Re: [RFC] mm: Proactive compaction

[Nitin Gupta <nigupta@nvidia.com>]

On Tue, 2019-08-20 at 10:46 +0200, Vlastimil Babka wrote:
> > This patch is largely based on ideas from Michal Hocko posted here:
> > https://lore.kernel.org/linux-mm/20161230131412.GI13301@dhcp22.suse.cz/
> > 
> > Testing done (on x86):
> >   - Set /sys/kernel/mm/compaction/order-9/extfrag_{low,high} = {25, 30}
> >   respectively.
> >   - Use a test program to fragment memory: the program allocates all
> > memory
> >   and then for each 2M aligned section, frees 3/4 of base pages using
> >   munmap.
> >   - kcompactd0 detects fragmentation for order-9 > extfrag_high and starts
> >   compaction till extfrag < extfrag_low for order-9.
> > 
> > The patch has plenty of rough edges but posting it early to see if I'm
> > going in the right direction and to get some early feedback.
> 
> That's a lot of control knobs - how is an admin supposed to tune them to
> their
> needs?


Yes, it's difficult for an admin to get so many tunable right unless
targeting a very specific workload.

How about a simpler solution where we exposed just one tunable per-node:
   /sys/.../node-x/compaction_effort
which accepts [0, 100]

This parallels /proc/sys/vm/swappiness but for compaction. With this
single number, we can estimate per-order [low, high] watermarks for external
fragmentation like this:
 - For now, map this range to [low, medium, high] which correponds to specific
low, high thresholds for extfrag.
 - Apply more relaxed thresholds for higher-order than for lower orders.

With this single tunable we remove the burden of setting per-order explicit
[low, high] thresholds and it should be easier to experiment with.

-Nitin