation change.
> kcompactd's full compaction would be re-enabled when lower order
> triggers kcompactd wake-up or time limit (a second) is passed.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

So, as you expected, I'm not thrilled about the tunables :) And also the
wakeups from allocator hotpaths. Otherwise I'll wait with discussing
details until we get some consensus on usecases and metrics.

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